Battery Ups Earthing

EEEEEETTTPPP///BBBSSSNNNLLL SILVER

CERTIFICATION COURSE TELECOM SUPPORT INFRASTRUCTURE

BATTERY UPS AND EARTHING

Version 2 June 2014

Telecom Support Infrastructure (TSI) Battery, UPS & Earthing

EETP/ BSNL Silver Certification Course /Ver.02/June 2014 Page 1 of 36

For Restricted Circulation

BATTERY UPS & EARTHING

INDEX

2.1 Introduction ..................................................................... 2

2.2 Objective ........................................................................... 2

2.3 Different methods of working of a battery .................... 3

2.4 Secondary cells (conventional) ....................................... 8

2.5 Maintenance free battery (VRLA) ................................. 9

2.6 Working and maintenance of UPS............................... 17

2.7 Earthing system in telecom ........................................... 22

2.8 Summary ........................................................................ 33

2.9 Self Assessment questions ............................................. 34

2.10 References and suggested further readings ................ 34




2 BATTERY UPS & EARTHING

STRUCTURE

2.1 INTRODUCTION

2.2 OBJECTIVE

2.3 DIFFERENT METHODS OF WORKING OF A BATTERY

2.4 SECONDARY CELLS (CONVENTIONAL)

2.5 MAINTENANCE FREE BATTERY (VRLA)

2.6 WORKING AND MAINTENANCE OF UPS

2.7 EARTHING SYSTEM IN TELECOM

2.8 SUMMARY

2.9 SELF ASSESSMENT QUESTIONS

2.10 REFERENCES AND SUGGESTED FURTHER READINGS

2.1 INTRODUCTION

The secondary cell batteries can be utilized in many ways. However presently

float working is adopted. The two type of batteries used are Conventional batteries and

Maintenance Free VRLA batteries. Maintenance free, valve-regulated lead-acid (VRLA)

batteries ensure a reliable, effective and user friendly source of power. It is spill proof,

leak proof and explosion resistant and there is no need to add water or to clean terminals.

Uninterrupted Power Supply (UPS) is required to provide stable AC in the event

of failure of Mains. It comprises of Battery banks coupled with inverter. An inverter is an

electrical device that converts direct current (DC) obtained from battery to alternating

current (AC); the converted AC can be at any required voltage and frequency with the use

of appropriate transformers, switching, and control circuits.

Earthing is an essential part of any electric /electronic system to prevent damage

and for proper operation of telecom equipment. Utmost care is required to be taken for

provision of proper and effective earthing for various telecom installations.

2.2 OBJECTIVE

The objective of this chapter is:

To know the different methods of battery working

To understand operating procedure of of float scheme

To explain initial charging and discharging

To enumerate different meters used

To explain the VRLA battery construction

To enumerate the chemical reactions




To explain the method used for charging

The principle of working of UPS system

The alarms in the UPS System

To list the objective of Earthing

To explain the requirement of Effective Earthing

To know the Earthing System for Electrical Installation

To know the Earthing System for Telecom Installations

2.3 DIFFERENT METHODS OF WORKING OF A BATTERY

The secondary batteries can be utilized in any of the following ways:

The charge discharge system:

In this system the electrical energy derived from the supply mains is first

converted into chemical energy and stored in the battery Subsequently the chemical

energy is reconverted into electrical energy when the battery discharges into the exchange

load. In the charge discharge system the entire DC supply for the equipment is taken from

the battery.

ii) Standby battery system:

In this system, the DC electrical energy for the operation of the equipment is

drawn directly from the public AC power supply system through mains conversion

equipment and the standby battery is kept fully charged and idle, i.e. not connected to the

load. Whenever there is mains failure the equipment load is immediately transferred from

the mains conversion equipment to the standby battery.

Fig1. Charge discharge system




(iii) Float System:

It is a variation of the standby battery scheme as in this system also, the battery

supplies only the emergency reserve requirements of the equipment during mains failure.

Unlike the standby battery scheme, the battery is always kept connected across the load in

the float system in a fully charged state. But the operating conditions are so maintained

that the floated battery does not discharge into the load as long as the mains power supply

is available. The main distinction between the floating battery scheme and the standby

battery scheme is that in the float scheme, there is absolutely no break in the DC power

supply when the mains supply fails as the emergency battery takes over the load

instantaneously. It is not the case for the standby scheme.

In this system the battery is always kept connected across the load in the float

system in a fully charged state. But the operating conditions are so maintained that the

floated battery does not discharge into the load as long as the mains power supply is

available. There is absolutely no break in the DC power supply when the mains supply

fails as the emergency battery takes over the load instantaneously.

The charge/discharge system was the standard system for manual exchange and

telegraph power plant operation for a number of years but has been superseded by float

systems for all the installations from late 60s onwards.

The float system can be worked on a fully automatic basis or on a semiautomatic

basis with a minimum of maintenance attention. Float systems are economical since

Fig2. Standby battery system

Fig3. Float Working




practically all the DC power requirements are taken directly from the public AC mains

supply through converters. With the battery connected in parallel almost full battery

capacity is available to carry the equipment load under mains failure conditions. It is

usually to provide a battery capacity equivalent to six times the busy hour load in float

systems.

2.3.1 Operating procedure

The operating procedure for floating batteries depends to a great extent on the

floating voltage. The floating voltage cannot be higher than 2.35 volts per cell since

beyond this limit, overcharging and vigorous gassing of the battery occurs. The lower

limit for the floating voltage of a lead acid cell is 2 volts, since below this voltage, the

battery will discharge. Within the permissible range of floating voltage from 2.0 to 2.35

volts per cell, three distinct types of full float schemes have been evolved. They are as

follows:-

Batteries floated between 2.02 to 2.07 volt per cell or divided battery float system.

Batteries floated between 2.15 to 2.20 volt per cell or parallel battery float system.

Batteries floated between 2.24 to 2.30 volt per cell or end cell switching system.

2.3.2 Requirements of float scheme

Voltage should be maintained between prescribed limits.

The capacity of the battery should be adequate to meet emergency.

It should last longer.

The energy losses should be minimum.

2.3.3 Details of float schemes

In automatic telephone exchanges the permissible range of voltage is from 46 to

52 V. Instead of referring these float schemes by the floating voltage per cell, they are

often named after the total number of cells in each set of the floated battery. From this

standpoint these float scheme are usually referred as the 25 cell, 24 cell and 23 cell

schemes.

The 24 cell float scheme or parallel battery float scheme has now been prescribed

as the standard method of supplying power to telephone exchanges. It is also adopted for

60V plus and minus supply in telegraph offices and telex exchanges.

