House electrics -very strange- stumped me

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tim_ber

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Mother called me round (I tried to get her to fix it over the phone...)

Being the sort that has always managed to fix my own electrics I am ashamed to have to admit defeat.

Does anyone have an idea, before she has to call in a pro tomorrow?

Old Wylex consumer unit has MCBs retro fitted (as oppposed to wired fuses). No RCD (that is fitted to another newer consumer unit above this older one powering the new extension and shower)

One of the 32A ones is tripping. I turned all appliances off (including kettle!). MCB to ON and started turning appliances on - they all worked even kettle for a while (say 5 mins).

BUT then the strange thing, after it eventually tripped. All appliances off, MCB back on, turned microwave on (just the LED clock) and MCB tripped.

All appliances in off position again, and then MCB on with NO CURRENT DRAW that I can figure and the MCB tripped immediately. Did it 3 times in a row, blew in a second.

Swapped that MCB for the main cooker one and all was well with both MCBs in their new locations. The 'dodgy' MCB powered the cokker fine.

Now MCBs back in original locations and all is working fine again, lights, kettle all going well !!

BUT I can't leave it at that (I was always taught that if a fuse goes, find the problem, so any ideas before I leave mother and her fire extinguisher to hand?)

Would be most grateful for any insight. A friend suggested swapping the MCBs because he said they could fail (and I hadn't thought of that).

What else am I missing?
 
Perhaps the 'faulty' MCB was not making a good contact and consequently heating up.

Removing and replacing it improved the connection and now all is well once more.

Is the MCB or the springy connectors it plugs into discoloured by heat?
 
I have had the same problem once and I think it was with the same type of MCBs that replace the original wired fuses in a Wylex consumer unit.

The breaker would trip for no apparent reason and was sometimes hard to get to reset as you describe. In the end, I found no fault with the connected cuircuits like you. So I put the old wired fuse back in. It was fine like that for weeks so I bought a new MCB and all was well.

Maybe it's a common failure mechanism

Ian
 
Thanks - feeling less worried about leaving now.

The prongs of MCB looked fine; will go and check the female recepticles now.

Fire extinguisher in place, torches dotted around.

Will buy new MCB tomorrow (must buy some to replace my old wired fuses too now I think about it) Typical -at my last house I fitted a nice new consumer unit with RCD when I installed a power shower (and had the work certified), bought a new house with old consumer unit and haven't gotten round to replacing it (but don't have a power shower there).
 
More than likely an intermittent short circuit, such as a pinched wire in the back of a socket that flashes over occasionally.

If you feel competent, with the power off, remove each socket and check the wiring very carefully, looking for damaged conductors, signs of pinched or damaged wires, or even the black splat marks that will confirm you have found the fault.
 
we had the same problems about 8 years ago in a new to us house. couldn't find a fault anywhere, and neither could an electrician. its been fine since i asked the missus to point the kettle spout away from the under cupboard lights in the kitchen. at least that's my theory, but never found out for sure. sockets were all fine.
 
Right, sockets to check too.

Heating element, had a few of those go in my time - but I'm pretty sure she was using the boiler today.

Leak to earth, that'll take some time to check without proper tools - I'll think on that one - don't think there is an RCD on the Wylex consumer unit.

Thanks for help
 
Twister_Ken said:
Intermittent earth leak somewhere? I know nothing about electricks, but that's what a sparks found when we had idiosyncratic tripping on a circuit. So it might be that. OTOH...
Click to expand...

I'd expect that from an RCD-protected circuit, but I think it would have to be quite a hefty leak to trip an MCB. All "trips" are not alike.

Pete
 
Swap the plug in breaker for another from the board, of equal or lower amps 32 20 16 will be fine. Main switch off, unscrew cover, simply unplug/plug in. If the nuisance tripping still occurs, it's obviously a fault on that circuit and not a faulty breaker.

If it still trips, a fault current to earth exists in either the wires or outlets themselves, i.e some damage somewhere, or moisture in the back of a socket for example, or the same in an appliance connected to the circuit.

The wylex upgrade to the old fuse wire jobbies, were intended to stop homeowners from inserting the wrong grade of wire, obviously with dire consequences if your cable is the weakest link as opposed to the fuse. The MCB only offers over current protection (short circuit conditions) and is not ultra sensitive, investigate further as ring circuits poorly installed and tested can be dangerous.
 
Don't mean to highjack.

I've been thinking of replacing the wired fuses with mcb's, but they are more proud, so the orignal cover won't fit, unless I make a skirt/lip to extend it.

Any thoughts???
 
Agree with intermittent leak/short circuit theory. We had the same symptoms once. Turned out small trickle of water was leaking under the sink, running erratically along bottom of sink and wall, thereby sometimes running along the socket for the dishwasher plug.

Very slight short circuit. I believe the moisture present probably also boiled a bit whenever there was contact (causing tripping), causing it to evaporate a minute amount, causing the symtoms to go away for seconds, minutes or days at a time.

The socket was charred just inside the plug holes.
 
capetown said:
Don't mean to highjack.

I've been thinking of replacing the wired fuses with mcb's, but they are more proud, so the orignal cover won't fit, unless I make a skirt/lip to extend it.

Any thoughts???
Click to expand...

At £16 or thereabouts each, assuming a typical 8 way fuse box, that's £128

Waste of money, throwing good money after bad.

A completely new consumer unit with MCB's and RCD's can be bought for about £60

Granted it's not a DIY job to replace it unless you are very very competent, but depending where you are, it won't cost a great deal more to get an electrician to do a proper job and fit a new consumer unit rather than patch up the old one.
 
My thoughts are that the breaker is probably faulty.
I would change it.
If you look up the current/time graph for it to trip if it's in spec, that's probably a bang you will hear, or at least some scorching if that energy is going in a short.
I have tripped a main breaker by shorting a light circuit at the switch (wire snapped and arced) it was fairly obvious where the damage was!
I would be looking at getting a current probe and a scope to check there is no current surge when it trips, but that's because I have access to such stuff. A clamp ammeter might show a peak if there is one.
I might be tempted to bench test the breaker with a couple of kettles to prove the point?
Does this show I don't have a yacht to fiddle with right now :-( ?
If you have any doubts about your electrics, get an RCD fitted to all circuits.
 
Thanks again.

Picked up new MCBs today (2 for £15), but agree a new consumer unit and RCD is more economical (and safer)

Finding time is another matter...
 
A difficult one to troubleshoot but

I had a similar problem several years ago switching everything off and yet the problem persisted. This might seem odd but what it turned out to be was a loose screw down connection. This resulted in a high resistance joint which generated some degree of heat and consequently an imbalanced current in/out.

As a longshot, try tightening up the screw connections to any MCB's and also at the bases of any of the slow blow screw-in fuse holders to ensure good electrical contact.

Good luck, Dave

PS: If you have a main slow blow fuse also ensure that the internal fuse holding ring is tightened!
 
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Might be an idea to check out all your power sockets for correct wiring using one of the plug-ins that light up showing a fault type if any. After finding one wrongly wired using this plug-in I physically checked all the connections in every socket and their plugs for tightness and correct wiring. The reason for the check was similar, the electric shower in a park-home was overloading the MCB; eventually got a new board fitted, but the checks were easy to do and eliminated at least one dodgy socket.

ianat182
 
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