How to track down tripping circuit?

Hi,
we hold the key for our neighbour in case the alarm goes off etc.
Went off again this morning because the power had failed - main trip had gone.
I thing the wiring is fairly old, although there is a new(ish) CU with RCD.
[I reckon he would be better with a split CU with the lighting and alarm not on the RCD, to avoid this kind of problem, however he is romantically involved with his money....]
Problem: Something trips out the RCD and all the power goes off. If I try to reset the trip, it pops again. If I knock off all the individual trips, the main RCD resets OK and if I then bring the individual trips back up one at a time everything comes up O.K. I presume that there is a marginal circuit or device somewhere which manages to trip the RCD when the system is loaded up in a particular way e.g. when starting all the devices at once by re-powering up the whole CU.
I guess you could diagnose by turning off the trip then powering up all but one circuit, but I am not sure how accurate this would be, and there are a lot of combinations.
So is there any easy way to test individual circuits?
I assume whoever fitted the new CU should have checked each circuit before reconnecting, but then again price was probably the main reason for selecting the contractor :-)
Cheers
Dave R
--




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The first job is to tell the RCD with an RCD tester. This will indicate if the device is oversensitive.
Then, everything can be unplugged (including any RCDs) and a insulation resistance check performed. This will indicate if there is a wiring fault. The initial test is on the entire installation. If a fault is found, the circuits can be tested individually.
If not, then it is probably an appliance. You can then go round PAT testing everything. Obviously, it is blows quickly and readily, it is easy to find the culprit without one. This doesn't sound like the case, though.
Unfortunately, all these testers are horrendously expensive.
Christian.
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Would there be any merit in going round the house and disconnecting appliances that ain't absolutely necessary, just in case it is one of them that is causing the fault? Not the most useful of suggestions methinks, but if it helps isolate the fault...
Mungo
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Mungo Henning wrote:

Thats exactly the method suggested in the user instructions with my new consumer unit.
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David W.E. Roberts wrote:

Dave, my whole house behaved lkike this, and in teh end it wasn't one thing, but a combination of lots of little things, so I put the whole house on 100mA RCD and its been perfect ever since.
This may or may not be the case, but an earth nuutral short is a possibility that would behave as described. The eassiest way is to knowck the whole house off, and unplug all the soctes so thee are no bits of equipment, and then put a resistance meter between earth and neutral busbars. It should read VERY high ersitsance. If it doesn't, disconnect either neutral or earth tails one by one and test them.
If they all come out OK, then it could be simply too much leakage from perfectly legal bits of kit - like electronic stuff with capacitance between earth and nuetral and earth and live. Enough cable in the house will almost do this of its own accord anyway.
The fact that you can switch stuff on by itself and it holds, but all on together trips it suggests its capacitatve anyway - the surge gets you. Mine was like this - switch on anything - a light, a thermostat - would trip the bloody thing with all the kit plugged in.
Only solution here is to go to 100MA RCD, and split load the box where you feel the extra protection of 30mA is needed..
Try unplugging all TV's comouters etc etc for a temporary bodge.
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Disconnect all plugs and lightbulbs etc, and then put them back on one by one.
Some high initial drain appliances - fridge motor, heater etc, may trip over sensitive MCBs but the appliance can be on [standby] for a while before it actually draws the load. So you need to monitor these as the circuit may not trip straight away.
dg

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