@mohamed sleem said:
If we are doing TN-C-S system and the cable protection is a breaker, would the 1x300m2 be acceptable with a feeder 8 sets of (4x1c 630mm2)? & if all of the sets installed in group the 8x1c300m2 will be ok?
Dear Mr. Mohamed ;
First at all, if you use ” TN-C-S ” as Earthing System , that means at the 1st level or maybe at the 2nd level also you have ” TN-C ” and ” PEN “, for that it will be better to do as follow :
– Check the Single-phase loads if they are distributed on 3 phases ( even if there’s some difference ).
– Check if the ” multiple third Harmonics ” are existed, if yes, you should know the percentage of each one.
Then, according to the results of these checkings, we can decide the section of ” PEN ” as follow :
– If the Single-phase loads are well distributed, and the percentages of the ” multiple third Harmonics ” are small and/or the total current of them is small, we can assume that ” 1x300m² ” or ” 2x240m² ” are enough, then we should calculate the Earth Fault Current for one of these 2 sections, and check if the chosen section can withstand the ” Thermal Stress ” of the Earth Fault Current during at least 1 or 2 sec ( certainly not more than 5 sec ), it not we should oversize the PEN’s section to ” 3x240m² ” or ” 2x300m² ” and repeat the procedure.
– If the percentages of the ” multiple third Harmonics ” aren’t small, it will be better to know the total current of them either by calculation or from the catalogues of the equipment generating these Harmonics, then we can use the same procedure that we use to define the Phases’ sections to define the section of ” PEN “, do the calculation of the Earth Fault Current, then check if the PEN cable can withstand the Thermal Stress.
By the way, and for ” TN-C ” system, please note that in all case the setting of the ” Magnetic or Short-time Protection ” should be smaller than the Earth Fault Current value by ” ≥ 20% “.