CONDUCTOR INSULATION

The NEC® requires that all conductors be insulated,310.2 (A). There are a few

exceptions, such as the permission to use a bare neutral conductor for services, and

bare equipment grounding conductors.

The classification of insulation, either thermoplastic or thermoset, generally

determines its suitability for use under various ambient conditions. Thermoplastic

materials, typically plastic resins, soften and flow when they are heated and

subjected to pressure, but they become rigid when cooled. Thermoset materials, by

contrast, are initially set or cured when heated, but after curing they will not soften,

flow, or distort.

Some kinds of insulation are approved for wire only if it is to remain in a dry

environment, preferably within a building, but other insulating materials have been

formulated to stand up to environments that are dry and damp, dry and wet, or wet,

and still other materials can withstand extreme temperatures. For example, some

insulated wire can perform effectively only up to 60°C (140°F), while others can

perform effectively at temperatures as high as 90°C (194°F).

All modern electrical wire insulation is rated as flame-retardant, but only a few of

these insulation materials are capable of remaining effective insulators following

long-term exposure to sunlight (ultraviolet radiation), ozone, or nuclear radiation.

PURPOSE

Conductors need to be electrically isolated from other conductors and from the

environment to prevent short circuits. Insulation is applied around a conductor to

provide this isolation. Most wire and cable insulations consist of polymers

(plastics), which have a high resistance, to the flow of electric current. A jacket is

the outermost layer of a cable whose primary function is to protect the insulation

and conductor core from external physical forces and chemical deterioration.

TYPES AND APPLICATIONS

Thermoplastic Insulation: Thermo means heat and plastic means formable,

thermoplastic is soften and melt if heated above its rated temperature. It can be

heated, melted, and reshaped. Thermoplastic insulation will stiffen at temperatures

colder than 14°F (minus 10°C). Thermoplastic is lower in cost and lighter in

weight and most commonly used. Thermoplastic compounds are materials that go

soft when heated and harden when cooled.

PVC (Polyvinyl Chloride): – is the most commonly used thermoplastic insulator

for cables. It is cheap, durable and widely available. However, the chlorine in PVC

(a halogen) causes the production of thick, toxic, black smoke when burnt and can

be a health hazard in areas where low smoke and toxicity are required (e.g.

confined areas such as tunnels). Sometimes referred to simply as ―vinyl,‖ PVC

does not usually exhibit extremely high- and low-temperature properties in one

formulation. Certain formulations may have a –55°C to 105°C rating, while other

common vinyls may have a –20°C to 60°C rating. The many varieties of PVC also

differ in pliability and electrical properties. The price range can vary accordingly.

Typical dielectric constant values range from 3.5 to 6.5.

When properly formulated, thermoplastic jackets of PVC provide cables with the

ability to resist oils, acids, alkalis, sunlight, heat, weathering and abrasion. This

range of properties makes PVC a suitable outer covering for such cable types as

underground feeders (Type UF), control, aerial, street lighting and cables for direct

burial.

PE (Polyethylene) – is part of a class of polymers called polyolefins.

Polyethylene has lower dielectric losses than PVC and is sensitive to moisture

under voltage stress (i.e. for high voltages only).

Polyethylene has excellent electrical properties. It has a low dielectric constant, a

stable dielectric constant over a wide frequency range, and very high insulation

resistance. However, polyethylene is stiff and very hard, depending on molecular

weight and density. Low density PE (LDPE) is the most flexible, with high-

density, high-molecular weight formulations being least flexible. Moisture

resistance is excellent. Properly formulated PE has excellent weather resistance.

The dielectric constant is 2.3 for solid and 1.6 for cellular (foamed) insulation.

Flame retardant formulations are available, but they tend to have poorer electrical

properties.

Fluoropolymers

Fluoropolymers, with the exception of PTFE Teflon(sometimes called TFE), are

extrudable thermoplastics used in a variety of low-voltage insulating situations.

Fluoropolymers contain fluorine in their molecular composition, which contributes

to their excellent thermal, chemical, mechanical and electrical characteristics. The

most commonly used fluoropolymers are Teflon(PTFE, FEP and PFA), Tefzel

(ETFE), Halar (ECTFE) and Kynar or Solef (PVDF).Teflon has excellent electrical

properties, temperature range and chemical resistance. It is not suitable where

subjected to nuclear radiation and does nothave good high-voltage characteristics.

FEP Teflon is extrudable in a manner similar to PVC and polyethylene. This

means that long wire and cable lengths are available. PTFE Teflon is extrudable in

a hydraulic ram type process. Lengths are limited due to the amount of material in

the ram, thickness of the insulation and preform size. PTFE must be extruded over

a silver- or nickel-coated wire. The nickel- and silver-coated designs are rated

260°C and 200°C maximum, respectively. The cost of Teflon is approximately 8 to

10 times more per pound than PVC compounds.

