Strange MCB trip

[Steve T]

Hi all,
Had my downstairs ring final circuit trip the B32 MCB today. The missus was in the kitchen cooking and had apparently put the kettle on and as it was close to boiling the MCB tripped.
I therefore assumed it would be due to the kettle so I checked the 13A fuse in the kettle plug but it was fine. So with the kettle unplugged I reset the MCB no problem. Tried boiling the kettle a few more times and did not trip anything.

So I'm now trying to figure out what caused the MCB to trip. It seems unlikely it was the kettle as the fuse in the plug did not blow. At the time the only things that were actively on were the kettle, an undercounter fridge, a fridge freezer, a PC and screen, and a few DC power supplies for the WiFi, phone chargers etc. So it really couldn't be an overload. Neither fridge had just turned on or just turned off (can tell by the amount of frost on the back wall) and no timers or other devices had just come on or off. Maybe worth noting that there is also RCD protection, which did not trip.

I have also checked the particular socket the kettle was plugged into, wiring behind was all fine but one of the switches is a bit loose compared to the other suggesting bad contact, so I will replace the socket anyway, but it seems unlikely that this would trip the MCB.

I can't seem to recreate any sort of fault with the kettle or the socket so I'm struggling to see why the MCB would have tripped randomly.
Hope I have given all relevant info but please ask if there's anything I missed. Any ideas would be greatly appreciated!
Thanks

 

[buzzlightyear]

tea Ernie faulty element. water or steam build up around the socket.

 

[hasel5]

Think you need a rewire mate ???
Just keep a eye on it could of be water in the base from a overboil or anything tbh

 

[Steve T]

tea Ernie faulty element. water or steam build up around the socket.

Think you need a rewire mate ???
Just keep a eye on it could of be water in the base from a overboil or anything tbh

Was certainly my first though but in the case of a faulty element that should blow the fuse shouldn't it? And for any water build up I would have expected the RCD to trip way before an MCB?

Actually are elements known to go faulty in a straight short circuit? That could explain why the MCB tripped first but does not explain why I can't recreate it.

 

[Dartlec]

Hi all,
Had my downstairs ring final circuit trip the B32 MCB today. The missus was in the kitchen cooking and had apparently put the kettle on and as it was close to boiling the MCB tripped.
I therefore assumed it would be due to the kettle so I checked the 13A fuse in the kettle plug but it was fine. So with the kettle unplugged I reset the MCB no problem. Tried boiling the kettle a few more times and did not trip anything.

So I'm now trying to figure out what caused the MCB to trip. It seems unlikely it was the kettle as the fuse in the plug did not blow. At the time the only things that were actively on were the kettle, an undercounter fridge, a fridge freezer, a PC and screen, and a few DC power supplies for the WiFi, phone chargers etc. So it really couldn't be an overload. Neither fridge had just turned on or just turned off (can tell by the amount of frost on the back wall) and no timers or other devices had just come on or off. Maybe worth noting that there is also RCD protection, which did not trip.

I have also checked the particular socket the kettle was plugged into, wiring behind was all fine but one of the switches is a bit loose compared to the other suggesting bad contact, so I will replace the socket anyway, but it seems unlikely that this would trip the MCB.

I can't seem to recreate any sort of fault with the kettle or the socket so I'm struggling to see why the MCB would have tripped randomly.
Hope I have given all relevant info but please ask if there's anything I missed. Any ideas would be greatly appreciated!
Thanks

Takes quite a bit to trip a B32 so as you say I doubt it was an actual overload - it would suggest a direct short somewhere - possibly related to steam from cooking or the kettle getting into a socket or junction box on that circuit..

It could also be the kettle element on its way out and failing at a certain temperature intermittently...

Check that the MCB does still isolate the circuit as its possible the MCB could be faulty (though unlikely)

And also if you haven't recently check the RCD trip button to ensure that it is still functioning, since that would normally go first with anything other than a direct Live-Neutral short.

 

[Steve T]

Takes quite a bit to trip a B32 so as you say I doubt it was an actual overload - it would suggest a direct short somewhere - possibly related to steam from cooking or the kettle getting into a socket or junction box on that circuit..

It could also be the kettle element on its way out and failing at a certain temperature intermittently...

Check that the MCB does still isolate the circuit as its possible the MCB could be faulty (though unlikely)

And also if you haven't recently check the RCD trip button to ensure that it is still functioning, since that would normally go first with anything other than a direct Live-Neutral short.

MCB definitely appears to be working correctly as does the RCD, I make a habit of testing it when the clocks change but have also just tested it now. Although of course the test button doesn't actually confirm it is working correctly. I'll try boiling it again a few times.

 

[Strima]

Was certainly my first though but in the case of a faulty element that should blow the fuse shouldn't it?

All depends on the size of the fault current, those fuses tend to hang on pretty well.

 

[pc1966]

What @Strima said.

The "instant" magnetic trip for a 32A B-curve MCB is between 96-160A and is usually < 0.01s
At 96A a 13A fuse takes between 0.01-0.4 seconds, so most of the time the fuse will survive.
At 160A a 13A fuse is between 0.004-0.07 seconds, so half the time the fuse will survive.

I had my fridge's defrost heater go "bang" and trip the MCB (admittedly 20A B-curve) but the fuse survived.

 

[Steve T]

What @Strima said.

The "instant" magnetic trip for a 32A B-curve MCB is between 96-160A and is usually < 0.01s
At 96A a 13A fuse takes between 0.01-0.4 seconds, so most of the time the fuse will survive.
At 160A a 13A fuse is between 0.004-0.07 seconds, so half the time the fuse will survive.

I had my fridge's defrost heater go "bang" and trip the MCB (admittedly 20A B-curve) but the fuse survived.

Thanks, makes sense, must be an intermittent fault as it hasn't happened again yet. I'll buy a new kettle?

 

[LastManOnline]

Thanks, makes sense, must be an intermittent fault as it hasn't happened again yet. I'll buy a new kettle?

By all means do that but personally I would wait. Its an intermittent fault after it occurs more than once. You are likely correct that the kettle is most likely at fault. If water has worked its way in somewhere it is now definitely no longer there as the short circuit will have vaporised it. However if it is definitively the kettle than you won't have long to wait until the problem occurs again. Normally speaking, once an element starts to deteriorate the issue the problem rapidly escalates. You are then 100% sure where the problem lay.