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
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 2nd 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.