MCB tripping, another thread...

[richmclaughlin]

Hello all,

There is an MCB in my fuse board that has just started tripping a couple of months after moving in. It's almost certain I'll get a pro in to fix, but I want to try and save them some time by trying no narrow down where the fault is.
The MCB is for a lighting ring that includes most upstairs lights, bathroom shaving socket, porch lights (on motion sensor) and a couple of downstairs lights.
Even if they're all off, it's still tripping out after a minute or so.
Can I at least start trying to figure out the location of the fault? My idea is to disconnect light switches and see if that helps?

Cheers!

 

[Harry Bloomfield]

The MCB is for a lighting ring that includes most upstairs lights, bathroom shaving socket, porch lights (on motion sensor) and a couple of downstairs lights.
Even if they're all off, it's still tripping out after a minute or so.

The cause could be absolutely anywhere, on that circuit. The only sensible way to find it, is using a Meggar, and isolating sections of the circuit.

 

[richmclaughlin]

from another thread:
"The cause of an MCB tripping is the current on the circuit is bigger than the MCB's value.

So an overload causes the MCB to trip.

On lighting circuits this is usually caused by a filament lamp blowing, or some other fault with a lamp or light switch. It can also be caused by too many lights being on the circuit.

More rarely, the MCB can trip due to a wiring fault."

There is heck of alot in this circuit so mayeb that's something to consider...

Harry. is it worth me disconnecting light switches to try and narrow it down?

 

[AndyPRK]

Is it an mcb or rcbo(with test button)

Is the outside light dry?

 

[Harry Bloomfield]

There is heck of alot in this circuit so mayeb that's something to consider...

Harry. is it worth me disconnecting light switches to try and narrow it down?

No - Items on a circuit, do not cause MCB's to trip, only turning them on can potentially produce an overload and cause the trip. You suggested it trips, with nothing turned on - which means a fault.

The only way to trace such a fault, is generally with the use of a Meggar, some experience of how to operate it, and splitting the circuit into sections.

You might be able to help tie it down some, by logically working out what exact circumstances cause the trip - is there really nothing switched on at the time? Does it trip when someone walks over a certain spot upstairs?

 

[ericmark]

The only way to trace such a fault, is generally with the use of a Meggar

I do not see how an insulation tester will find an overload problem?

There is an MCB in my fuse board

So not talking about a RCBO or RCD.

lighting ring

That is unusual, lighting rarely is wired as a ring, but as a radial circuit, which in your case can help, as likely one can disconnect a part of the circuit to find the fault.

A multi-meter is likely good enough to find the fault, most have an ohm range, so 6 amps at 230 volt = 38 ohms.

Although we can test insulation with 250, 500, or 1000 volt, the running amps/ohms is tested at 2 to 9 volt, so a short due to water may not show up, but just one item still live will cause an insulation tester to show zero ohms.

shaving socket

One hopes this has a built-in switch to isolate when nothing is plugged in, but an AC winding with a DC tester will show a short circuit. So putting a meter on the supply, should show if there is an overload, the meters shown do not require one to open anything, clamped around one supply lead it should show current being used, this could be the main supply lead at the consumer unit, and long as all other MCB's are turned off.

I like the clamp on meter shown, as there is no scale that can be selected which will cause a short circuit, errors can damage the meter, but not the user. So for DIY they are the preferred type.

The one on left I bought in Hong Kong when working there, internet one can pick one up for around £12, the one on right does DC as well, and goes down to 1 mA and cost me £35 you are unlikely to find one which will do DC for less, but you don't really need DC.

But you will need a tester of some sort, you may find a fault by trial and error, but unlikely.

 

[Harry Bloomfield]

I do not see how an insulation tester will find an overload problem?

Nor do I, but the OP said the MCB is tripping, when everything is off. That suggests a damaged cable to me, rather than an overload - don't you think, Eric?..

 

[Harry Bloomfield]

That is unusual, lighting rarely is wired as a ring, but as a radial circuit, which in your case can help, as likely one can disconnect a part of the circuit to find the fault.

Careful Eric, the OP may be using completely the wrong terms!

 

[Stuckinarut]

Have you hung anything up on the walls recently? Any issues with vermin?

 

[ericmark]

Point taken, maybe they can confirm it is a MCB tripping.

 

[Murdochcat]

If the mcb trips after a minute or 2 after switching it back on , without any lights on, a competent spark with experience and proper test kit should be able to narrow down the fault .

Do you have rodents?
Do you have any water leaks?
Have ANY changes been made to the wiring OR the fittings?
How long has this been going on?
How are you coping without lights ?

 

[JohnD]

You mention porch lights

Outdoor wiring and fittings are very prone to rainwater entry.

If you are able to isolate the outdoor circuits, preferably by adding a DP isolator (not a light switch) indoors, close to the point the cable goes through the wall, it will be informative.

 

[ericmark]

Even if they're all off, it's still tripping out after a minute or so.

Does not point to a short circuit, but an overload. If it takes a minute or so, then not a crushed cable, but could be a faulty bulb, transformer, PIR, however since all switched off, not them either, so it would seem.
1) Something not switched off.
2) The installation was faulty from the day it was installed.
I will expand, we should ensure that a MCB can trip on its magnetic part which with a type B is 3 to 5 times higher than the thermal part, so for a 6 amp (B6) MCB over 30 amp (6 x 5) and it will trip in a fraction of a second, but below 30 amp but above 6 amp, it will take some time for the MCB thermal part to get warm enough to trip. We should use a loop impedance tester and 230/30=7.66 Ω so we should not pass the installation if we get a reading over 7.66 Ω. But we are assuming, installed by an electrician who knows the rules.

If, however, something has been installed using very long or undersized cable, then there could be a short circuit which does not trip the magnetic part, 30 meters of 1 mm² cable is the maximum we can use to comply with volt drop rules, but around 180 meters before the loop impedance will be high enough not to trip the magnetic part of the trip.

So in real terms looking for something which has not been switched off, so I would start by removing all bulbs, in case one you think is switched off, is not switched off. Again, in theory every bulb should have a built-in fuse, as a BA22d bulb holder is only rated at 2 amp, but in practice many bulbs don't have the built-in fuse, specially compact fluorescent lamps.

Outside lights should be supplied from a double pole switch inside the house, but I know most of mine are not, so any PIR is possible the cause. Personally, I would disconnect outside lights just in case, as they are likely the cause.

 

[Harry Bloomfield]

Does not point to a short circuit, but an overload. If it takes a minute or so, then not a crushed cable, but could be a faulty bulb, transformer, PIR, however since all switched off, not them either, so it would seem.

No Eric, and you have now twice suggested this.... An MCB tripping (if indeed it is an MCB, rather than RCD), with everything turned off, suggests a short somewhere, possibly an intermittent one, not an overload.

 

[richmclaughlin]

Thanks for your help all, I'll have a proper read through later. No changes have been made and I've not hung anything up. Rodents could well be an issue as could weathering. I'm pretty sure it's an MCB as it's almost identical to the MCB in the image above.
The issue has been happening the last 3 weeks, and i moved in mid september.
I think I'll start with disconnecting the outside lights in this case. Most of the bulbs need swapping anyway as many of them are old high wattage.
It's not a major issue as the upstairs sockets are fine so I can just use lamps for now (although a couple of the upstairs lights do still work).
I'm having plenty of electrical work done next year so I can bundle this in.