Overcurrent fault tripped two MCBs

[JosephandAmber]

So customer reporting she plugged in a faulty LED light and it tripped both upstairs and downstairs lighting MCBs indicating that the circuits are connected (the board has no RCDs - yet). Before I go and do a CU change to RCBOs I need to identify the source of this fault. I'm interested to hear how others might approach this scenario in terms of the fault-finding method? Is it just a case of testing continuity between MCB 1 and the fixtures of circuit 2 until you find a leg that shows a connection is present?

 

[Mike Johnson]

After checking the appliance that was plugged in, an electrician would use their MFT and go through all the testing sequence's to identify what/where the fault was, if any.

 

[SJD]

If there is some cross-connection of circuits, likeliest place is a light with switches present both upstairs and downstairs. Typically this means the landing light (e.g. borrowed N, or accidental cross-connection at hall or landing 2-gang switch).

Switch everything on, then disconnect cables at the CU and test between L of one and N of the other.

 

[streamer]

It seems strange that plugging something in trips a lighting circuit.

 

[littlespark]

It seems strange that plugging something in trips a lighting circuit.

Well noticed.
Maybe they meant an LED bulb... or they meant up and down sockets??

I’m sure after testing, the solution will become clear.

 

[stevethesparks]

You might start off with a global insulation test, and if any problems work down to individual circuit insulation tests. You also need to ensure there are no borrowed neutrals or interconnecting circuits. As already said in #2, go through all testing sequences, loose connections may also contribute.

 

[pc1966]

It is worrying as it implies they are linked. Cross-coupled rings is one of the C2s mentioned in the BPG#4 but it takes a bit more 'bad effort' to link other types of circuit!

Approach fault-finding with logic and care, in particular you should never assume it has been done properly! So usual of IR testing but also see if the circuits have linked L or borrowed N, and be aware it might only happen for certain light switch position combinations!

 

[stevethesparks]

it takes a bit more 'bad effort' to link other types of circuit!

Nothing would surprise me anymore

 

[Mike Johnson]

The OP is going to be changing a CU to all RCBO's but does not know how to find a fault on a lighting circuit, am I the only person that finds that a bit strange, I wonder who will be issuing the notifications.

 

[telectrix]

The OP is going to be changing a CU to all RCBO's but does not know how to find a fault on a lighting circuit, am I the only person that finds that a bit strange, I wonder who will be issuing the notifications.

think it's more that OP is seeking other ideas as forearmed is forewarned. we all know the situation with landing lights 2 way switching (L from 1 circuit, N from another) but there's also diy dave'sinput with his sticky paws where they should keep away.

 

[Mike Johnson]

think it's more that OP is seeking other ideas as forearmed is forewarned. we all know the situation with landing lights 2 way switching (L from 1 circuit, N from another) but there's also diy dave'sinput with his sticky paws where they should keep away.

That's the nub of the matter I am alluding too, if the question had been asked after going to the premises and not finding the fault that would make sense, but the original question/hypothesis was "is it just testing continuity" it rang alarm bells for me.

 

[westward10]

Let's not condemn them yet until we have more info this happened yesterday and the OP never returned.

 

[telectrix]

OP must have Irish MCBs.... to be sure, to be sure.

 

[Mike Johnson]

Let's not condemn them yet until we have more info this happened yesterday and the OP never returned.

That's a very moderate response, Oh I suppose it would be. ?

 

[Lucien Nunes]

If any circuits are interconnected in a way that causes two single-pole* MCBs to trip on a single fault, the fact that the circuits are interconnected at least on the line side will be immediately obvious, because both will need to be turned off before the circuit goes off.

*Not relevant here, but I qualified this with 'single pole' because with DP MCBs, a short circuit at a point with a borrowed neutral could cause the same symptom. One breaker sees the fault current on the line and the other on the neutral and hence both trip.