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SRI LANKA INSTITUTE of ADVANCED TECHNOLOGICAL EDUCATION
Training Unit
Protective Measures against Excessive Contact
Voltage Theory
No: EE 017
INDUSTRIETECHNIKINDUSTRIETECHNIK
ELECTRICAL and ELECTRONIC ENGINEERING
Instructor Manual
1
Training Unit
Protective Measures against Excessive Contact Voltage
Theoretical Part
No.: EE 017
Edition: 2008 All Rights Reserved Editor: MCE Industrietechnik Linz GmbH & Co Education and Training Systems, DM-1 Lunzerstrasse 64 P.O.Box 36, A 4031 Linz / Austria Tel. (+ 43 / 732) 6987 – 3475 Fax (+ 43 / 732) 6980 – 4271 Website: www.mcelinz.com
2
PROTECTIVE MEASURES AGAINST EXCESSIVE CONTACT VOLTAGE
CONTENTS Page
LEARNING OBJECTIVES...................................................................................................5
1 GENERAL ....................................................................................................................7
1.1 The protection of humans and animals against direct and indirect contact .........7
1.1.1 Direct contact ...................................................................................................7
1.1.2 Indirect contact.................................................................................................7
1.2 Legal regulations..................................................................................................7
2 DEFINITIONS...............................................................................................................9
2.1 Active components...............................................................................................9
2.2 Inactive components ............................................................................................9
2.3 Types of fault .......................................................................................................9
2.3.1 Housing contact .............................................................................................10
2.3.2 Short circuit ....................................................................................................10
2.3.3 Earth fault.......................................................................................................10
2.4 Electrical equipment...........................................................................................10
2.5 Causes of excessive contact voltage on the housing of electrical equipment ...11
2.6 Operational insulation ........................................................................................11
2.7 Fault voltage ......................................................................................................11
2.8 Contact voltage ..................................................................................................11
2.9 Area of risk (Resistance area) ...........................................................................12
2.10 Earth voltage......................................................................................................13
2.11 Surface voltage gradient US ...............................................................................13
2.12 Specific earth resistance....................................................................................14
2.13 Reaching distance (arms reach) ........................................................................14
2.14 PEN conductor (combined protective conductor and neutral conductor)...........15
2.15 PE-conductor or protective conductors or earth continuity conductor................16
2.16 The term "nominal".............................................................................................16
3 TYPES AND APPLICATIONS OF PROTECTIVE MEASURES.................................17
3.1 Summary of protective measures ......................................................................17
3.2 The requirements when putting an Installation into operation...........................17
3.3 Protective insulation (double insulation) ............................................................18
3.3.1 Double insulation of electrical equipment.......................................................18
3.3.2 The Symbol for double insulated equipment..................................................19
3
3.3.3 Double insulation............................................................................................19
3.3.4 Types of AC plug............................................................................................19
3.3.5 Protective insulation of the working area .......................................................21
3.4 Protective low voltage (safety extra-low voltage or SELV)...............................22
3.4.1 Diagram symbols for safety transformers ......................................................22
3.4.2 Permissible voltage supplies for SELV ..........................................................23
3.4.3 Non permissible supplies for SELV................................................................23
3.4.4 Regulations for extra-low voltages .................................................................24
3.5 Protective isolation (protection by electrical separation) ....................................24
3.5.1 Regulations for protective Isolation ................................................................25
3.6 Protective earthing .............................................................................................26
3.7 Neutralisation or protective multiple earthing.....................................................29
3.7.1 Regulations for protective multiple earthing ...................................................30
3.7.2 Dimensions of connectors in protective multiple earthing systems................31
3.7.3 Neutralisation in complete electrical installations...........................................31
3.8 Protective conductor systems ............................................................................31
3.8.1 Regulations for protective conductor systems ...............................................32
3.9 Voltage-operated earth-leakage circuit breaker .................................................33
3.9.1 Auxiliary earth ................................................................................................34
3.10 Current-operated earth-leakage circuit breaker .................................................35
3.10.1 How the current-operated circuit breaker functions ...................................35
3.10.2 Calculation of the earth electrode resistance .............................................36
3.10.3 The regulations for connecting current-operated circuit breakers..............37
3.11 Combined current-voltage-operated circuit breaker...........................................38
3.11.1 The regulations for connecting combined current-voltage-operated circuit
breaker 38
3.12 Controlling the earth potential ............................................................................39
4 EARTHS.....................................................................................................................40
4.1 General ..............................................................................................................40
4.2 Earth electrode resistance (Reel) ........................................................................40
4.2.1 The approximate value for specific earth resistance (ρe)[Ωm] ......................40
4.3 Types of earth ....................................................................................................41
4.3.1 Strip earth (horizontal earth) ..........................................................................41
4.3.2 Foundation earths ..........................................................................................42
4.3.3 Earth rods (deep earths - vertical earths).......................................................44
4.3.4 Earth plates ....................................................................................................45
4.4 General information for insulating earths ...........................................................45
4
4.4.1 Water pipes....................................................................................................45
4.4.2 Other pipe systems ........................................................................................46
5 TESTING THE PROTECTIVE MEASURES...............................................................47
6 EXPLANATION OF FORMULA SYMBOLS AND FORMULARE ...............................48
6.1 Explanation of symbols ......................................................................................48
6.2 Formulary...........................................................................................................49
5
PROTECTIVE MEASURES AGAINST EXCESSIVE CONTACT VOLTAGE
LEARNING OBJECTIVES
The student should...
