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PostPosted: Mon Jul 16, 2018 12:00 pm 
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Hopefully you can assist.
Never really got to grips with these, all that sticks in my head is .4 or 5 seconds, if anything greater, then you are likely to get a nasty shock.
Many homes these days have MCBs, can I assume that a MCB is a BS EN 60898?

Can I assume (in most circumstances), if a home has the above, then page 58 of the regs (Table 41.3) is the table to use to find the ZS value ensuring the above trips within time / or less of course.

What is the difference a distribution and a final circuit?

Gong back to my first note, a MCB tripping within .4 sec (I get this, as extremely quick) However, 5 seconds seems a long time to wait!

On the noted page 58 / Table 41.3, it highlights .4secs and 5 seconds - why?

Keen to read all your return Reponses

Apologies to be wet around the ears!

Your answers will certainly help.


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 Post subject: Re: dISCONNECTION TIMES
PostPosted: Mon Jul 16, 2018 6:32 pm 
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A distribution circuit is one that powers another CU or a distribution board.
A final circuit powers lights, sockets, cookers etc.

The maximum earth loop to trip a MCB (ie a device to BS 60898 for modern MCBs, older models had different BS numbers and different maximum earth loops) is actually the same value for 0.4 and 5 seconds disconnection times. So as long as the maximum earth loop given in table 41.3 is met then the MCB will always trip in 0.4 seconds if there is a fault.

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 Post subject: Re: dISCONNECTION TIMES
PostPosted: Tue Jul 17, 2018 5:36 am 
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MCB's come as B, C, and D, it was 5x, 10x and 20x so a 5A B type MCB will need 5 x 5 = 25 amp to trip in 0.1 seconds, now we add 5% so 230/25 = 9.2 ohms plus 5% = 8.74 ohms earth loop impedance. It seems the 5% was added because of volt drop, a B type magnetic part should trip between 3 and 5 times rating. The thermal part is far slower there are graphs.

Today the RCD means the ELI is in real terms less important, however since often the volt drop is exceeded before the tripping times are exceeded it is only to comply with regs we measure ELI I seem to remember to be within volt drop the loop impedance line - neutral is around 0.95 ohms for a ring final but ELI is 1.366 so it should never be limiting factor with a TN-C-S supply.

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