Figure 1. Typical low voltage PVC cable

Figure 1. Typical low voltage PVC cable

Figure 2. LV/MV cable for outdoor usage with PE sheath

Thermoset: Thermo =heat and set = fixed, thermoset materials do not melt when

heated. Onced cured cannot be recycled like thermoplastic. Once the ingredients

have been mixed, heated, and formed, it can never be reheated and reshaped. If

heated above its rated temperature, it will char and crack. Thermoset is more

flexible at lower temperatures.

XLPE (Cross-Linked Polyethylene) – has different polyethylene chains linked

together (―cross-linking‖) which helps prevent the polymer from melting or

separating at elevated temperatures. Therefore XLPE is useful for higher

temperature applications. XLPE has higher dielectric losses than PE, but has

better ageing characteristics and resistance to water treeing. Normal operating

temperatures are typically between 90C and 110C. Temperature limit is 250C.

EPR (Ethylene Propylene Rubber) – is a copolymer of ethylene and

propylene, and commonly called an ―elastomer‖. EPR is more flexible than

PE and XLPE, but has higher dielectric losses than both. Normal operating

temperatures are typically between 90C and 110C. Temperature limit is

250C.

Figure 3. 3-phase EPR insulated cable for MV

Figure 4. MV cable with XLPE insulation, 33 kV

Comparison of Materials

A comparison of common insulating materials is as follows:

Material Advantages Disadvantages

PVC

Cheap

Durable

Widely available

Highest dielectric losses

Melts at high temperatures

Contains halogens

Not suitable for MV / HV cables

PE

Lowest dielectric

losses

High initial dielectric

strength

Highly sensitive to water treeing

Material breaks down at high

temperatures

XLPE

Low dielectric losses

Improved material

properties at high

temperatures

Does not melt but

thermal expansion

occurs

Medium sensitivity to water treeing

(although some XLPE polymers are

water-tree resistant)

EPR

Increased flexibility

Reduced thermal

expansion (relative to

XLPE)

Low sensitivity to

water treeing

Medium-High dielectric losses

Requires inorganic filler / additive

Paper /

Oil

Low-Medium dielectric

losses

Not harmed by DC

testing

Known history of

reliability

High weight

High cost

Requires hydraulic pressure / pumps for

insulating fluid

Difficult to repair

Degrades with moisture

A letter code has been established to simplify the selection of the optimum

insulated wire for specific tasks while considering operating temperatures and

application environments. A knowledge of this code will permit the user suffucient

background information to make the best selection. Examples of these code

designations are RHH, THHN, TW, and XHHW.

Copper and aluminum wire are identifiable on sight by their color—reddish brown

for copper and silver for aluminum. However, it is difficult to identify copper-clad

aluminum wire on sight without making a clean cut through the wire to examine

the core.

On the other hand, even experts have trouble identifying wire insulation. For this

reason, manufacturers print an identification code directly on the wire insulation,

giving:

■The trade name of the insulation

■Maximum operating temperature (H for 75°C, HH for 90°C, none for 60°C)

■Environments for safe application (dry, damp, wet, or combinations of these)

■Year of manufacture

■Maximum voltage rating

Cable selection

Generally, all element s relating to cable selection can be broadly categorized as

electrical and physical. Electrical consideration range from the more obvious

ampere and voltage selection. The physical elements include consideration of

tensile strength.

Voltage

The operating voltage for a given circuit is a primary factor in the choice of

insulating material. Incorrect selection of the insulating material may result in

insulation breakdown and may cause short circuit.

Current

Circuit current requirement are used to determine the conductor size. The

maximum current carrying capacity for a given size of conductor is determine by

the heat dissipation.

Frequency

When conductors are selected for dc or low frequency application, frequency need

not be considered. However, when conductors are used as RF transmission lines,

skin effect become a consideration.

Environmental Considerations

Animal Life

Cables covered by lead sheathing and armor are not always adequately protected.

Rodents have sometimes destroyed even these protective covering. To prevent

damaged, protective sheaths are often painted or impregnated with arsenic or other

poisons and repellents.

Sunlight

Some synthetic rubber sheaths and insulations are subject to damaged by

prolonged exposure to sunlight. If prolonged exposure cannot be avoided, the

following preventive measures are recommended:

1. Perform frequent inspection

2. Treat cable surface with silicone compound

3. Paint cables with a rubber base preservative paint.

Ozone

Ozone(O3) is a form of oxygen which, unlike oxygen in its normal state (o2), is

harmful to rubber and rubber compounds. The preventive measure should be taken

such case.

Extreme climatic conditions

Extreme cold, salt air, hot humid air, and other climatic conditions must be

considered when the type of cable to be used is determined.

Insulation Resistance Testing

Insulation starts to age as soon as it's made. As it ages, its insulating performance

deteriorates. Any harsh installation environments, especially those with

temperature extremes and/or chemical contamination, accelerates this process. This

deterioration can result in dangerous conditions in power reliability and personnel

safety. As such, it's important to identify this deterioration quickly so that

corrective steps can be taken. The most important reason for testing insulation is to

insure public and personal safety. By performing a high dc voltage test between

de-energized current-carrying (hot), grounded, and grounding conductors, you can

eliminate the possibility of having a life-threatening short circuit or short to

ground. One of the simplest tests and its required test instrument are not

universally understood.