state the purpose of protective measures
state the highest allowable contact voltage for humans and animals
state what is meant by the following three types of fault: insulation fault, short circuit
and accidental earthing and describe. the difference between them
define the terms active components, inactive components, electrical equipment, area
of risk and reaching distance
state the difference between a neutral conductor and a protective conductor
classify active and inactive protective measures
understand the difference between insulation of electrical equipment and insulation of
the working area
state the highest allowable values of low voltage
name the permissible and non permissible voltage supplies for safety extra low
voltage
explain how protective Isolation functions
state the rules for Installation of protective Isolation
explain how protective earthing functions
calculate the protective earthing resistance
explain how neutralisation functions
name the rules for Installing neutralisation
name the rules for Installing protective conductor systems
state what is meant by a voltage operated earth leakage circuit breaker
describe the Operation and make a drawing of a current operated earth leakage circuit
breaker
calculate the earth electrode resistance
6
state the rules for the installation of a current-operated circuit breaker
state the use of a protective switch, neutralization
explain the rules for the installation of a voltage-operated circuit breaker
state the purpose of controlling earth potential
define the term "earthing"
state the way in which earth resistance varies with distance from the earthing
electrode
describe different methods of earthing
know the rules for Installing earthing systems
7
PROTECTIVE MEASURES AGAINST EXCESSIVE CONTACT VOLTAGE
1 GENERAL
1.1 The protection of humans and animals against direct and indirect contact
1.1.1 Direct contact
This means contact with the active conductors in electrical circuits. Protection is provided
by all equipment for normal sale. In most cases appliance and equipment will be marked
with a safety symbol.
1.1.2 Indirect contact
This refers to contact with inactive components of electrical equipment which are at a high
voltage due to a defect (i.e. insulation fault). The laws regarding Electrical Engineering
specify protective measures in which the maximum permissible contact voltage may not
exceed:
50 V for humans and 24 V for animals.
(Various protective measures are listed in paragraph 3, page 17).
1.2 Legal regulations
The material design of all high-voltage installations must meet the latest standards of
electrical engineering technology.
In all industrialized countries, laws and regulations have been established to protect
persons working with electrical equipment and appliances.
8
It has been assumed that, in the countries where this text book will be used, laws and
regulations exist which are the same or similar to the regulations outlined and the
instructors will be able to point out the differences without difficulty.
The laws for Electrical Engineering make the observation of certain regulations
compulsory. Failure to comply with these regulations is often punishable (by fine).
The law shows little leniency towards trained people, such as electricians, who should be
aware of such dangers.
The specialist must protect the layman!
The following regulations must always be observed when installing and operating
electrical installations:
- Life must not be endangered.
- Electrical Installations may not cause any damage (fires).
- Safety standards must be as high as possible in order to ensure trouble-free operation
of the plant.
9
2 DEFINITIONS
2.1 Active components
Active components are all those components (parts) or conductors (wires) which have a
current-carrying capability and which make up the electrical circuit.
Direct contact with active conductors is prevented by operational insulation (insulated
conductors in cables), protective covering (covered terminals), or installations which are
out of reach.
The following are examples of active conductors:
Electric lines, switches, fuses, neutral conductors, plugs and terminals.
2.2 Inactive components
Inactive components are conducting components which are not a part of the working
circuit, e g a motor housing.
2.3 Types of fault
10
2.3.1 Housing contact
This is a connection, caused by an insulation fault, between an active conductor with an
inactive component or housing of electrical equipment.
2.3.2 Short circuit
This is a connection between active conductors having different potentials.
2.3.3 Earth fault
This is a conducting connection, caused by an insulation fault, between a live conductor
and earth or earthed components.
2.4 Electrical equipment
Electrical equipment includes items which, as a whole or in part, are intended for
generating, transmitting or using electrical energy.