In this scheme the battery voltage is maintained within very close limits (51.5 0.5). The required battery capacity, (ordinarily for 6 to 8 busy hours ) is provided equally

in two sets of batteries to facilitate maintenance attention to any one battery set. Both the

battery sets are floated in parallel simultaneously. Ref. Fig.3. as the floating voltage of

2.15 V per cell is within the trickle charging range, a steady trickle charge current flows

and recoups all capacity losses due to local action, partial discharge on peak loads etc.,

incurred during the floating periods. Thus the floated battery is maintained at its full

capacity, which is available as reserve for mains failure conditions. Refresher charges at

three monthly intervals and yearly reconditioning cycle are given to the battery to ensure

that the battery is kept in a healthy condition. Under mains failure conditions the battery

voltage drops down to 48 volts for 24 cells and the lower operating voltage of exchanges

is limited to 46 volts thus the end point for the discharge of the cells has to be limited to

1.92 volt per cell. Thus only about 52% of the rated capacity of the cell is available for

use. Therefore higher capacity batteries are required.




2.3.4 Initial Charging

Initial charging is the charge received by any battery for the first time after

installation. The following procedure is a general guideline for the first or initial charge.

The instructions given by the manufacturer is to be followed strictly.

General:

Make sure that the battery room is well ventilated. Install batteries so that they are

not exposed to direct rays from the sun and are away from steam pipes, radiators and

other heat generating equipment, as a warm battery tends to have a self discharge at an

accelerated rate, increasing the load on the power supply and shortening life of the

battery. Align the cells and make sure all connections are tight.

Initial Charge

Before placing the battery on charge, check and record the open circuit voltage of

every cell. After this connect the battery for charging continuously at the rate specified

for 80 hours or as specified by the manufacturer by using direct current. The voltage

output from the charger should be minimum 2.7 volts per cell. Another method of

charging prescribed is start the charging @ 14% of AH capacity and as the cells start

gassing reduce the rate of charge to 7% till the end of the charge.

As soon as the battery is put on charge, take another set of voltage readings and

enter these readings opposite to open circuit voltage readings. This is to check if there are

any reversed cells and to avoid reverse connection of the entire battery. If the battery is

connected properly and there are no reversed cells, proceed with charging. During first

charge take individual cell voltage, S. G. and temperature readings for every eight hours

and record

The cells are considered to be fully charged when values of cell voltage and S.G.

of electrolyte corrected to 27o C remain constant for three consecutive hours at the end of

approximately 80 hours of charging. However in spite of achieving constancy of S.G. and

voltage readings, initial charge has to be continued for 80 hours (in the case of 80 hrs of

charge method). The voltage at the end of initial charge will be 2.65 to 2.7 volts per cell

The temperature of electrolyte in the cells at any period of charge should not be

allowed to rise beyond 50o C

At the end of charging the S.G. of electrolyte is to be adjusted to 1.200 0.005 at 27 o C. If the S.G. at the end of charging is above 1.200, add distilled water and if it is

below 1.200 add 1.400 S.G. acid. If any of the cells show abnormally low S.G. / voltage

at the end of 80 hours initial charging inform the supplier.After ensuring that the cell is

fully charged, give a rest of 12 to 24hrs and conduct test discharge to ensure the

following.

The capacity of the battery.

The efficiency of the battery.

The specific gravity range of the battery.

Normally the capacity does not reach 100% until 4 or 5 cycles of charge

discharges.

2.3.5 Test Discharge

Test discharge is conducted at 10 hr rate.




Instruments and accessories required

(i) Artificial resistance load

An artificial resistance load made up of lamp resistance or coils of wires is used in

case of low capacity batteries. For large cells electrodes immersed in water with little acid

or wire coils (300 lbs GI) are used. Care is taken to avoid contact between the electrodes.

(ii) Voltmeter

Two voltmeters with suitable ranges are required to measure the terminal voltages

across the individual cells and the battery. The voltmeter should have a resistance of not

less than 1000 ohms per volt and an accuracy of 1% of the full scale reading. AVO meter

/ Digital Multi meter can be used in place of voltmeter.

(iii) Ammeter

Each division of the ammeter scale should denote not more than 1% of the full

scale reading. It should be accurate to 1% of the full scale reading. The scale of the meter

should be so chosen that the value of the currents to be measured comes to 2/3rd of the

full scale of the meter.

(iv)Hydrometer

The hydrometer should be capable of indicating S.G. reading in the steps of 0.002

(2 points) . In any event the steps should not be at intervals longer than 0.005 (5 points)

(v)Thermometer

A thermometer capable of reading correct to 0.5c or 1F should be used.

2.3.6 Discharging procedure

Connect cable lugs to the cable for connecting artificial load to the battery.

Prepare artificial load.

Connect the connecting cable to the artificial load.

Disconnect the battery from the float /charger.

Connect the connecting cable to the ammeter in the circuit.

Take the open circuit voltage of the battery and of each individual cell.

Connect the connecting cable to the battery fuse output by nut and bolts in battery room.

Adjust the artificial load to get 10hour discharge rate. Adjust the artificial load frequently to maintain constant current.

The total voltage of the battery, the voltage, specific gravity and temperature of each cell should be read at the beginning of the discharge

and at hourly intervals up to the 8th hour and these readings are taken

every 15 minutes after 8-1/2 hours of discharge. These readings should be

recorded in the form given.

Stop discharge if the electrolyte temperature exceeds 37.8 c (100F) and

start discharge when the temperature falls sufficiently below 35 c.

The test discharge shall be stopped as soon as the overall battery voltage reaches a value equivalent to 1.85 V x no. Of cells in series (44.4 V for 24

cell Battery) or when the voltage across any cell falls to 1.70V whichever

is earlier.

Cells have to be recharged soon after discharge




2.3.7 Recharging

Recharging is done at two rates. The staring rate being maintained till the cells

reach 2.4 Volts per cell, after which at the finishing rate till end of charge. The high rate

of charge is normally 14 % and finishing rate is 7 % of battery capacity. Charging should

be completed at 7% of the capacity till full charge. Alternatively, the cells can be charged

at 10 hour rate till constancy. The battery is considered fully charged when all the cells

gas freely for 2 hours and the cell voltage and S.G. remain constant for 3 consecutive

hours thereafter.

It is recommended to conduct cycles of discharge and charge as above before

connecting to the associated equipment. Find out the efficiency of the battery. If 100%

capacity is attained in the 1st discharge itself then there is no need to discharge it for a

second time. Strict adherence to the instructions and maintenance manual supplied by

manufacturer is a must for making any complaint to the supplier. All records as per the

suppliers is instructions must be maintained.

