ARW wrote:

On 06/03/2018 20:49, Roger Hayter wrote:
ARW wrote:

On 05/03/2018 20:19, Jim K wrote:
ARW Wrote in message:
Take a 3 phase incoming supply to a main distribution DB with a C100A
rated main switch.

This DB has a C63A 3PH MCB to power DB1 that is about 15m away with some
16mm SWA.

You then use different phases with B20 MCBs at DB1, feed them through
13A SFCUs with 2.5mmT&E and join them at the load side of the SFCUs
about 30m away with some more 2.5mm T&E.

Then you turn on the 20A MCBs?

So which goes first?

One of the 13A BS1362 fuses, both of the 13A BS1362 fuses, a B20A MCB
etc............?

Educated guesses welcome.

--
Adam


The 20A MCB?
(uneducated in electricals)




I did not know the answer to this one myself until someone actually
tried it:-)

And the answer is "that every fuse and MCB tripped/blew including the
C100A main switch".

Only the main incoming fuses did not blow and I have no idea what rating
they are and I have no way of finding out even if I wanted to as someone
has installed a steel pillar in front of the incomer.

Is that unsurprising? There may be a fault current high enough for the
breakers to be disengaged before the springs have time to accelerate the
contacts apart?


I would not have expected the C100A main switch to have blown due to the
resistance of the 2.5.

Quick back of envelope (but calculator aided) calculation gives total
resistance between phases 540mOhm, current 740A, divided by 63 equals
11.8 so it is within spec for the C63 to trip as fast as it likes (no
min time). So even if it's surprising it is within specs. BICBW

--

Roger Hayter