NOTE:
Electrical equipment must be protected against mechanical damage to from high
temperature, which could cause damage to the insulation. lf, as the result of an insulation
fault, connection is made between an active conductor and an inactive component in an
item of electrical equipment, then there will be a potential difference between the housing
of the equipment and earth.
11
2.5 Causes of excessive contact voltage on the housing of electrical equipment
Overloading, failure to allow for expansion, dust and dampness, incorrect handling and
repair by unqualified persons can cause excessive voltage on the housing of equipment.
2.6 Operational insulation
This is achieved by covering conductors along their full length with insulating material
(Varnish, enamel or oxide coatings are not permissible. Such conductors may only be
used on the Inside of electrical equipment or behind a protective covering).
2.7 Fault voltage
This is the voltage which occurs when contact is possible
2.8 Contact voltage
This is the part of the fault voltage which can be bridged by a human.
12
Re = consumers earth electrode Ro = power supplier's earth
resistance electrode resistance
Rf = fault resistance Rtoc = location contact resistance
Rh = human resistance Uc = contact voltage
Uf = fault voltage
Ut = line voltage
2.9 Area of risk (Resistance area)
It is that part of the general mass of earth where, in case of a short circuit to earth, a
noticeable voltage exists between any point in this area and a reference point of the earth
(a sufficient distance away from earth electrode).
13
2.10 Earth voltage
This is the voltage which appears between a current-carrying earth (earth Installation) and
a reference point of the Earth which is a sufficient distance away from earth electrode.
2.11 Surface voltage gradient US
This is that part of the earth voltage between two points 1 m apart (an average stride).
14
2.12 Specific earth resistance
This is the resistance between opposite faces of a cube of earth with 1 m edges. The
specific earth resistance ( ) is calculated by using the formula for conductor resistance.
2.13 Reaching distance (arms reach)
This is the area which a person can reach with his hands from the point or place where he
is standing.
15
NOTE:
A distance of 2500 mm must exist between the standing area (floor) and a live conductor
or conducting element.
2.14 PEN conductor (combined protective conductor and neutral conductor)
This is the neutral conductor of an electrical network, which is connected to the earth
continuity conductor on the customer's premises and also to the earth electrode. Should
an earth fault occur, the fault current flows back to the source of supply through the PEN
conductor in parallel with earth. Since the path resistance is very low, the fault current is
large enough to blow the fuse on switch off a protective circuit breaks.
16
2.15 PE-conductor or protective conductors or earth continuity conductor
The protective conductor serves to prevent the housing of electrical equipment against
excessive contact voltage and must, therefore, be installed and clamped with the greatest
of care.
Symbol for the terminal for the protective conductor:
The colour of the insulation (covering) on the protective conductor is yellow and green
(from the manufacturer). In older installations it can be red or for English cables, brown.
The protective conductor must never carry operational current; it only carries leakage
current (fault current) in case of a fault. The protective conductor for portable electrical
equipment must be included in the connecting cable for the equipment.
NOTE:
The protective conductor must never be switched with the live or neutral conductors. This
could endanger life!
2.16 The term "nominal"
The term "nominal" identifies the values used in the design of electrical equipment and
installations. Operational characteristics and limit test values are related to these values.
17
3 TYPES AND APPLICATIONS OF PROTECTIVE MEASURES
3.1 Summary of protective measures
The following are examples of active protective measures, apart from the connection of
the protective conductor to the equipment, which will excessive contact voltage:
- Protective insulation (double insulation).
- Protective low voltage (safety Extra Low Voltage, SELV).
- Protective Isolation for one item of equipment.
The following are examples of inactive protective measures, when a protective conductor
is connected to the equipment, which prevents excessive contact voltage from remaining:
- Protective isolation for several items of equipment.
- Safety earthing.
- Neutralization (Protective Multiple Earthing, PME).
- Protective conductor system.
- Voltage-operated earth leakage circuit breaker.
- Current-operated earth leakage circuit breaker.
- Combined current-voltage-operated earth leakage circuit breaker.
- Controlling the earth potential.
3.2 The requirements when putting an Installation into operation
- Before every electrical installation is put into operation, the fault- free Operation of the
protective measures used must be thoroughly tested.
- It is entirely forbidden to simulate protective measures or to render them inoperative.
- The following practices are forbidden:
Installing an earthed AC plug without a protective conductor (exception: when the
equipment is double-insulated) .
Altering a normal AC plug so that it will fit into an earthed AC socket.
18
Bypassing protective switches.
Using unauthorized adapter plugs which interrupt the protective conductor.
3.3 Protective insulation (double insulation)
The function of protective insulation is to protect all inactive components in the equipment
by additional insulation when a fault (housing contact) occurs which would result in
excessive contact voltage.