2.4 SECONDARY CELLS (CONVENTIONAL)

One of the primary requirements of any telephone system is that service shall be

available to the subscribers at all times. The electrical energy required for signaling,

switching, speech transmission etc. in telephone exchanges is derived either directly or

indirectly from the public electricity system. In order to provide uninterrupted service, the

exchange power supply system is designed to give continuous energy to the system. So

provision is also made for alternate source of supply in the event of mains failure. This

emergency energy is derived from Batteries of secondary cells or a combination of

battery and prime mover generator sets.

The secondary cells in general use in our dept. are of lead acid type. Secondary

cells are electrolytic cells for generation of electric energy. These cells can be restored to

its original condition after they are discharged. This restoration is done by passing a

current in a direction opposite to the flow of current in the cell during the discharge.

2.4.1 Type of secondary cells

There are three types of storage (secondary) cells in use. They are (1) lead-lead-

acid type (2) Nickel-iron-alkaline and (3) Nickel-Cadmium alkaline type. In telecom only

lead-lead-acid type is used. They are commonly known as lead acid type cell. These cells

have electrodes of lead immersed in an electrolyte of dilute sulphuric acid in a suitable

container

There are two lead-acid designs in use today: the flooded and the valve regulated

(VRLA). Of the two designs, the flooded battery is the more reliable and should be the

battery of choice for mission critical applications. VRLA batteries have the advantage of

lower cost and lower space requirements




2.5 MAINTENANCE FREE BATTERY (VRLA)

Maintenance free, valve-regulated lead-acid (VRLA) batteries ensure a reliable,

effective and user friendly source of power. It is spill proof, leak proof and explosion

resistant and there is no need to add water or to clean terminals. It has low self-discharge

rate which eliminates the need for equalizing charges.

2.5.1 VRLA Technology

The container of VRLA Battery is made of polypropylene. Each plate is

individually wrapped by a highly absorbent, micro porous glass separate developed

specially for VRLA batteries. The chemically inert glass ensures lifelong service. The

absorbed electrolyte ensures that there is no spillage even in the unlikely event of

puncture of the cell. Gas evolution under float conditions is negligible. The water loss

throughout life due to gassing is roughly 0.1% of the total electrolyte present in the cell.

This will in no way affect performance and also eliminate the need for specially

ventilated battery room and acid resisting flooring. As the batteries can be installed in

stacks, there will be considerable space saving also.

Various capacities of Batteries are 120 AH, 200 AH, 400 AH, 600 AH, 1000 AH,

1500 AH, 2000 AH, 2500 AH, 3000 AH, 4000 AH and 5000 AH.

2.5.2 A brief review of Chemical Reaction

The electrode reaction in all lead acid batteries including VRLA battery is

basically identical. As the battery is discharged, the lead dioxide positive active material

and the spongy lead negative active material react with the sulphuric acid electrolyte to

form lead sulphate and water. During charge, this process is reversed. The Columbic

efficiency of the charging process is less than 100% on reaching final stage of charging or

under over charge conditions, the charging energy is consumed for electrolytic

decomposition of water and the positive plates generate oxygen gas and the negative

plates generate hydrogen gas.

Fig4. Secondary Cell (Conventional)




Under typical charging conditions, oxygen at the positive plate occurs before

hydrogen evolution at the negative. This feature is utilized in the design of VRLA

batteries. In flooded cells, the oxygen gas evolved at the positive plate bubbles upwards

through the electrolyte and is released through the vents. In MF-VRLA batteries the

oxygen gas evolved, at the positive plate, instead of bubbling upwards is transported in

the gas phase through the separator medium to the negative plate. The separator is a

highly absorbent glass matrix type with very high porosity, designed to have pore volume

in excess of the electrolyte volume (starved electrolyte design), due to which the oxygen

gas finds an unimpeded path to the negative plate.

The oxygen gas gets reduced by reaction with the spongy lead at the negative

plate, turning a part of it into a partially discharged condition, there by effectively

suppressing the hydrogen gas evolution at the negative plate. This is what is known as the

oxygen recombination principle. The part of negative plate which was partially

discharged is then reverted to the original spongy lead by subsequent charging.

Thus, a negative plate keeps equilibrium between the amount which turns into

spongy lead by charging and the amount of spongy lead which turns into lead sulphate by

absorbing the oxygen gas generated at the positive plate. The oxygen recombination

principle can be shown by the following reaction:

Reaction at positive plate:

H2O = O2 + 2e (1)

Reaction at negative plate :

Pb + 1/2O2 = PbO (2)

PbO+H2SO4 = PbSO4 + H2O (3)

To reaction (1)

PbSO4 + 2H+ + 2e = Pb + H2SO4 (4)

To reaction (3)

To reaction (2)

The total reaction at negative plate

O2 +2H+ = H2O

Thus, the oxygen recombination technology makes the battery virtually

maintenance Free.




2.5.3 Freshening Charge

Batteries lose some charge during transportation as well as during the period prior

to installation. A battery should be installed and given a freshening charge after receipt as

soon as possible. Battery positive (+) terminal should be connected to charge positive (+)

terminal and battery negative (-) terminal to charger negative (-) terminal.

The charge intervals for storage are given below.

Temp in centigrade Charging interval in months

32 6.0

37 4.5

42 3.0

47 2.25

52 1.5

Storage beyond this period without freshening charge can result in excessive

sulphation of the plates.

Fig5. Power Stack Cell Cut Section




2.5.4 Constant Voltage Method

Constant voltage is the only charging method recommended. Most modern

chargers are of the constant voltage type.

Determine the maximum voltage that may be applied to the system equipment.

This voltage, divided by the number of cells connected in series, will establish the

maximum volts per cell (VPC) that may be used.

Table B lists recommended voltages and charge times for the freshening charge.

Select the highest voltage the system allows but not exceeding 2.37 volts per cell to

perform the freshening charge in the shortest time period. The charging current should be

limited to a maximum of 20% of the rated capacity in Amps.

Table 1

Cell Volts Time

2.25 30 hrs

2.30 12 hrs

Note: Time periods listed in Table 1 are for temperatures from 15oC to 40C. For

temperatures below 15C double the number of hours. Charging

The charging current should be limited to a maximum of 0.2 times of AH

Capacity. Widely accepted charging methods use a current of 0.1xC10 (C10 = AH

Capacity when discharged at 10 hr rate) Example:

A 2000 AH VRLA battery is to be charged as under

0.2 x 2000

0.2 x 2000 = 400 Amps or

0.1 x 2000 = 200 Amps ( as per TEC recommendation)

Raise the voltage to the maximum value not exceeding 2.37 volts per cell

permitted by the system equipment. When charging current has tapered and stabilized (no

further reduction for three hours), charge for the hours shown in the above table or until

the lowest cell voltage ceases to rise. Correct charge time for the temperature at the time

of stabilization. To determine lowest cell, monitoring should be performed during the

final 10% of the charge time. All POWER STACK batteries are rated to an end cell

voltage of 1.75 VPC at all rates of discharge.

