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Needing to meet mcb zs with ln loop.
Former Community Member
Where an rcd is relied on for fault protection due to high zs it has been suggested to me that the loop between live Conductors needs to be less than the mcb max zs. I can't find anything on this anywhere but he said he had this confirmed by an niceic assessor. Have I got it wrong ?
Working with the typical B type MCBs found in most domestic and many other types of electrical installations may not call for precise testing .
Referring to Fig3A4 in BS7671 gives operating times for type B MCBs across a range of prospective currents.
There is also a table giving the current for time 0.1 sec to 5 secs required to operate the MCB. The table is really a prompt to remind you that the current for a B type MCB is five times it's rating .
So for example if you have a shower circuit protected by a B40 MCB so long as the prospective short circuit current is more than 200 amps measured by a live neutral loop test and the PSSC calculated from the result it's job done isn't it?
Apart from ticking the box to say the voltage drop has been verified. ?
Working with the typical B type MCBs found in most domestic and many other types of electrical installations may not call for precise testing .
Referring to Fig3A4 in BS7671 gives operating times for type B MCBs across a range of prospective currents.
There is also a table giving the current for time 0.1 sec to 5 secs required to operate the MCB. The table is really a prompt to remind you that the current for a B type MCB is five times it's rating .
So for example if you have a shower circuit protected by a B40 MCB so long as the prospective short circuit current is more than 200 amps measured by a live neutral loop test and the PSSC calculated from the result it's job done isn't it?
Apart from ticking the box to say the voltage drop has been verified. ?
