Cable Selection13

8/11/2019 Cable Selection13

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Calculate the design current (Ib).

Select the type and nominal rating of protective

device (In). Determine and apply the correction factors to In.

Select Cable from the table IEE Regulation Iz.

Calculate the Voltage Drop and check forcompliance.

Check the circuit complies with shock protection.

Check the circuit complies with thermal constraints.


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The amount of current which flows to the load.

That current is knows as Designing current Ib.

The factors which are used to determine thedesigning current are Power factor, Rated Voltage &

Power of the connected load.

KW=KVA*P.F.

Ib=(KVA*1000)/(1.732*V)


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The Protective device is used to protect

the load from overload current and fault

current.

So the Cable size with respect to

protective device rating depends upon

Amplitude and duration in time.


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Ca: Correction factor for ambienttemperature.

Cg: Correction factor for grouping.Ci: Correction factor for thermal

insulation.

So based on these factors the size of thecable is chosen from the table based onIEE Regulation.


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A cable's conductor can be seen as an impedance and

therefore whenever current flows through a cable, there

will be a voltage drop across it, which can be derived by

OhmsLaw (i.e. V = IZ).

The impedance of the cable is a function of the cable size

(cross-sectional area) and the length of the cable.

Most cable manufacturers will quote acables

resistanceand reactance in /km.


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Shocks occur when electric currents flow through the body

between points at different voltage.

Earth fault occurs when some phase conductor touches

neutral (for earthed system) or ground potential.

The shock voltage is calculated by multiplying the

resistance of the earth conductor over the length of the

cable by the current that will trip the protective device in

five seconds . The disconnection time is checked by finding the current

that will trip the device in the required time (0.4 or 5

seconds), and determining whether the current flow in an

earth fault will reach this level.


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When the large amount of fault current occurs , the

temperature of the cable increases which would then

melt and fire would start. so to reduce the heat the

proper size of the cable should be selected.

The size is found using the factors such as K(The

factors depending on the conductor and insulating

materials), t(The operating time of the disconnectingdevice in seconds), I(The value of fault current).

S=Sqrt(It)/K

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Cable Selection13