2.5.5 Floating Charge Method

In this type of operation, the battery is connected in parallel with a constant

voltage charger and the critical load circuits. The charger should be capable of

maintaining the required constant voltage at battery terminals and also supply normal

connected load where applicable. This sustains the battery in a fully charged condition

and also makes it available to resume the emergency power requirements in the event of

an AC power interruption or charger failure.




2.5.6 Float and Boost Voltages

Given below are the float and boost voltage recommended for the POWER

STACK battery system. The average Volts per cell (VPC) value of the series string should be set to the recommended voltage under Float and Boost conditions.

RECOMMENDED FLOAT VOLTAGE 2.25 VPC AT 27C

RECOMMENDED BOOST VOLTAGE 2.30 VPC AT 27C

Modern constant voltage output charging equipment is recommended for the

floating charger method of operation of batteries. This type of charger, properly adjusted

to the recommended float voltage and following recommended surveillance procedures,

will assist in obtaining consistent serviceability and optimum life. The charging current

for the battery should be limited to 20% of its nominal AH capacity.

After the battery has been given its freshening charge (refer to section 4), the

charger should be adjusted to provide the recommended float voltage at the battery

terminals. Do not use float voltages lower or higher than those recommended. This will

result in reduced capacity and/or reduced battery life.

Pilot Cell- A pilot cell is selected in the series string to reflect the general condition of all cells in the battery. The cell selected should be the lowest cell

voltage in the series string following the initial charge.. Reading and recording

pilot cell voltage monthly serves as an indicator of battery condition between

scheduled overall individual cell readings.

Temporary Non-use- An installed battery that is expected to stand idle for over 6 months should be treated as follows. Give the battery an equalizing charge

as per section 6. Following the equalizing charge, open connections at the

battery terminals to remove charge and load from the battery. Every six

months, temporarily connect battery to charger and give it an equalizing

charge. To return the battery to normal service, re-connect the battery to the

charger and load, give an equalizing charge and return the battery to float

operation.

2.5.7 Determination of State of Charge of VRLA Batteries

Sealed Maintenance Free Valve Regulated Lead Acid Batteries represent the state

of the art in Lead Acid technology.

The maintenance-free feature of these batteries often raises a practical problem in

the field. How can the battery bank be monitored? In conventional flooded batteries, the

specific gravity of the electrolyte gives a fairly good indication of the state of charge of

the battery. However, in a VRLA battery, it is not possible to measure the specific gravity

of the electrolyte since it is completely absorbed in the spun glass microporous separator.

The terminal voltage of the battery is directly related to the concentration of the

electrolyte. Therefore, if one were to measure the open circuit voltage of the battery, the

state of charge can be determined. The Open Circuit Voltage (OCV) readings should be

taken 24 hrs after charging is discontinued. The OCV value is co-related to the state of

charge of VRLA batteries as per the table enclosed.

Sometimes, it may not possible to disconnect the batteries from service for 24 hrs.

and then check the OCVs. Then the pattern of charging current delivered by a




temperature compensated voltage regulated charger after a discharge provides the alternate method for determining the full state of charge. The temperature compensation

factor is 3 mV per cell C rise from ambient temperature of 27C.

Under normal conditions the batteries are floated at around 2.25 volts per cell, i.e.

in a DOT System 24 cells are floated at 53.5 volts. During charging as the cells approach

full charge, the battery voltage rises to approach the charger output voltage, i.e. 53.5 volts

and the charging current decreases to the float current value of around 50 mA/100 AH for

VRLA batteries. So, when the charging current has stabilized at the float current for three

consecutive hours or the voltage across the battery bank terminals is constant for six

consecutive hours, then the battery bank can be considered as having reached full state of

charge.

If the charging voltage has been set at a value higher (but equal to or less than

2.30 VPC) than normal float voltage (so as to reduce charging time), it is normal practice

to reduce the charging voltage to the float value of 2.25V after 12 hrs. Then the float

current will soon stabilize and the above methods can be adopted for determining the state

of charge.

C10 0.43

C5 0.58

C3 0.68

CAUTION:

It was noticed in some of the exchanges during the normal course of day to day

working, the exchange failing due to low voltage condition even though the batteries

appear to be in healthy condition while they are on float as indicated by the daily readings

recorded. To avoid this it is recommended that the batteries be discharged periodically;

say once in a month by switching off the FR and the exchange load may be allowed to be

taken by the battery for half an hour. The individual cell readings are to be taken and no

cell voltage should fall below 2.10 V. Some cells which are faulty, may show negative

values with reference to the other cells. If the voltage of any individual cell differs from

others in this manner it is recommended that the cell be replaced.

Table 3

% State of Charge Open Circuit Voltage

100 2.15

90 2.13

80 2.11

70 2.09

60 2.07




50 2.05

40 2.03

30 2.01

20 1.97

0 1.95

Float charging is at 2.23 VPC and the recommended boost charge voltage is 2.30

VPC. If the charger does not have a float cum boost mode, it is important to switch over

to float after boost not later than 24 hours under steady current conditions.

2.5.8 Safety precautions

In normal use, VRLA batteries will not release hydrogen and oxygen gasses, will

not release acid mist and will not leak acid. Thus they are safer than conventional lead

acid batteries. However, under abnormal conditions, or as a result of damage, misuse or

abuse, these potentially hazardous conditions can occur. Hence the instructions given by

the supplier is to be strictly followed.

Stacking limitation- The recommended limits on stacked battery configurations are as follows.

Module arrangements Max Modules

Horizontal single stack 8 High

Horizontal multiple stack 8 High

Module assembly- This design is arranged to provide the shortest connections between modules using rigid lead coated copper strip connectors to maximise

system performance Modules are identified with a label located at the end of

the module. Each module is provided with a protective cover to prevent

accidental contact with module live electrical connections.

2.5.9 Monitoring Of VRLA Batteries

The points to be taken care to observe the health of the battery and expected

residual capacity

Periodic physical inspection of each cell of the battery for cracks and leaking etc.

Discharge of battery for a short duration and recording the voltages of each cell in the string.

Measurement of a mark deviation (>30%) in the impedance or conductance of the cell as compared to the one recorded at the time of commissioning.

Measurement & recording of cell temp periodically.

Float Voltage of cells & its comparison with the midpoint voltage.

