power cable systems

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KZ 3144

Power Cable systems


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Introduction

A cable is defined as a single conductor or an assembly ofconductors covered by solid electrical insulation.

Cable specification generally starts with the conductor andprogress radically through the insulation and covering.


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Introduction

The following is a typical list of specifications:

Number of conductors in cable

Conductor size and material

Insulation type

Shielding system

Outer finishes (sheath)

Insulation

Alternatively cables are specified by its ampacity, voltage,

frequency and other important system data


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Cable construction

Conductors

The two conductor materials in common use are copper and

aluminum.

Copper is used for conductors of insulated cables primarily

for its desirable electrical and mechanical properties.

The use of aluminum is based mainly on its favorable

conductivity to weight ratio, availability and stable low cost.


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Copper vs Aluminum

Copper-Advantages

Copper has a higher conductivity than aluminum.

It is more ductile (can be drawn out).

Copper has relatively high tensile strength (the greatest stress asubstance can bear along its length without tearing apart).

It can also be easily soldered.

Copper-Disadvantages

However, copper is more expensive and heavier than aluminum.


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Copper vs Aluminum

Aluminum-Disadvantages

Compared to copper, aluminum has worse conductivity per unitvolume, but better conductivity per unit weight. In many cases, weightis more important than volume making aluminum the 'best' conductor

material for certain applications. For example, it is commonly used forlarge-scale power distribution conductors such as overhead powerlines.

Aluminum-Adantages

Although aluminum has only about 60 percent of the conductivity ofcopper, its lightness makes long spans possible.

Its relatively large diameter for a given conductivity reduces corona.


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Cable construction

Classes of conductors

Conductors are classified as solid or standard. A solid conductor

is a single conductor of solid circular section.

A standard conductor is composed of a group of small

conductors in common contacts.

A standard conductor is used where the solid conductor is too

large and not flexible enough to be handled readily.


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Cable construction

Conductor size Conductor sizes are ordinarily expresses by two different

numbering methods: the AWG and the circular mil.

The AWG conductor sizes are numbered from 30 to 1, then

continuing with 0, 00, 000, 0000.

Conductors larger than 4/0 AWG are designated in circular

mils.

A circular mil is defined as the area of a circle having a diameter

of 1/1000 of an inch.


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Insulations

In general, current-carrying conductors must not beallowed to come in contact with one another, theirsupporting hardware, or personnel working nearthem.

To accomplish this, conductors are coated orwrapped with various materials, commonly knownas insulating materials.

Two fundamental properties of insulating areinsulation resistance and dielectric strength


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Cable construction: Insulation

INSULATION RESISTANCE Insulation resistance is the resistance to current leakage through the insulation

materials. Insulation resistance can be measured with a megger withoutdamaging the insulation. Information so obtained is a useful guide in appraisingthe general condition of insulation. Clean, dry insulation having cracks or otherfaults may show a high value of insulation resistance but would not be suitablefor use.

DIELECTRIC STRENGTH

Dielectric strength is the ability of an insulator to withstand potentialdifference. It is usually expressed in terms of the voltage at which the insulationfails because of the electrostatic stress. Maximum dielectric strength values canbe measured only by raising the voltage of a TEST SAMPLE until the insulationbreaks down


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Cable construction-: insulation

Insulations can be classified as solid, taped or special purposeinsulations. The following is a list of insulations commonly

used:

Thermosetting compounds (solid dielectric)

Thermoplastic compounds (solid dielectric)

Paper laminated tapes

Varnished cloth laminated tapes

Mineral inorganic insulation (solid dielectric granular)


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Cable construction-: insulation

Insulations in general use for voltages above 2 KV:Thermosetting compounds (solid dielectric)

Cross linked polyethylene (XLP or XLPE)

Ethylene propylene rubber (EPR)

Styrene butadiene rubber (SBR) Silicon rubber, oil base rubber

Thermoplastic compounds (solid dielectric)

Polyethylene (natural)

Polyvinyl chloride (PVC)Paper laminated tapes

Varnished cloth laminated tapes


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Cable construction-: Shielding

Insulation shielding is required for all non matallic, sheathed,single (> 2KV) and all metallic sheathed cables (> 5KV) :

Shielding confines the electric field of the cable to the insulationsurrounding the conductor by means of conducting or semi

conducting layer.

There are many purposes of shielding Confine the electric field within the cable Equalize the voltage stress within the insulation Protect cable from induced potentials Limit electromagnetic and electrostatic interference Reduce shock hazard


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Cable construction-: Outer finishes

Cable outer coverings are used to protect the underlying cablecomponents from the environmental and installation conditionsassociated with intended service.

The choice of cable outer finishes for a particular application is

based on Electrical Thermal Mechnical considerations.

Combinations of metallic and non metallic finishes are usuallyrequired to provide the total protection needed for the installationand operation.


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Cable Rating and Selection Criteria

The selection of power cables involves the consideration ofvarious electrical and environmental conditions including:

Quantity of power distributed

Degree of exposure to adverse mechanical and thermal stresses.

