strange fault tripping out garage

[ianphilip007]

hi everyone

went to a call today reguarding a tripped rcd and loss of power to garage. When i arrived the the rcd had been reset and all mcb's were on and all circuits working normally. one 16amp Mcb was off and no power to garage. She said her son had reset rcd but thought the problem was with the 16amp Mcb. Nothing strange there.

when entering the garage i found the wiring to be in a terrible state with sockets and lights all on the same circuit some lights hard wired thru junction boxes and some plugged in to sockets by flex. All in poor condition. there was also an old freezer, poor condition that i initially thought might have a leakage problem that was causing the rcd to trip. But that was now beeing run via an exstension lead to the kitchen ring main and had caused no tripping.

i unplugged the freezer and as many of the lights i could find from the circuit. i expected the 16 A mcb to not reset as i expected the cause of the trip may be a E to L fault but to my surprise it did reset and the rcd held. i then plugged in the freezer and other lights and it continued to hold. then after about 6 mins the rcd and mcb tripped. i then carried out a basic IR test on the garage circuit and found L to E 0.00 ohms. E to N 0.00 and L to N 0.00 basically a short between all cables. Now the N to L reading could probably of been a connected load just strange that the rcd wouild reset at all let alone hold with the readings obtained. any thoughts as to why the rcd reset ?

 

[ianphilip007]

as it is customer did not want the exspense of what may have turned out to be an exstensive fault find so circuit has now been dissconected and made safe. But just surprised that with what appeared to be a LE and LN fault the rcd held at all.

 

[Octopus]

Was it/had it been raining?

 

[ianphilip007]

not that i know of are you thinking water ingress which may have caused a short between NE and LE causing the rcd to trip. could be possible but doesnt explain why the rcd would reset and hold when the IR test readings are indicating 0 ohms between LE and NE

 

[ianphilip007]

without further investigation its hard to tell but the ir readings indicate a direct short so why the mcb and rcd reset at all is very strange.

 

[Rockingit]

You need to consider how an RCD works - it measures the imbalance between phases, so what goes out must equal what comes back in.

Now, consider the resolution on your IR meter - there will have been SOME resistance, just not enough to register on a MOhm scale.

So, do some maths and assume that there might have been say 10 ohms resistance equally between all three cables.... what do you get?

 

[Octopus]

I would agree that it sounds odd but as you quite rightly say further investigation is the way forward. Were there any neon indicators on the circuit?

 

[ianphilip007]

yes your right rocknight the actual resistance readings would not be 0.00 ohms due to the sensitivity of the meter. are you saying that the rcd reset due to their being no actual imbalance between live and neutral and no direct short to earth due to the actual resistance of 10ohms between LNE as you suggested.

 

[ianphilip007]

there may have been murdoch so the LN result could be a neon or undiscovered load. But the LE and LN were also virtually 0 ohms

 

[Happyneiller]

Did you test with the rcd disconnected

 

[ianphilip007]

yes happy circuit was disconnected from cu LNE

 

[ianphilip007]

had another think and may have a possible explanation for why the rcd was resetting and tripping again just after a few mins.

30ma RCD 230V supply. R= V/I 230/30ma = 7666.6 Ohms

so if the IR readings were very low but not actually indicating a dead short then the RCD would reset.

if the IR of the circuit is close to the 7666.6 ohms then it is possible that the rcd will trip due to leakage in excess of 30ma.

might be totally wrong but its a theory.

 

[ianphilip007]

but as has been said as customer didnt want further investigation i guess ill never know lol

 

[Rockingit]

Ian, go back to my earlier post: 230 / 10 = 23A IN balance. So there could, with my hypothetical figures, be a constant load of 23A on that cable regardless of anything else. BUT, because it is being caused by the breakdown of insulation it is therefore not a very stable resistance, so will change and fluctuate. If the leakage L-E suddenly became 11 ohms, then the result as far as an RCD is concerned is massive.

Kind of, I've simplified for the sake of us still having the time to drink beer!

 

[ianphilip007]

thanks rocknight intresting point. so you are saying that there may be a constant load of 23 amps that is balanced due to the low resistance between L,N,E, but as you say due to the breakdown of the insulation on the circuit it would only take a small change in resistance between LE to trip the rcd. Do you think this is why the rcd holds for a few minuates but when the circuit is energized the resistance between LE changes after a few minutes therefore tripping rcd? hope thats what you mean

 

[Rockingit]

yes, that's basically it. You have a 'high load' situation rather than a direct short, so an MCB will warm up slowly and trip after a while (time/current characteristics), and as the whole thing gets warmer the insulation values and therefore the resistance values change too.