Which fuse or MCB trips first?

Question

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.

spuorgelgoog · Accepted Answer

Join what to what? Phase to phase? I'll guess the 3PH MCB as it has overcurrent on two phases. Owain

Roger Hayter · Answer

Purely from domestic experience I reckon one or both B20 MCBs while the

13A fuses are in a brief molten/vaporised state. But also no expert. It may be possible to calculate whether the C63A should trip before the others have time to open.

John Rumm · Answer

What was the PSSC at the supply end? (although I am guessing since you asked the question probably the 63A  3ph MCB ;-)

Robin · Answer

The apprentice who joined them?

ARW · Answer

Greater than 12KA according to my test meter. The same as my house supply! Not saying ATM :-)

dennis · Answer

Only if he is connected to the company fuses.

harry · Answer

What you are talking about is called "discrimination". Google electrical discrimination Eg  A whole sub-subject of electrical distribution

Brian Gaff · Answer

You trying to blow yourself up or what?  Brian

John Rumm · Answer

That would be a woosh then from harry ;-)

philipuk · Answer

Whichever will be the biggest pain in the arse when it blows/trips. Philip

ARW · Answer

Probably got his phases mixed up:-)

Which fuse or MCB trips first?

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