Discuss Does anybody know of this new regulation that omits the ZS test. in the UK Electrical Forum area at ElectriciansForums.net

MCBS are for short circuits. There has been cases of Live and Neutral touching and it just trips the MCB and not the RCD. It seems that the regs are saying we can't rely on the zs being low enough to help trip the MCB in time which may cause the cable to be permanently damaged or appliance, but we still have protection of persons and livestock by the use of RCDs. This is my thoughts at the moment.
 
MCBS are for short circuits. There has been cases of Live and Neutral touching and it just trips the MCB and not the RCD. It seems that the regs are saying we can't rely on the zs being low enough on an TT system to help trip the MCB in time which may cause the cable to be permanently damaged or appliance, but we still have protection of persons and livestock by the use of RCDs. This is my thoughts at the moment.
 
Where does it say 7667 ohms?
Looking at table 41.5 for a 30mA rcd it’s max zs is still 1667 ohms.
Also see regulation 411.4.204

I don’t have a book to hand but I’m pretty sure that for a TN system they’ve changed it from 50V to 230V giving 7667ohms.
TT systems have remained at 50V
 
I
Well yes, RCDs don’t work on L - N faults
I'm saying that the Regs when it comes to TTs, it that we can't always garantee the disconnection of the mcb circuit on time and that it may damage property but as soon it leaks to livestock or people , it must trip within the time and the continuity of CPC must be sound. And results must be recorded.
 

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