Insulation testing components

The insulation resistance (IR) test (also commonly known as a Megger) is a spot

insulation test which uses an applied DC voltage (typically either 250Vdc, 500Vdc

or 1,000Vdc for low voltage equipment <600V and 2,500Vdc and 5,000Vdc for

high voltage equipment) to measure insulation resistance in either kΩ, MΩ or GΩ.

The measured resistance is intended to indicate the condition of the insulation or

dieletric between two conductive parts, where the higher the resistance, the better

the condition of the insulation. Ideally, the insulation resistance would be infinite,

but as no insulators are perfect, leakage currents through the dielectric will ensure

that a finite (though high) resistance value is measured.

The megohmmeter( megger)

Megohmmeter (sometimes referred to as a megger) is a special type

of ohmmeter used to measure the electrical resistance of insulators. Insulating

components, for example cable jackets, must be tested for their insulation strength

at the time of commissioning and as part of maintenance of high voltage electrical

equipment and installations. For this purpose megohmmeters, which can provide

high DC voltages (typically in ranges from 500V to 2kV) at specified current

capacity, are used. Acceptable insulator resistance values are typically 1 to 10

megohms, depending on the standards referenced.

A basic megohmmeter hook-up schematic is shown in Fig. 1 (above). The

megohmmeter is similar to a multimeter, when the latter is in its ohmmeter

function. There are differences, however.

First, the megohmmeter's output is much higher than that of a multimeter. Voltages

of 100, 250, 500, 1,000, 2500, 5,000, and even 10,000V are. The most common

voltages are 500V and 1,000V. Higher voltages are used to stress an insulation to a

greater degree and thus obtain more accurate results.

econd, the range of a megohmmeter is in megohms, as its name implies, instead of

ohms as in a multimeter.

Third, a megohmmeter has a relatively high internal resistance, making the

instrument less hazardous to use in spite of the higher voltages.

Ground Resistance Measuring

Why grounding?

Lack of good grounding is dangerous and increases the risk of equipment failure.

Without an effective grounding system, we could be exposed to the risk of electric

shock, not to mention instrumentation errors, harmonic distortion issues, power

factor problems and a host of possible intermittent dilemmas. If fault currents have

no path to the ground through a properly designed and maintained grounding

system, they will find unintended paths that could include people.

However, good grounding isn’t only for safety; it is also used to prevent damage to

industrial plants and equipment. A good grounding system will improve the

reliability of equipment and reduce the likelihood of damage due to lightning or

fault currents. Billions are lost each year in the workplace due to electrical fires.

This does not account for related litigation costs and loss of personal and corporate

productivity.

Why test grounding resistance or why determine the soil resistivity?

Soil Resistivity is most necessary when determining the design of the grounding

system for new installations (green field applications) to meet your ground

resistance requirements. Ideally, you would find a location with the lowest possible

resistance. But as we discussed before, poor soil conditions can be overcome with

more elaborate grounding systems. The soil composition, moisture content, and

temperature all impact the soil resistivity. Soil is rarely homogenous and the

resistivity of the soil will vary geographically and at different soil depths. Moisture

content changes seasonally, varies according to the nature of the sub layers of

earth, and the depth of the permanent water table. Since soil and water are

generally more stable at deeper strata, it is recommended that the ground rods be

placed as deep as possible into the earth, at the water table if possible. Also,

ground rods should be installed where there is a stable temperature, i.e. below the

frost line. For a grounding system to be effective, it should be designed to

withstand the worst possible conditions.

Over time, corrosive soils with high moisture content, high salt content, and high

temperatures can degrade ground rods and their connections. So although the

ground system when initially installed, had low earth ground resistance values, the

resistance of the grounding system can increase if the ground rods are eaten away.

That is why it is highly recommended that all grounds and ground connections are

checked at least annually as a part of your normal Predictive Maintenance plan.

During these periodic checks, if an increase in resistance of more than 20 % is

measured, the technician should investigate the source of the problem, and make

the correction to lower the resistance, by replacing or adding ground rods to the

ground system.

Earth resistance is measured in two ways for two important fields of use:

1. Determining effectiveness of ―ground‖ grids and connections that are used with

electrical systems to protect personnel and equipment.

2. Prospecting for good (low resistance) ―ground‖ locations, or obtaining measured

resistance values that can give specific information about what lies some distance

below the earth’s surface (such as depth to bed rock).

What is a good ground resistance value?

There is a good deal of confusion as to what constitutes a good ground and what

the ground resistance value needs to be. Ideally a ground should be of zero ohms

resistance. There is not one standard ground resistance threshold that is recognized

by all agencies.

However, the NFPA and IEEE have recommended a ground resistance value of 5.0

ohms or less.

The NEC has stated to ―Make sure that system impedance to ground is less than 25

ohms specified in NEC 250.56. In facilities with sensitive equipment it should be

5.0 ohms or less.‖

The Telecommunications industry has often used 5.0 ohms or less as their value

for grounding and bonding.

The goal in ground resistance is to achieve the lowest ground resistance value

possible that makes sense economically and physically.