Blown Sockets - Diagnostic Assistance Needed Pleeeeease?

[Bob Burrell]

Can anyone apply some logic to what's happened?
Background: We have two plug socket MCBs.
Summary: Two separate kitchen appliances seem to be causing an MCB to blow (DIshwasher and Freezer)
Detail:
1. A socket MCB blew.
2. With the dishwasher off, I was able to reset the MCB.
- changed fuse in the dishwasher plug. Still blew MCB
- to confirm, I used an extension lead to connect the dishwasher to a socket on 2nd MCB. The 2nd MCB blew
- "belt and braces", I also checked using a second extension lead
3. With dishwasher disconnected, everything working again
But Alas....a few hours later...
4. Turned the freezer on...runs for about 10 seconds, gets up to speed...and low and behold: MCB blows!
Turn it off, Reset MCB , On....and blows again! WTF???
5. Same process - check via extension lead, now it appears I have a household appliance pandemic!!

Current situation: Running the house without the Freezer until I can understand the problem.

(P.S. My 11 year old Dishwasher is also now in the garage, assumed dead, but maybe I was too quick off the mark)

Has anyone got a vaccine (or good idea what the problem is) ?
Thank you in advance, Bob

 

[CBW]

I would say you may have already partially diagnosed the fault(s) with 2 appliances, especially if running them on different circuits causes the mcb’s to trip. It’s probably just sheer bad luck/coincidence that they have failed at the same time.

 

[ericmark]

My first thoughts are is it a MCB? I would have expected a fuse to rupture before a B32 MCB so wonder if it is a RCBO or other type of RCD?

they look very similar and a RCBO would make more sense, note the test button.

 

[ericmark]

My guess is a RCBO and a neutral to earth fault. This would explain why it seems multiple items fail together.

 

[Bob Burrell]

Thanks guys. They are MCBs. Note that both the dishwasher and Freezer were tested against two separate MCBs, and in turn tripped both. So unlikely to be an MCB fault.

Chris (CBW) the co-incidence thing is noted, thank you. I can very definitely see your point, but I have to assume they're not both failing....for the moment )

Eric is there a simple way (for a relative novice like me) to check for a neutral to earth fault, or will I need to call someone out?

 

[JohnD]

1. A socket MCB blew.

Please show us a photo of the things that tripped, close enough to read the letters and numbers on them.

 

[Bob Burrell]

Picture

 

[JohnD]

Well that's interesting

I did once have something similar after a burst pipe got several appliances, sockets and plugs wet.

 

[Bob Burrell]

I understand. In this case, there was no leak (that I know about!) and the appliances are in different rooms.

 

[Bob Burrell]

I just tested it again. The Freezer is now blowing the RCD, NOT the MCB. Does that change things?

 

[securespark]

Does the freezer trip the other RCD if plugged into the other circuit?

 

[Bob Burrell]

Yes. After testing on the 2nd circuit, it blew the RCD but not the MCB too [ my apologies - it appears I may have fed you duff information earlier ]

 

[Bob Burrell]

So both circuits just blow the RCD not MCB [ apologies, I gave duff information earlier ]

 

[securespark]

And what about the dishwasher?

 

[ericmark]

OK RCD makes more sense, so the main problem is neutrals are rarely switched, and with no load neutral and earth are the same voltage, so ohms law 0/ohms = 0 amps, so even if neutral connected to earth the RCD will not trip.

However once we put a load on the circuit the neutral voltage raises above the earth voltage, so just 2/67 ohms = 30 mA, what I am saying a fault with a toaster can cause the RCD to open when the kettle is used.

So step one is unplug all you can, not just switch off, does it still open the RCD with high load items?

There are some items which are well known for problems, in the main they use mineral insulated heating elements, and the mineral attracts water, so cooker, dish washer, kettle, washing machine, and frost free freezers, the latter has a defrost element and it is common for it not to have the neutral switched.

We use some thing like this the one shown costs £32 from Amazon but google VC60B and likely cheaper versions.

The main thing is they use 500 volt to test, the 3 - 9 volt a multi-meter uses in not normally good enough, and clearly you test neutral to earth so even if controlled by a relay likely the neutral still connected.

As to if worth buying one, not sure, don't want kids playing with it with 250 - 1000 volt, so may be better to get some one in, a standard PAT tester has one built in, and often the fault is picked up when PAT testing.