3.3.1 Double insulation of electrical equipment
This additional insulation is produced by enclosing components in insulation and by
incorporating insulating, intermediate sections in the gears at the factory.
19
3.3.2 The Symbol for double insulated equipment
3.3.3 Double insulation
Double insulated equipment need not have a protective conductor connection. However,
the AC plug on the connecting cable of a piece of double insulated equipment must fit into
an earthed AC socket.
3.3.4 Types of AC plug
Contour AC plugs or flat AC plugs (with partially insulated pins) must be inseparably
connected with the AC cable. When such plugs or the cable are to be replaced, earthed
plugs should be used with the protective contact not connected.
20
Protective insulation is only recognised as a protective measure if the item of equipment is
marked with the appropriate
NOTE:
Electrical hand tools, electric razors and household appliances etc.; are items which are
commonly constructed with double insulation.
21
3.3.5 Protective insulation of the working area
This is sometimes known as "Protection by non-conducting location". This is allowed for
fixed electrical installations only. In addition to the standing area, all components which
are current-conducting and situated within reaching distances of ground must have
insulating covers, (i.e. floors, water-pipes and radiators etc.).
The covering must be sufficiently large so that the electrical equipment can only be
reached from an insulated area. The covers must also be robust and firmly fixed to the
base. If several fixed, electrical items can be reached from one location, then all of their
inactive components must be connected with an equalizing conductor (earth free local
equipotent bonding).
22
3.4 Protective low voltage (safety extra-low voltage or SELV)
The protective low voltage may not be more than 42 volts; in children's toys it may not
exceed 24 volts nominal rate. This voltage is not high enough to allow a dangerous
amount of current to flow through the human body.
3.4.1 Diagram symbols for safety transformers
- enclosed safety transformer
- safety transformer non-enclosed (built in)
- transformer for toys
- transformer for bells
- transformer for hand-light
- transformer for de-frosting
23
3.4.2 Permissible voltage supplies for SELV
- Protective transformers with separate windings
- Rotary converters with separate windings (motor and generator are separate)
- Primary cells
3.4.3 Non permissible supplies for SELV
24
Prohibited: because of electrical connection between the primary and secondary sides.
A potential source of danger exists.
- Autotransformers
- Series-resistors (voltage droppers)
- Voltage dividers (potential dividers)
- Rotary converters with common windings
3.4.4 Regulations for extra-low voltages
- Safety extra-low voltage electrical circuits must not be earthed.
- Plugs for low-voltage equipment must not be capable of being fitted into sockets for
higher voltage.
- Extra-low voltage circuits and conductors must be insulated for a nominal 250 V.
- For children's toys, driven by electric motors, the supply must be 24 V or lower
- Extra-low voltage equipment for treating and keeping animals must have a rated
voltage of 24 V or lower.
- Protective conductors may not be connected to low-voltage equipment (danger of
voltage from other faulty electrical equipment passing over the protective conductor to
the low voltage equipment).
The use of safety extra-low voltage:
Children's toys, hand lamps, electrical hand tools, under-water lights and wet grinding
machines etc. all use safety extra-low voltage
3.5 Protective isolation (protection by electrical separation)
The purpose of protective Isolation is to isolate the electrical circuit completely from the
electrical mains and earth, through an Isolation transformer with a maximum 660 V
nominal rating for the primary and a secondary with from 42 V to a maximum 380 V
nominal rating.
25
Diagram symbol for isolation transformer
Connection of electrical equipment to an Isolation transformer
3.5.1 Regulations for protective Isolation
- The movable connecting cable on the secondary must have a minimum performance
equal to GMM or YMM cables.
- Any portable isolation transformer must be double-insulated.
- The housing of electrical equipment (non isolated electrical hand drills) silvated in
electrically conducting areas must be connected in an easily visible manner to this
area with a minimum of 4 mm² copper cable.
26
- The connection to the secondary electrical circuit must not be too long. The maximum
conductor length is 250 m at 250 V.
- Should several items of equipment be driven with an isolation transformer, then their
housings must be connected with a potential equalizing conductor.
Connection of several items of electrical equipment to one isolation transformer
The use of protective isolation:
Protective isolation is used in razor sockets (AC), electrical hand tools and in boilers.
It is also used when repairing open electrical equipment in a repair shop.
3.6 Protective earthing
All inactive components of electrical equipment which might receive excessive contact
voltage in case of a housing contact are connected through a protective conductor to an
earth electrode installed in the earth.
27
Example of protective earthing
Rs = safety earthing resistance
The fault current flows through the protective earth electrode to earth. The resistance of
the safety earthing Rs must be sufficiently low, so if a fault voltage of 50 V (24 V for
animals) is reached, the preceding protective component operates and shuts off the
supply.
