max disonnection times

dochisholm · Mar 22, 2011

hi all in brb it states that on a tt system the max time is 0.2 for under 32 amps IF the circuit is protected by mcb and has main bonding in place however the tn values can be used. am i reading this correct can i use the tn values as the circuits have mcb and rcd protection thanks. this is for paperwork values are nowhere near either for the record

telectrix · Mar 22, 2011

don't quite understand what you are asking. if its max. values of Zs on a scedule of test results, on a TT system with 30mA RCD you can enter 1667ohms.

dochisholm · Mar 22, 2011

not zs max disconnection times and dont i apply 80% rule to the 1667 ohms

telectrix · Mar 22, 2011

ah, got you. max. dis times are 0.4sec up to 32A and 5secs over 32A ( except on a TT when the 0.4 becomes 0.2 ). as for the max. Zs values i always enter the uncorrected value as that is the max. but in practice would make sure the measured value was under the 80% value. Reason is never known cable to be so heavily loaded so as to run at 70deg.

dochisholm · Mar 22, 2011

thanks

Chr!s · Mar 23, 2011

hi all in brb it states that on a tt system the max time is 0.2 for under 32 amps IF the circuit is protected by mcb and has main bonding in place however the tn values can be used. am i reading this correct can i use the tn values as the circuits have mcb and rcd protection thanks. this is for paperwork values are nowhere near either for the record

This is where the mcb provides Earth fault current protection, which i think you will agree in most circumstances wont happen. So an rcd is used for this purpose, and the mcb for overcurrent protection.

IQ Electrical · Mar 23, 2011

Chr!s said:
This is where the mcb provides Earth fault current protection, which i think you will agree in most circumstances wont happen. So an rcd is used for this purpose, and the mcb for overcurrent protection.

Spot on, I know only too well how difficult it can be to get an Ra sub-200 Ohms never mind meet requirements for disconnection by an OPD.