Mccb tripped

cliffed · Apr 9, 2020

Simple question but maybe harder to explain.
Whats you take on resetting a Mccb say 100 amps 3p@ Main panel board.
You’ve been called Out to a DB TPN, that’s got no voltage to it,you look inside DB,seems ok,& by experience you can see a reason why this happened.
You find the MCCB & reset,go back to DB, test voltages,switch mcbs back on.
Everything is now ok.
Would you suggest FI & maybe monitor the situation.
i find it’s always harder to explain to the client.

pc1966 · Apr 9, 2020

So is the MCCB only providing over-current protection to the TPN board?

If it is not an RCD then either it is faulty or (more likely) something has managed to exceed the maximum current. Is there any metering to see what the board is taking per phase normally to get a feel for the operating margins?

Is there enough high surge current loads (big motors, etc) that if they simultaneously started it might trip things?

cliffed · Apr 9, 2020

It’s MCCB & yes there are motors involved, big cold store units x 2 supplied from this DB.

buzzlightyear · Apr 9, 2020

that DB needs monitoring just in case it goes again ,it the first time. ?

cliffed · Apr 9, 2020

Yea thanks,got a report going,& suggesting FI

pc1966 · Apr 9, 2020

As buzzlightyear said something to monitor.

Was anything else tripped at the time (i.e. MCB or blown fuse on something)?

Is the MCCB one with adjustable magnetic trip or a fixed type? If fixed is it B/C/D (K?) characteristic?

If normal current is below the 100A board rating (presumably that is the MCCB resting as well) it might just be bad luck on motors cutting in simultaneously, but before adjusting/replacing the MCCB characteristic you need to be sure of what the reason is.

cliffed · Apr 10, 2020

This is the type of mccb

[automerge]1586500179[/automerge]
63 amp,not 100amp.

pc1966 · Apr 10, 2020

Looks like it is adjustable 50-63A thermal trip and a fixed 500A magnetic trip, but I can't make out what the thermal trip has been set to from that photograph.

Do you know if more circuits, especially any higher current single phase ones, have been added to the end DB over the years? It might be one phase is running closer to 100A and/or the MCCB has been set to the 50A end of its range so a thermal trip is on the books. As buzzlightyear said worth monitoring e.g. putting a current clamp(s) on the phases at the end DB to see what s typically used.