Rs = Safety earthing electrode resistance.
Ib = Fault current through the over-current
protection component which will cause
it to operate.
In = Nominal rating of the over-current protection
component.
k = Short circuit factor (the factor by which In must
be multiplied in order to obtain the smallest
lb in case of a short circuit).
28
Operation of over-current protection components
Over-current protection
component
Fault current Ib in
consumer plant In x k
Fuse, quick-acting In x 3.5
Fuse, rating up to
50 A (slow-acting) over
63 A (slow-acting)
In x 3 5
In x 5
Circuit breaker
Type L
Type G
In x 5
In x 10
Calculate the highest permissible earthing electrode resistance, using a quick-acting 20
amp fuse.
For a house Uc = 50 V.
For a farm Uc = 24 V.
Calculate the highest permissible earthing electrode resistance in a mains circuit. An
automatic circuit breaker, type L10 A, is installed as an over-current protection
component.
29
As the calculations have shown, the earth electrode resistances must be very low.
It is very expensive to lay safety earthing and almost impossible to keep the regulations in
earth which conducts poorly.
NOTE:
With the safety earthing, there must be a very low resistance to earth. It is forbidden to
replace an over-current protection component with one of a higher nominal rating.
3.7 Neutralisation or protective multiple earthing
All inactive components of an electrical Installation which might reach an excessive
contact voltage in the event of a fault (housing contact) are connected to the neutral
conductor.
The fault current flows through the neutral conductor.
30
3.7.1 Regulations for protective multiple earthing
- When the conductor cross section is under 10 mm² the PEN-conductor must be
separated into a neutral conductor and protective conductor. These may not be
reconnected later on.
- The PEN-conductor must never be fused or interrupted.
- The PEN-conductor must be connected to the water pipes and must also bridge the
water meter in every household.
31
3.7.2 Dimensions of connectors in protective multiple earthing systems
- Steel strip: 90 mm² cross section, galvanized, minimum 3 mm thick.
- Steel rod: 10 diameter, galvanized.
- Copper: same cross section the PEN-conductor, for conductor sizes up to 16 mm².
All of the conducting components (foundation earth, central heating and oil tank etc.) are
also connected to the potential equalizing strip.
3.7.3 Neutralisation in complete electrical installations
Neutralisation must only be used in the complete installations when the following four
neutralisation conditions can be met:
- The cross section of the conductor between power supply and consumers must be of
a sufficient size, so that in case of short circuit between the live and PEN-conductors,
a current, larger that the operating current of the preceding over-current protection
circuit breaker, flows.
- The PEN-conductor must be earthed near the power source (transformer) and near
the terminal of the mains branch.
- The PEN-conductor may not be interrupted (switched) and must also be laid to the
same regulations as the live wire.
- All available earths in the area of the installation are to be connected to the PEN-
conductor.
3.8 Protective conductor systems
These protective measures may only be used in factories or workshops which have a
power generator or a transformer with the windings isolated from earth (neutral point of a
star connection must not be connected to earth!).
32
3.8.1 Regulations for protective conductor systems
- All exposed metalwork in buildings and construction sites and all inactive components
of electrical equipment must be connected to the protective conductor.
- A supervisory device must be installed to monitor the insulation condition between live
conductors and earth.
- The total earth electrode resistance of the protective conductor system must not
exceed 20Ω.
Electrical equipment remains operative in the event of an insulation fault in one phase.
There is no contact voltage on the inactive components. The fault, however, will be
indicated on the insulation supervisory equipment. Only when there is a second housing
contact in an electrical component in another phase will the over-current protection device
be activated.
33
3.9 Voltage-operated earth-leakage circuit breaker
Should there be a high fault voltage on inactive components of electrical equipment,
current flows through the coil of the voltage-operated earth leakage circuit breaker, which
is earthed to an auxiliary earth and the voltage-operated circuit breaker switches off
automatically.
34
With the test button one can only determine whether the breaker is operating or if the
auxiliary earth conductor and the auxiliary earth are good. The condition of the protective
conductor and its proper connection is not tested by the Operation of the test button.
3.9.1 Auxiliary earth
Earthing rods driven 1.5 m into the earth may serve as auxiliary earth, or an earthing band
3 m long laid below the frost line can be used. The connecting cable from the auxiliary
earth to the voltage-operated circuit breaker must be insulated when laid
With a contact voltage of 50 V, the auxiliary earth resistance may be up to a maximum of
800 Ω and the circuit will operate correctly.
NOTE:
The voltage-operated circuit breaker will not function when there is a direct connection
between the protective conductor and the auxiliary earth. In new installations the voltage-
operated circuit breaker is no longer authorized for use.
