Phase Conductor

[Sudeep]

What is the basis of selection of different sizes of PE conductor w.r.t. Phase conductor sizes ?
If the phase conductor is incresed to restrict voltage drop, is it also necessary to increase the PE conductor size by the same ratio?

Question by eproengg

 

– Moderator Team

[Spir Georges GHALI]

Sudeep said:

What is the basis of selection of different sizes of PE conductor w.r.t. Phase conductor sizes ?
If the phase conductor is incresed to restrict voltage drop, is it also necessary to increase the PE conductor size by the same ratio?

Question by eproengg

 

– Moderator Team

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Dear ;

 

The basic rule to select the PE's section is as follow :

     – SPh < 16mm²  we should use the same section for PE

     – SPh > 16mm²  we can use for PE ” 1/2 section of pahse “, but not more than ” 300-400mm² ” because the effect on the Earth Fault Current value will be too small.

But it's not suitable for all cases, as the selection of PE's section is depending on the Earthing System because :

For ” TN-C ” :

As we can't use the Earth Leakage Protection with this system, we should, after selection of PE, calculate the Earth Fault Current ” If ” where this value should be bigger than the adjustment value of the Magnetic Protection ” Im ” of the Circuit Breaker, if not, we have 2 possibilities : either minimize the magnetic adjustment ” Im ” ( if it's possible ) to be smaller than ” If “, or over-size the PE's section, and in some case the pahses' section, till the  ” If ” be bigger than the magnetic adjustment ” Im “.

For ” TT ” :

As the resistances' values of the Earth Points are big ( a few ohms ) and are integrated in the Earth Fault Impedance Loop, and the cables' impedanse are too small ( … mΩ ), the value of Earth Fault Current is limited, and the effect of changing the PE's section is negligible. For this reason the maximum section of PE in TT system is ” 50mm² ” for phases' section more than ” 50mm² “, and ” 1/2 section of pahse ” for smaller than ” 50mm² “.

 

For the 2^nd question : it's not necessary to modify the PE's section when we chang the pahses's section, because the Earth Fault Current will be bigger.

 

In all case, the Earth Fault Current ” If ” should be calculated, then we can decide what we should do.

 

Regards.

[erickench]

The earth should never be used as a ground fault current return path. I's resistance is too high and may therefore be unable to operate the overcurrent device in case of a fault.

[Spir Georges GHALI]

erickench said:

The earth should never be used as a ground fault current return path. I's resistance is too high and may therefore be unable to operate the overcurrent device in case of a fault.

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Dear Erickench ;

 

1 – Excuse-me to say, I can't understand where the Earth Fault will be circulated, asume that we use ” TN-C ” as Earthing System, that means ” 4 Wires  :  L1, L2, L3, PEN “, or we use ” TN-S ” that means ” 5 wires  : L1, L2, L3, N, PE “, and in both the earth fault is with ” L2 “, please let me know where the Earth Fault Current will be circulated ?

 

2 – Normally, once we have an ” Earth Fault “, and accordignly to the Earthing System, we have 2 possibilities to trip the Circuit Breaker that are :

     A – Use the ” Magnetic Protection ” built-in the Circuit Breaker, if the value of the Earth Fault Current is bigger than the setting of this protection, or, if possible, modify the setting to be smaller than the Earth Fault Current.

     B – Use an ” Earth Leakage ” Protection when we can't realize any condition mentioned in A.

Noting that :

     – We can use the ” Earth Leakage Protection ” with ” TN-S  ;  TT  ;  IT “.

     – We can't use the ” Earth Leakage Protection ” with ” TN-C “.

     – The value if the ” Impedance Fault Loop ” is big ” … Ω “ in ” TT ” and ” IT with many different Earth Points “, where in these cases the Earth Fault Current's value is small and we should use the Earth Leakage Protection to trip the circuit breaker.

 

Regards.

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