Float current in fully charged battery.




Periodic Physical Inspection

Check for any crack or leakage every month. If not every month, at least once in two months.

Battery Partial Discharge Test- Put battery to a test discharge for 30 minutes by shutting power plant so that 20% of the battery is discharged. This can be

decided by the table supplied by the manufacturer. Record the Voltage of each

cell. Any cell showing more than 5% variation compared to voltage of other cell

can be potential weak cell.

Impedance Measurement- Take impedance measurement when the charger is on and the battery is on float. Any change in impedance/conductance of the cell

more than 40% shows imminent failure of the battery/cell. A change of




Do not mix the batteries of different capacities or makes.

Do not combine ordinary conventional batteries with VRLA batteries.

Do not install physically damaged cells.

Do not dump any waste materials in the battery room.

2.6 WORKING AND MAINTENANCE OF UPS

UPS stands for Uninterrupted Power Supply. It is required for providing

uninterrupted and stable power supply, even during the failure of Mains.

An inverter is an electrical device that converts direct current (DC) to alternating

current (AC); the converted AC can be at any required voltage and frequency with the use

of appropriate transformers, switching, and control circuits. Solid-state inverters have no

moving parts and are used in a wide range of applications, from small switching power

supplies in computers, to large electric utility high-voltage direct current applications that

transport bulk power. Inverters are commonly used to supply AC power from DC sources

such as solar panels or batteries. The inverter performs the opposite function of a rectifier.

There can be different types of inverters. Modified sine wave Pure sine wave & Grid tie

etc.

2.6.1 DC power source utilization

An inverter converts the DC electricity from sources such as batteries, solar

panels, or fuel cells to AC electricity. The electricity can be at any required

voltage; in particular it can operate AC equipment designed for mains operation,

or rectified to produce DC at any desired voltage.

1. Micro-inverters convert direct current from individual solar panels into alternating current for the electric grid. They are grid tie designs by default.

2. Uninterruptible power supplies

An uninterruptible power supply (UPS) uses batteries and an inverter to supply

AC power when main power is not available. When main power is restored, a

rectifier supplies DC power to recharge the batteries.

3. Induction heating

Inverters convert low frequency main AC power to higher frequency for use in

induction heating. To do this, AC power is first rectified to provide DC power.

The inverter then changes the DC power to high frequency AC power.

4. HVDC power transmission

With HVDC power transmission, AC power is rectified and high voltage DC

power is transmitted to another location. At the receiving location, an inverter in a

static inverter plant converts the power back to AC.

5. Variable-frequency drives

A variable-frequency drive controls the operating speed of an AC motor by

controlling the frequency and voltage of the power supplied to the motor. An

inverter provides the controlled power. In most cases, the variable-frequency drive

includes a rectifier so that DC power for the inverter can be provided from main




AC power. Since an inverter is the key component, variable-frequency drives are

sometimes called inverter drives or just inverters.

2.6.2 Principle of UPS system

The Block diagram of UPS is given above single phase A.C. supply is first

rectified to 120V D.C. supply. Across these 2 sets of 40 AH, 120 volts battery sets

are floated. The combined D.C is then fed to the inverter, which converts into A.C

of 220 volts. During the available period of input AC mains, the battery sets will

be trickle charged and the load will be taken by rectifier only. During the absence

of input A.C. mains, the battery set will be taking the load. Another type of UPS

system used is designed to work on -48 V D.C supply fed from the exchange

power plant.

2.6.3 Circuit Description

Basic designs

In one simple inverter circuit, DC power is connected to a transformer through the

centre tap of the primary winding. A switch is rapidly switched back and forth to

allow current to flow back to the DC source following two alternate paths through

one end of the primary winding and then the other. The alternation of the direction

of current in the primary winding of the transformer produces alternating current

(AC) in the secondary circuit..

The electromechanical version of the switching device includes two stationary

contacts and a spring supported moving contact. The spring holds the movable

contact against one of the stationary contacts and an electromagnet pulls the

movable contact to the opposite stationary contact. The current in the

electromagnet is interrupted by the action of the switch so that the switch

continually switches rapidly back and forth. This type of electromechanical

inverter switch, called a vibrator or buzzer, was once used in vacuum tube

automobile radios. A similar mechanism has been used in door bells, buzzers and

Rectifier

/ charger

Inverter

X

100 %

Battery

AC mains DC DC AC Output

Fig6. Block diagram of UPS System




tattoo guns. As they became available with adequate power ratings, transistors and

various other types of semiconductor switches have been incorporated into

inverter circuit designs.

UPS units are divided into categories based on which of the above problems they

address and some manufacturers categorize their products in accordance with the

number of power-related problems they address.

Specifications Of UPS Systems

a) Input 230V, 1 , A.C. supply

b) Input BTY 120V, 40 AH, two sets of

Batteries.

c) Output 220 volts, A.C

d) Output current For 5 KVA it is 21.5 Amps.

Alarms In UPS System

1) Low Battery - If the battery voltage falls below a specific voltage.

2) Over load - If the load on UPS is more than its capacity UPS will be tripped

and overload alarm comes

Fig7. Basic design of UPS




3) Output High - When output voltage becomes high UPS will be tripped and this

alarm comes.

4) Output low When output voltage becomes low UPS will be tripped and this

alarm comes.

5) Trip This LED glows once if the inverter trips.

2.6.4 UPS Maintenance

Maintenance of the UPS consists of preventive and corrective maintenance.

Preventive maintenance consists of a scheduled list of activities. Performing these

activities keeps the UPS in good working order and helps to prevent failures.

Corrective maintenance is performed as a result of a failure. Corrective

maintenance fixes the problem and gets the unit working again.

A general guide for the maintenance requirements of the UPS systems modules,

static switches, and controls is provided. Although electronic components are not

subject to wear in the same degree as electromagnetic (EM) components, they do

require systematic maintenance.

Preventive maintenance

Periodic maintenance is required to maintain the integrity and lifetime of the

battery. Power electronic equipment also requires scheduled maintenance even

though solid-state devices are used. Preventive maintenance may require that the

UPS system be shut down. A transfer of the critical load which may not provide

the power enhancement capabilities of an UPS system is something that the user

must tolerate in order to obtain maximum reliability and minimize downtime and

repair costs.

Equipment record

This record should list the basic information on the equipment itself, e.g.,

manufacturers identification, style, serial, size, location, etc., and incorporate inventory-control data for spare parts. Warranty requirements covering

uninterruptible operating conditions should be abstracted from the users manual.