35
3.10 Current-operated earth-leakage circuit breaker
All conductors which serve the purpose of supplying current for the electrical equipment to
be protected are controlled through the balanced current transformer of the current-
operated circuit breaker.
3.10.1 How the current-operated circuit breaker functions
If the insulation resistance of the electrical installation is good, the sum of the current
flowing through the balanced current transformer will be equal to zero (Kirchhoff's First
Law).
Should a housing contact exist in an item of electrical equipment which is connected after
the current-operated circuit breaker, then fault current flows from protective earth to
supply transformer earth. As a result, the sum of the currents flowing through the current
transformer is no longer zero.
36
Depending on the amount of fault current in the primary, a current will be induced in the
secondary. This current reduces the strength of a permanent magnet in the switch, the
switch will operate and turn off the supply.
3.10.2 Calculation of the earth electrode resistance
For the protection of humans
For the protection of domestic animals
Re = earth electrode
I∆f = Fault release current of the current-operated circuit breaker
Calculate the following earth electrode resistance Re for the protection of humans and
domestic animals using a current-operated circuit breaker.
Fault release
current I∆f [A]
0.015 003 0.1 0.3 0.2 2
Max. Re [Ω]
with Uc ≤ 150 V
4333
Max. Re [Ω]
with Uc ≤ 24 V
1600
37
3.10.3 The regulations for connecting current-operated circuit breakers
- All metal installation components are to be protected and must be earthed according
to regulations. The minimum cross section of the protective conductor is 1.5 mm².
- Fuses must be installed in front of the current-operated circuit breaker.
- All of the conductors after the current-operated circuit breaker must be well insulated
from earth.
- All energy-carrying conductors, including the neutral conductor, must be connected
through the current-operated circuit breaker.
- The current-operated circuit breaker must be tested monthly. The following information
is to be posted with good visibility.
NOTE:
This test cannot determine whether the electrical equipment is properly connected and
earthed. A current-operated circuit breaker with a fault release current up to 100 mA will
also protect one against earthing in case of contact with active components (live parts).
38
3.11 Combined current-voltage-operated circuit breaker
This type of protective breaker is intended for use in installations where the neutralizing
requirements are not fully satisfied.
The combined current-voltage-operated circuit breaker contains a current operating and a
voltage operating unit.
3.11.1 The regulations for connecting combined current-voltage-operated circuit breaker
- All live conductors and the neutral conductor must be wired through the circuit
breaker.
- The neutral conductor after the switch must be insulated from earth.
39
- All of the electrical equipment to be protected must be connected through a protective
conductor to terminal "K" of the protective circuit breaker.
- The auxiliary earth conductor will be connected to terminal "H" of the protective circuit
breaker. The auxiliary earth conductor is to be insulated from earth.
3.12 Controlling the earth potential
This is used mostly in combination with other protective measures. All metal parts and an
earth, must be electrically connected.
Example
- Foundation earth
- Water pipes (metal)
- Drain pipes (metal)
- Pipe systems from central heating
- Gas pipes
- Protective conductors etc
Potential equalizing strip
40
4 EARTHS
4.1 General
Earthing means creating an electrical, conducting connection between earth and the
earthing system. The earth electrode resistance near the earthing system is very high.
The greater the distance from the earthing electrode, the larger the earth's cross section
available for current flow.
After a distance of approximately 20 m from the earthing system, the earth can be rated
as an excellent current conductor because of the large cross section.
4.2 Earth electrode resistance (Reel)
This is dependent on the specific earth resistance (see paragraph 2.11, page 13) and on
the cross section and length of the grounding item.
4.2.1 The approximate value for specific earth resistance (ρe)[Ωm]
Wet, marshy soil 30 Ωm
Damp clay and humus 100 Ωm
Damp sand 300 Ωm
Damp gravel with sand 500 Ωm
Dry sand 1000 Ωm
Dry gravel 3000 Ωm
Rock 10000 Ωm
Area of risk:
See paragraph 2.9, page 12.
41
4.3 Types of earth
The type of earth which will be used depends on the composition of the earth and on the
actual requirements (protective measures) in each case.
4.3.1 Strip earth (horizontal earth)
Made of galvanized steel band, the minimum cross section is 90 mm²; dimensions 3 x 30
mm.
Made of galvanized steel cable, the minimum cross section is 95 mm²; diameter of each
wire minimum 2.5 mm.
Made of copper cable the minimum cross section is 35 mm².
The earthing strip should be buried a minimum of 0.5 - 1 m deep, depending on the
properties of the soil.
The strip earth may be laid as a ray, ring or mesh.