The selection of conductor size is based on the following criteria

Voltage rating

Load current criteria

Emergency overload criteria Voltage drop limitations

Fault current criteria.


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Cable Selection: Voltage rating

The selection of power cables insulation (voltage) rating is basedon the phase to phase voltage of the system, system type(grounded or non grounded) and the time in which a ground faultis cleared by the protective equipment.

Ungrounded cable system must have greater insulation thicknessthan the cable used on a grounded system.

100% voltage rated cables are applicable to grounded system(clearing time 1 min).

133% voltage rated cable are applicable to ungrounded system(clearing time 1 hour)


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Cable Selection: Load current criteria

The manufacturers ampacity recommendation should be used asload current criteria.

Ampacity tables indicates the minimum size conductor required.

Conservative engineering practice, future load growth, voltagedrop, short circuit consideration may requires the use of largerconductor

Other considerations to be honored in load current criteria are:

Skin and proximity effect

Ambient temperature

Surrounding medium


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Cable Selection: Emergency overload

criteria

The nominal loading limits of insulated cables are determinedbased on many years of practical experience. These limits accountfor a rate of insulation deterioration that results in the useful lifeof cable system.

The life of cable insulation may be and the average thermalfailure rate almost doubles for each 5 to 15 degree increase innormal daily load temperature.

Maximum emergency overload temperatures for various types ofinsulation can be found in practical guides.

Operation at these emergency overload temperatures should notexceed 100 hours / year and such 100 hour period should notexceed five during the life of the cable.


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Cable Selection: Voltage drop

The supply conductor, if not of sufficient size, will causeexcessive voltage drop in the circuit, and the drop will be in

direct preposion to the circuit length.

Proper starting and running of motors, lighting equipment,

and other loads having heavy inrush current must be

considered.

It is recommended that the steady state voltage drop in

distribution feeders be no more than 5%.


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Cable Selection: Fault current criteria

Under short circuit conditions the temperature of the conductorrises rapidly then, due to the thermal characteristics of theinsulation,sheath, and surrounding materials, it cools off slowlyafter the short circuit is cleared.

A transient temperature limit for each type of insulation for shortcircuit duration not in excess of 10 sec has been established andmany times this criterion is used to determine mimimumconductor size.

IPCEA standard defines the maximum conductor temperaturelimits allowable under worst case fault condition.


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This section will consider the following areas used in cable

selection:

Design current (Ib)

Rating of the protective device (In) Reference methods

Correction factors

Application of correction factors

Voltage drop

Shock protection

Thermal constraints

Diversity


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To find the design current, you may need to use the

following equations:

Single phase supplies:

Uo=230 V

Three phase supplies:

Where PF is the power factor of the circuit concerned.


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When you have worked out the design current (Ib), you

must next work out the current rating or setting (In) of the

protective device. The IEE wiring regulation says that

current rating (In) must be no less than the design current(Ib) of the circuit.

The reason for this is that the protective device must be

able to pass enough current for the circuit to operate atfull load, but without the protective device operating and

disconnecting the circuit.


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You need to decide at this stage in the cable selection

process which method of installation to use. This will make

sure that the correct cable column is chosen in the later

stages of cable selection.

This choice is also important when you calculate

correction factors for thermal insulation.


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Diagram


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Ambient temperature

Grouping factors

Thermal insulation

BS 3036 fuse

Mineral insulated cable


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This is the temperature of the surroundings of the cable.

When a cable carries current, it gives off heat.

Hotter the surroundings of the cable, the more difficult it is

for the cable to get rid of this heat. If the surrounding temperature is low, then the heat given off

could be easily let out and the cable could carry more current.


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If a number of cables are run together and touching each other,they will all produce heat when they are carrying current-Separation necessary

If each cable is separated by a clearance to the next surface of atleast one cable diameter:- 'spaced' - correction factorimportant.

Where the horizontal clearance to the next cable is more than

2Dia then no correction factor is needed.

It is important that you apply these correction factors to thenumber of circuits or multicore cables that are grouped and notthe number of conductors.


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Thermal insulation has the effect of wrapping a cable in a fur

coat on a hot summer's day. The heat produced when the cable

carries current cannot escape.

There are two ways; thermal insulation on one side of the cable

totally enclosed in thermal insulation


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BS 3036 fuses

always use a factor of 0.725 when calculating current

carrying capacity.

Mineral insulated cable For bare cables (i.e. no PVC outer covering) exposed to

touch, the tabulated values should be multiplied by 0.9.


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When the right correction factors have been applied, we then

know the effective current carrying capacity of the conductor

(Iz).

This value of current tabulated in Appendix 4 of BS 7671 is

given the symbol Itwhich must be greater than Iz. Hence:


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the voltage drop between the origin of the installation

(usually the supply terminals) and the socket outlet or the

terminals of the current using equipment should not

exceed 4% of the supply voltage.

Voltage drop formula


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power cable systems

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