Repair cost record

This record should provide a history of repair and associated costs of maintenance

for the UPS system. It is an essential diagnostic record for avoiding future

difficulties, especially for systems determined to be of poor quality, misapplied, or

marginal for the application.

Inspection check list

This list should provide necessary and pertinent information on points to be

checked and establish the recommended recurring dates when these checks should

be made. Since shutdown may require a sliding window period, the amount of

time for which this request must precede the shutdown window should also be

stated.




Periodic maintenance schedule

This schedule provides a complete listing of the day-to-day, weekly, monthly, and

annual duties which should be reviewed on the same periodic time basis so that

potential trouble situations can be investigated and corrected as soon as possible.

Maintenance inspection and repair records

These necessary and vital documents should be completed in detail by the

inspector or an assigned individual in the maintenance department. Maintenance

personnel usually report their findings and corrective action on assignment sheets

or job cards. Permanent records are a useful guide to each UPS systems general condition and reliability. It is important to know the frequency and type of repair

and how often there is a need for a complete overhaul.

Use of records

These records provide for a workable preventive maintenance program. The

information obtained from the necessary periodic inspections can be quickly lost.

This is particularly true when test results are required. Unless records and data on

the test and performance of equipment are retained, the maintenance program will

be defeated. Unless records are updated at each succeeding test period, valuable

information is lost. Comparative test data materially assists an UPS specialist in

defining problems, especially when test results differ from manufacturers recommended settings or actual factory test data. Significant changes in

comparative test data can, in general, be related to the equipments condition.

Scheduling

Scheduling of UPS and battery maintenance is normally based on the

manufacturers recommendations. Since an UPS system is vital to the operation of critical loads, it may be considered advisable to provide more inspections than

those the manufacturer recommends. Certain items on the UPS should be

inspected daily or weekly. This inspection can be done by operating personnel, but

data should be recorded and sent to the appropriate maintenance point not more

than 5 days after being recorded. Battery maintenance should be done by

maintenance personnel. Visual monitoring data should be recorded daily for the

UPS, and if recorded for the battery, the recorded data should be handled and sent

on to the maintenance department on a weekly basis.

Periodic system status checks

The continued monitoring of the operating status of any electronic equipment

greatly enhances the probability that failure of that equipment will be prevented.

Daily observation is advised but do not provide less than weekly checks.

Personnel involved in status monitoring should be those most concerned with the

equipments proper functioning.

2.6.5 UPS Maintenance Checklist

Regular equipment testing should be part of a facilitys UPS maintenance schedule. Such a schedule might include the following elements:

Quarterly:




Visually inspect equipment for loose connections, burned insulation or any other

signs of wear.

Semiannually:

Visually check for liquid contamination from batteries and capacitors.

Clean and vacuum UPS equipment enclosures.

Check HVAC equipment and performance related to temperature and humidity.

Annually:

Conduct thermal scans on electrical connections to ensure all are tight and not

generating heat, which is the first and sometimes only indication of a problem. A

non-evasive diagnostic tool helps technicians identify hot spots invisible to the

human eye. Technicians should retorque if thermal scan provides evidence of a

loose connection. Provide a complete operational test of the system, including a

monitored battery-rundown test to determine if any battery strings or cells are near

the end of their useful lives.

Biannually:

Test UPS transfer switches, circuit breakers and maintenance bypasses. If a

generator is part of the buildings emergency-power system and feeds the UPS, it also will need to be tested monthly or quarterly. Most facilities have a generator-

maintenance schedule in place in which testing frequency is defined. A UPS

typically is sized to carry the load for a short period of time. Longer outages

require backup-power generation to maintain critical services. Each facility is

unique, and managers need to develop a maintenance schedule to suit each sites specific needs.

2.7 EARTHING SYSTEM IN TELECOM

Earthing is an essential part of any electric /electronic system. Utmost care is

required to be taken for provision of proper and effective earthing for various

telecom installations. This chapter discusses about various types of earthing which

are used in telecom installations.

The objective of Earthing system may be summarized as follows:-

Reduction of Crosstalk and Noise is achieved through proper Earthing System in the Telecom Network.

Earthing is used to afford convenience & reliability, in the operative path of the circuits involved in the switching apparatus of telecom circuits.

Used as return path in telegraph and voice circuits.

Earthing is used for protection of costly apparatus and persons against foreign voltages and leakage currents.

Earthing is used for protection of buildings and equipments from lightening strikes.




Earthing in power supply systems is used to effect reliability of power as it helps to provide stability of voltage regulations, preventing excess fluctuations and provides a

measure for protection against lightening.

To divert stray RF energy from sensitive audio, video control and computer equipments.

2.7.1 REQUIREMENTS FOR EFFECTIVE EARTHING

An earthing system must meet the following specifications:-

The resistance to the earth must be within allowable limit for the particular

application.

The electrode buried in ground must be:

Having good electrical conductivity to carry highest specified load current.

Immune to the corrosive action of the soil all along its path.

Of sufficient mechanical strength to enable them to install without any damage.

Inert i.e. must not be a source of corrosion within the system to be protected.

The earth electrode must provide as much as area of contact as possible with the soil to reduce the resistance of the current path.

The resistance of the earth connection must remain within the allowable limits.

2.7.2 EARTHING SYSTEM FOR ELECTRICAL INSTALLATION

Earthing or grounding means to connect the equipment or electrical system to the

general mass of the earth. There are two types of earthing.

System earthing.

Equipment earthing.

2.7.3 System Earthing:-

It means connecting to the earth, the neutral point i.e the star point of generator,

transformer, rotating machines, and of grounding transformer. If neutral point of a

system is earthed, the phase to ground voltage under earth fault condition do not

rise to high value

Let us consider a system in which neutral is not earthed. If there is Earth fault on

B-phase, the voltage of R and Y (wealthy) phases to the earth becomes equal to

the line voltage. The rise in voltage causes stress on the insulation. Earthing

provides protection against such rise.




It constitutes the connecting of non-current carrying metal parts of a equipment to

the earth. It provides protection to operating personnel and equipment by ensuring

operation of protective control gear.

Advantages of Neutral Earthing:-

Connecting the neutral of an electrical system to earth has following advantages.

Arcing grounds are eliminated.

Voltage of healthy phases with respect to earth does not rise 3 times to normal value.

Insulation is prevented from stress of high surge voltage and hence provides long life to insulation & equipment.

Stable neutral point.

Earth fault relay function becomes simple & reliable.

Greater safety to personnel & equipment.