With a ray earth, the spacing should be even.
The angles between the rays should not exceed 60°.
42
The connection to the earthing electrode must be wrapped with a corrosion-protective
band, alternatively the connecting point must be covered with cable-insulating compound
to protect against corrosion.
Approximate values for earth electrode resistance Reel for strip earth (horizontal earth)
when the specific earth resistance equals 100 Ωm.
Band length
[m]
10 25 50 100
earth electrode
resistance Reel [Ω]
16 7.5 4.5 2.5
4.3.2 Foundation earths
These are very economical as later digging is not required Foundation earths are laid as a
ring under the outer foundation walls of a building (when necessary under middle walls as
well).
43
No point in the ground level plan of the building may be more than 5 m from the earth.
Strip steel must be laid an edge and there must be a minimum of 5 cm of concrete
beneath the strip.
Concrete-cased steel components and reinforcing steel can be connected to the
foundation earth by welding, screws or clamps.
44
Specified dimensions for connecting a foundation earth
4.3.3 Earth rods (deep earths - vertical earths)
These are used when it is impossible to lay a horizontal earth or when good conducting
earth is found at a deep level. The material used is galvanized rod or sectional steel which
is vertically driven into the earth. Earth rods may also be connected in parallel. However,
the distance between the rods in the earth must be twice the length of the original rods.
Approximate values of the earth electrode resistance for earth rods when the specific
earth resistance equals 100 Ωm.
Effective rod length [m] 2 3 5
Earth electrode resistance
Reel [Ω]
40 30 20
45
4.3.4 Earth plates
These are sheet plates at least 3 mm thick and buried vertically 1 m deep in the earth.
They are rarely used as a main earthing electrode due to the high earth electrode
resistance but may be used as an auxiliary earth.
Approximate value of the earth electrode resistance Reel for earth plates when specific
resistance equals 100 Ωm.
Plate size [m] 0.5 x 1 1 x 1
Earth electrode
resistance Reel [Ω]
45
35
4.4 General information for insulating earths
The contact surface of the connecting points must be cleaned. A connecting strip must be
laid between the earthing terminal and the water pipes. All connections open to be
protected from corrosion. Earthing connectors should not be laid under driveways or
paths. The earth electrode resistance can vary during the course of the year by a ratio of
approximately 1 : 2.
4.4.1 Water pipes
Metal water pipes may only be used as a sole earth when their components are
electrically well connected with each other and the earth electrode resistance must be as
low as normal earth electrode resistance. It must also be ensured that a change from
metal to plastic pipes will not be made at a later date.
46
4.4.2 Other pipe systems
Central heating pipes, pipes from distant heating plants, pipes from the gas system and
drain pipes must not be used as sole earths. They must be connected to the protective
earth.
47
5 TESTING THE PROTECTIVE MEASURES
The applied protective measures are to be tested by the installer before the Installation is
made operative.
The test must include all measurements and a complete visual inspection of all of the
protective circuit components.
If line voltage is used during the test, attention should be paid to the danger arising
through contact voltage, i.e. surface voltage gradient. If the test with a low test current
already shows that the protective measures are not in order, then the current must not be
increased. The test is to be interrupted.
The installer must advise the consumer to have the protective measures tested at suitable
intervals.
48
6 EXPLANATION OF FORMULA SYMBOLS AND FORMULARE
6.1 Explanation of symbols
Formula Symbol Name Unit Abbreviation
Ib Breaking current Ampere A
In Nominal current Ampere A
I∆t Fault release current of the
current-operated circuit
breaker
Ampere A
k Short circuit factor
Reel Earth electrode resistance Ohm Ω
Re Earth electrode resistance of
customers
Ohm Ω
Rt Resistance at point of fault Ohm Ω
Rh Human resistance Ohm Ω
Ro Power supplier's earth
electrode resistance
Ohm Ω
Rs Safety earthing resistance Ohm Ω
Rloc Location contact resistance Ohm Ω
Q Specific earth resistance Ohm metre Ωm
Uc Contact voltage Volt V
Ue Earth voltage Volt V
Uf Fault voltage Volt V
Ul Line voltage Volt V
Us Surface voltage gradient Volt V
49
6.2 Formulary
Calculation of the protective earth electrode resistance
Calculation of the earth electrode resistance for current-operated circuit breaker
50
EE 017
Protective Measures against Excessive
Contact Voltage
Theoretical Test
51
EE 017
PROTECTIVE MEASURES AGAINST CONTACT WITH EXCESSIVE VOLTAGE
TEST 1
1. Name three possible ways in which humans may be protected from making contact
with active conductors.
2. What is meant by "short circuit"?
3. State three safety principles which must be observed when installing and operating
electrical installations?
