MCB tripping but nothing on ...

[Iggle Piggle]

hey, any electricians on here?
I have a half Amp reading on a lighting circuit but no lights switched on.
In this circuit I have a couple of conventional lights, one 12v transformer system and 2 switch-mode power supply 12v lights plus one outdoor light (which is dry before you ask).
Am I looking at faulty insulation?
Help !

[buildersmate]

Do you mean MCB?
MCBs trip on overcurrent, and an MCB controlling a lighting circuit is normally 6A.
RCDs trip on earth leakage - the differential current between that on the Live conductor and that on the neutral. Sorry if I'm teaching Grandmothers' etc.

Where are you measuring 0.5A at?

[Iggle Piggle]

Hi BM, I am measuring 0.6A in line between the MCB and the circuit.
It is a 'Hager' 6A MCB.
The wiring is original with the house, built 1991.
I think the answer is to pull all the light fittings and check again. Not an easy task with kids running loose ! <gulp>

[shooty]

do you have a bathroom fan on the same circuit ?

[Iggle Piggle]

hmmm. just what I was thinking, there is a timed fan in this circuit. It works fine but...

[shooty]

what is the issue ?
are you cost cutting or do you have a fault ?

[Iggle Piggle]

well there's got to be a fault if there's a half amp being used by a circuit with nothing switched on...
140W of power going somewhere other than a light bulb makes me think of smouldering in a wall, damp cabling or something else unwanted !

[shooty]

0.6 amps would make the circuit trip is it was a fault. think you may find that the pcb's in auto detect light outside and humidistat in fan etc will ammount to some current draw

[shooty]

what made you inline test for current draw if you dont mind me asking.
also are you using a clamp meter or inline probes. as a clamp meter does have a great deal of tollerence due to the fact it only measures the magnetic feild and not the true current flow

[Iggle Piggle]

errrm, yeah, that sort of PIR sensor or humidistat might pull a tenth of the current I am measuring or so, but then I don't have any on the circuit...

[johnk]

If it's a soft short not to earth it won't trip the RCB. You'll have to isolate/short out 1 item at a time and try your test.

[shooty]

Are your lights switched after rather than before the transformers ?
Do you a have light in the attic that is switched from your landing and has been accidently turned on. ?
do you have a shed or garage that has a light on?
do you have a pond pump that has been tapped off the lighting circuit. ?
(and yes, i have seen this been done before)

the only way to find out is to bell out with a meter until you find the last item on the radial circuit and insulation test all the way through for piece of mind.

[Iggle Piggle]

The MCB had tripped in the night with nothing on, I thought the MCB was possibly old and faulty so went to pull it out for testing, when I disconnected the wire it gave a good spark, so I took a current reading. It's a regular Fluke DVM I'm using.
If I go quiet for a while I'm taking all the light fittings down ... !

[BRIGHT SPARK]

When was the last condition report carried out Iggy

[Iggle Piggle]

Thanks for all the help, it turned out to be an ancient outside light I thought was disconnected, the connection box was full of rainwater...