2.7.4 Types of Earthing:-

Sub-station earthing is mainly of two types

a) Mesh earthing b) Electrode earthing

a) Mesh Earthing :-

It is formed by steel rods (30 to 40 cm dia) laid horizontally at a depth of about

0.5 meter below ground surface to give a shape of mesh. The length and breadth

are formed by welding of steel rods.

b) Multiple Electrode system

Where the required earth resistance cannot be obtained by a single driven

electrode, it will be necessary to install additional electrodes and connect them

together. To obtain the best results, multiple earth rods must be separated by a

distance at least equal to their depth in the ground. Drive the rod head to 100 mm

below the ground surface and connect together with earth wire buried at least 300

mm below ground. Hand tools are generally suitable in light soils and the

situations where it will not be necessary to drive the rods deeper than four meters.

For harder driving conditions and greater depths power tools provide quicker and

more effective means of installing the rods.

Good quality earth wires (stainless steel or copper) are used to connect together

the earth electrodes to the equipment, which must be earthed. Wherever possible

the earth wires should be run without joints between electrode positions. Sharp

bends should be avoided in the earth wires as these could be the source of faults

under lightning discharge conditions.

Fig8. Neutral Earthing




2.7.5 Earth Electrode:-

Earth electrodes, which are in use, are

a) Rod & pipe electrode

b) Strip electrode

c) Plate electrode

a) Rod or Pipe Electrodes:-

Pipe electrode shall not be smaller than 40 mm internal diameter if of galvanized

iron. The length of the pipe electrode shall be minimum 4.5 meter. If one

electrode fails to give the required resistance, no. of such electrodes shall not be

less than twice the length electrodes. The G.I. Pipe shall be cut tapered at bottom

and provided with holes of 12 mm dia drilled not less than 7.5 cm. from each

other up to 2 mt length from bottom.

b) Strip Electrodes:-

It shall not be smaller in size than 40 Sq.mm section if of copper and 25x4 mm if

of galvanized iron. For round conductors it should be not less than 3 mm2 in case

of copper and 6 mm2 if of galvanized iron. The length of buried conductor shall

not be less than 15 meter laid in trench not less than 0.5 meter depth.

c) Plate Electrode:-

In it, the plate is made of either copper or galvanized iron. This type of earthing

(as shown in Fig.) is mostly used in our department. Size of copper plate shall not

be less than 600 x 600 x 3 mm. And that of G.I. be 600 x 600 x 6 mm. Plate shall

be 3 meter. Where earth resistance is not sufficient with one plate, two or more

plate electrodes may be connected in parallel. This type is most suited for

generating stations & sub-stations.

2.7.6 Earthing Conductor :-

The conductor from Earth Electrode to Earth shall be of the material as Earth

electrode i.e. G.I. or copper and be in the form of strip or wire size of Earthing

conductor which shall not be less than the following

Four mm dia copper wire.

Five mm dia G.I. Wire.

25 x 4 mm the power of 2 G.I. Strip.

20 x 3 mm the power of 2 Copper Strip.

However Earthing conductor, shall not be more than 150 Sq. mm in Case of G.I.,

or 100 Sq. mm in case of copper, unless otherwise specified.

2.7.7 Types of Telecom Earthing

The various types of earthing practices adopted for earthing the Telecom

Equipments as per the relevant Engineering instructions are listed below:

Ring earthing

Plate earthing

Spike Earthing




Mesh earthing

Static earthing

Ring Earthing

A GI strip of 50 x 3mm is laid in a trench 30 cm wide and depth 1 to 1.5m.

The trench to be minimum 1m away from the building.

The layout of ring earth is shown in fig. (9)

The ring earth The ring earth to be provided in all transmission and switching system where

space is available. This type is mainly used for tower earthing.

In case of new bldg. ring earth to act as equipotential bonding and all earthing

equipments to be extended from ring earth.

Fig9. Ring earth




Commoning of Earths for Equipotential Bonding

For Telecom Installation the existing earths are to be commoned i.e. tower earth,

bldg.earth, lightning earth, equipment earth, power plant earth, engine alternator

body earth, etc., to bring equipotential situation.

The sheath / armour of underground cable bringing the LT supply inside the

building should be connected to the ring earth of building.

Frame of EA set is to be connected to ring earth. Neutral of alternator should be

extended to power board and connected to ring earth through a link.

DC positive from the power plant as well as LT lightning arrestors are to be

connected to ring earth.

Earthing of roof top antenna / Tower

Roof top tower to be earthed through 50 x 3mm GI strip down leads.

The leads are bonded to any two opposite tower legs and brought down along

outside of the building and connected to ring earth /plate earth.

Each leg of the tower is to be separately connected to ring earth by 50x3 mm GI

strip.

The sheath of warning light cable is to be connected to ring earth.




Plate Earth

Plate earth is to be done as illustrated in fig 11

Nos. of 600x600x6.5mm GI plate are buried in a pit of size 2m x 2m at a depth of

3m and interconnected by 4 No, 50mmx3mm GI strips using GI bolts & nuts.

50 x 3mm GI strips are used to connect to various equipments for earthing.

The bends in GI srtip should not be sharp but with a radius of 1m.

The Earth electrode to be placed in position and all interconnections made .

The pit should be filled with fine ash and finely sieved good quality soil in layer.

Fig10. Tower mounted on top of building




The soil and ash to be rammed so as to make it compact and cohesive.

Spike Earth

Fig11. Method of plate earthing

Fig12. Plate earth




The earthing system consists of 20 Nos. of 25mm or 38mm GI pipes, each of

275cm. or 375 cm. long, driven into the ground as shown in fig.

Each pipe is used as earth spike and is so driven into the ground that its top

remains at some depth below the ground surface.

The spacing between 2 spikes is not less than 375cm and in no case less than

250cm.

The spikes shall be connected together by a continuous main earth conductor of

size 19 x 1.6mm bare tinned copper protected by PVC pipe.

The earth conductor is brought into the building and terminated at a height of

30cm. above floor.

It may be used for MBM Exchanges equipment earthing.

Mesh type earthing

The radio and Multiplexing equipments are to be connected by means of mesh

earth with copper strips 20x1.5mm running on top of the bays and forming a

mesh. Each bay may be connected by means of copper cables to the copper strip.

The mesh to be connected to external ring earth in a shortest distance possible at

two opposite points of the mesh.

Fig13. Spike earth




Static Earthing

The primary source of electrostatic charge is contact electrification, where two

dissimilar metals are brought into contact and then separated. The two metals

become oppositely charged due to charge transfer. Whenever charged object

makes contact with uncharged object, the charge is shared between them to the

extent that their conductivities allow. Materials having surface resistivity more

than 1012 Ohm m are capable of retaining charge for longer period. These are

readily charged by contact electrification.