4. What is an active component?
5. What is meant by "earth fault"?
6. Which insulating materials may only be used on the inside of electrical equipment?
7. What is contact voltage?
8. State the formula for calculating specific earth resistance.
9. What distance must there be between a live conductor and the standing area, to
ensure safety?
10. What is the purpose of the PE-protective conductor?
52
EE 017
PROTECTIVE MEASURES AGAINST CONTACT WITH EXCESSIVE VOLTAGE
TEST 2
1. State the colour of the insulation on a protective conductor.
2. Name three examples of active protection measures apart from connection of the
protective conductor to the equipment to prevent excessive contact voltage.
3. State eight examples of inactive protective measures, using a protective conductor,
which prevents excessive contact voltage from remaining, when the equipment is
connected.
4. What is the function of double insulation?
5. What measures should be carried out before putting an installation into operation?
6. How can double-insulated equipment be recognised?
7. What is the maximum permissible voltage of a safety extra-low voltage system?
8. Name three permissible types of voltage supply for safety extra-low voltage systems.
9. Name three non permissible types of voltage supply for safety extra-low voltage
systems.
10. Why should protective conductors not be connected to low-voltage equipment?
53
EE 017
PROTECTIVE MEASURES AGAINST CONTACT WITH EXCESSIVE VOLTAGE
TEST 3
1. What is the purpose of double insulation?
2. What is the maximum fault voltage allowed on the neutral or protective conductor
before a component is shut off?
3. What are the minimum dimensions of neutralisation conductors made from steel strip;
steel rod; copper?
4. What is the maximum total earth electrode resistance permissible in a protective
conductor system?
5. With a contact voltage of 50 V, what is the maximum permissible auxiliary earth
electrode resistance using a voltage-operated earth-leakage circuit breaker?
6. What are the approximate values of specific earth resistance of the following
conditions: wet, marshy soil; damp sand; dry sand; rock?
7. What factors govern the type of earth used?
8. How deep should the earthing strip be buried?
9. Name three types of horizontal earthing strip.
10. What is the relationship between the length of vertical earth rods, and the distance
between the rods?
54
EE 017
PROTECTIVE MEASURES AGAINST CONTACT WITH EXCESSIVE VOLTAGE
TEST 1
(Solution)
1. By insulation of the conductor, protective covering of conductors and machinery, by
placing the conductors out of reach.
2. A conducting connection between active conductors with different potentials.
3. The safety to life may not be endangered.
Electrical installation must not cause any damage (e.g. fire).
Safety standards must be as high as possible.
4. Any component or wire used to carry current.
5. A conducting connection between a live conductor and earth or earthed (grounded)
component.
6. Varnish, enamel or oxide coatings.
7. The part of the fault voltage which can be bridged by a human.
8.
9. 2500 mm.
10. The PE-conductor is installed to protect the equipment housing from excessive
voltage.
55
EE 017
PROTECTIVE MEASURES AGAINSTCONTACT WITH EXCESSIVE VOLTAGE
TEST 2
(Solution)
1. Yellow and green.
2. Protective insulation; protective low voltage; protective isolation of one item of
equipment.
3. Protective isolation for several items of equipment; safety earthing; neutralisation;
protective conductor system; voltage-operated circuit breaker; current-operated
circuit breaker; combined current-voltage-operated circuit breaker; controlling the
earth potential.
4. To protect all inactive components from excessive contact voltage.
5. The Operation of all protective measures must be inspected and tested.
6. By the standard symbol (a square within a square) which all such equipment must
carry.
7. Maximum 42 volts.
8. Protective transformers with separated windings; rotary converter with separate
windings; storage batteries; dry batteries.
9. Autotransformers; series resistors; voltage dividers; rotary converters with
connections between windings.
10. Because of the danger of voltage from other faulty equipment passing over the
conductor to the low-voltage equipment.
56
EE 017
PROTECTIVE MEASURES AGAINSTCONTACT WITH EXCESSIVE VOLTAGE
TEST 3
(Solution)
1. To provide complete Isolation from the mains and earth.
2. 50 V
3. 90 mm² cross section
10 mm diameter
16 mm² cross section
4. 20 ohms.
5. 800 ohms.
6. 30 Ωm; 300 Ωm; 1000 Ωm; 10000 Ωm.
7. The composition of the soil (earth) and the requirements of the installation.
8. Between 0.5 and 1 m, depending on soil conditions.
9. Ring earth; ray earth; mesh earth.
10. The distance between them must be equal to twice their length.
57
KEY TO EVALUATION
PER CENT
MARK
88 – 100
1
75 – 87
2
62 – 74
3
50 – 61
4
0 – 49
5
