125A D Type MCB keeps tripping on a heater element circ

[Aaron the Spar]

I changed 3 x 125A fuses for 3 x 125A D type MCBs (ABB). They held for a day and then two started tripping, we didn't have any spare 125A, so we fitted 100A C type and the issue has disappeared, can anyone shed any light on this for me please?

 

[PEG]

No overload since?

 

[Pete999]

Is this a TP and N circuit and are you using 3 x SP MCBs? Does the heater bank require a N or is it a balanced load, maybe try a TP MCB

 

[James]

I would start by measuring the running current on each phase.

 

[Pete999]

Is this a TP and N circuit and are you using 3 x SP MCBs? Does the heater bank require a N or is it a balanced load, maybe try a TP MCB

Are you going to ignore post 3?

 

[pc1966]

There is no sensible reason for a 100A C-curve MCB to resist tripping that a 125A D-curve MCB would respond to, short of one or the other being faulty (125/D too sensitive, 100/C not responding as it should).

I would suspect an unfortunate timing of faults behind this. Something downstream with medium/high MCBs and a fault would also trip a MCB upstream (but probably not a fuse, or a suitably configured MCCB) so you should check for that sort of thing.

Also Pete999 has a very important point - in a 3-phase system you really don't want to lose a phase if any loads are 3-phase (motors, etc) so you ought to be looking at linked 3-pole breakers.

Yes, you had 3 separate fuses before. But they ought to have been coordinated with downstream breakers so you get selective and so very little chance of a fuse going on a minor fault.
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Another though is the 125A D-curve had a poor connection and was heating up causing it to trip prematurely, and a change fixed that. But I suspect you would have seen signs of thermal stress there (or found it hot shortly after power off when replacing).

 

[davesparks]

Why were the fuses replaced for circuit breakers in the first place?

 

[James]

I think the main point here, is not why some fuses or breakers are tripping or blowing quicker than others.

posts 2,3,4 & 5 are all trying to point you in the direction of finding the fault.

I would suggest that you stop straying from the obvious fault (fuses of different types, characteristics and ratings are all blowing)

FIND THE UNDERLYING FAULT and fix it.

 

[PEG]

Maybe it's an exam question type scenario? .....and we've been getting excited,too early?

 

[Aaron the Spar]

I would start by measuring the running current on each phase.

72A on phase 1 and 3 and 82A on phase 2, there are 21 elements on the circuit totaling 60kw. This is all correct, we are now getting excessive heat on the 35mm cables.

 

[pc1966]

72A on phase 1 and 3 and 82A on phase 2, there are 21 elements on the circuit totaling 60kw. This is all correct, we are now getting excessive heat on the 35mm cables.

Are they aluminium 35mm? I would be very surprised to see copper 35mm overheating at 82A!

Even Al would be roughly 23mm Cu equivalent and ought to cope with that sort of current.