Fig14. Earth distribution




The human body has a low enough volume resistivity to act as a conductor and

if insulated from earth it can accumulate electrostatic charge. A troublesome

consequence of the electrostatic potential on charged personnel is that it can be

high enough to cause damage to electrostatic sensitive devices, such as

semiconductors, when these are being handled or assembled. Antistatic or

conductive type materials can be used to avoid retention of static electricity. If

antistatic floor is not provided, anti static wrist strap, which is properly earthed,

shall be used while working on such equipments. In NT Exchanges with false

floor the jack s below the false floor are earthed for protection against static

electricity.

2.7.8 GENERAL CONSIDERATIONS FOR ELECTRICAL EARTH AND TELECOM EARTH

General considerations for Electrical Earth

Since material resistivity does not play a part in the resistance of earth electrode to

earth and the GI pipe electrode offers comparatively lower resistance than even a

copper plate electrode, GI / CI pipe earthing may be preferred to plate earthing

wherever possible. Also to improve the earth resistance further, as per

requirements, Bentonite mixture can be used with GI pipe earthing in places

where continuous attention is available .In such places, the condensate water from

air conditioners may be directly taken to the earth pit to keep the pit constantly

wet. This earthing with Bentonite powder can be preferred to some chemical

earthing since this Bentonite method is cost effective and gives the same results of

low resistance. This Bentonite earthing may not be used for BTS Stations or in

remote areas where continuous attention is not available.

The disadvantage with Bentonite is that if it is allowed to dry, it becomes hard and

revival by addition of water becomes difficult. Since resistance to corrosion of

copper is better than any other material, the existing method of copper plate

earthing may be adopted in the areas where the effect of corrosion is high. The

method of chemical earthing adopted by private operators can also be tried at one

or two places on experimental basis. It is learnt that this has been already

practiced at one or two places. The feedback received from this place is not

encouraging. The problem indicated was that the earth resistance shot up with

passage of time. This may probably require periodical maintenance such as

keeping the earth pit wet by pouring water.

General considerations for Telecom Earth

The engineering instructions issued by T&D Circle, Jabalpur in the Issue III

dt.30.04.2005 specifies that ring earthing is to be used for all transmission and

switching system.

It is gathered that the ring earthing is specifically used for tower earthing by

Telecom Wing. The engineering instructions further specifies that all the

equipments are to be connected to this ring earth for equi -potential bonding.It is

recommended that the ring earthing may be particularly used for tower earthing

and related BTS Station earthing with all the equipments connected to this ring

earth. Spike earthing requires lot of space. It is comparatively costlier. Hence the

method of spike earthing can be adopted in exceptional cases where very low and




reliable earth resistance values are required and this system of spike earthing is

not recommended for normal applications. Static earthing using anti-static floor

tiles in switch rooms of electronic exchanges are invariably being adopted and

hence no change is recommended.

RECOMMENDATIONS FOR EARTHING SYSTEM

Type of Exchange Telecom earthing Electrical earthing

Ground Based

Tower & BTS

Ring earthing with

all equipments

connected to ring.

For neutral & lightning conductors

separate GI pipe earthing.

Other equipments can be extended to

ring earthing.

Roof Top Tower

& BTS

04 Nos. GI plate

earthing.



Other equipments separate GI pipe

earthing.

Rural Exchanges

like CDOT SBM

04 Nos. GI plate

earthing.




earthing.

New tech

Exchanges (LT

connection)

04 Nos. GI plate or

spike earthing.


separate GI pipe earthing and bentonite

mixture wherever required.


earthing with bentonite mixture

wherever required.

New tech

Exchanges (HT

connection)

04 Nos. GI plate or

spike earthing.


separate Copper plate earthing.

Other equipments separate Copper plate

earthing (fault current density

consideration) .

Note: Where soil is corrosive, G.I pipe/plate earthing can be replaced by copper

plate earthing.

2.8 SUMMARY

This unit explains the different methods of battery working. This unit covers the

different types of inverters like modified sine wave, pure sine wave, grid type, etc. It also

gives a basic idea about the circuit diagram of inverters and the UPS. The chapter also

gives the maintenance schedule for preventive and corrective maintenance of the

uninterrupted power supply.




For any Telecom installation to be reliable, effective earthing system plays an

important role. Not only the earthing system, but what type of earthing system is used

also plays a significant role in effective operation and maintenance and longer life of the

installations.

2.9 SELF ASSESSMENT QUESTIONS

1. There is absolutely no break in the DC power supply in ____________ scheme. ((Float/Standby/charge discharge)

2. A battery capacity equivalent to ______ the busy hour load is provided in float systems.( four times/ Two times/ Six times)

3. The batteries of different capacities or makes can be connected together. ( T /F ) 4. The VRLA Battery can be connected in series with the Conventional Battery.(T/F) 5. The recommended charging of VRLA battery is to be done at constant

____________( Voltage/ Current)

6. Maintenance free battery works on_____________ principle. 7. The battery capacity is expressed in __________________ .( AH/ Watts) 8. The floating voltage cannot be higher than_____volts per cell.(2.3/2.35/2.25) 9. To divert stray RF energy from sensitive audio, video control and computer

equipments ___________ is used

10. System Earthing means connecting neutral point to earth ( T /F ) 11. Arcing grounds are eliminated due to_____ (System Earthing/ neutralEarthing) 12. The resistance of Earth Electrode shall not be more than (10 Ohms/5 Ohms) 13. ________type of earthing is mainly used for tower earthing(Mesh/Plate/Ring) 14. Where soil is corrosive, G.I pipe/plate earthing can be replaced by_______ 15. ________ is not a type of inverter 16. The output of the UPS System is___ 17. When output voltage becomes high UPS will be tripped and ______ alarm comes. 18. Micro-inverters are grid tie designs by default.( T / F ) 19. The Input given to the UPS System is ( 220V/230V/440v) 20. In HVDC power transmission, ______ is rectified and ____ is transmitted.

2.10 REFERENCES AND SUGGESTED FURTHER READINGS

en.wikipedia.org/wiki/Uninterruptable_ power_ supply

www.apc.com/products

www.ups.com/tracking

www.tec.gov.in

www.tnd.bsnl.co.in

intranet.bsnl.co.in/digital library

Participants for further understanding of Earthing System can visit Electrical and

Telecom Installation in the Telephone Exchange.




SELF ASSESMENT ANSWERS

1. Float 2. 2 times 3. False 4. False 5. Voltage 6. Recombination 7. AH 8. 2.35 9. Earthing 10. True 11. Neutral Earthing 12. 5 Ohms 13. Ring 14. Copper Plate 15. Saw-Tooth Wave Inverter 16. 220V AC 17. Output High 18. True 19. 230V AC 20. AC, DC

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