One ring main trips, the other doesn't...

[techbiscuit]

Hi all,

Here's a little puzzle. Two domestic ring mains in a house, nothing special, MK consumer unit (early 1990s 32A MCBs on each ring). No load on either ring, everything turned off and unplugged.

Connect large drill to ground floor ring main. Fire up drill, everything is fine.

Connect same drill to upstairs ring main. Fire up drill, power trips out almost instantly. Not on the MCB for that ring main, but the main breaker for the whole house.

Exactly the same thing occurs with an electric planer, but connecting a 2.8kW kettle to either one leads to no problems at all. There's been no work done to either ring main since we moved in, and other than a couple of upstairs sockets that have obviously been replaced (different front panels) it all seems original from when the place was built.

Does anyone have any clues as to likely culprits?

Cheers!

 

[LoveRocket]

By 'main breaker', I assume you mean that there's an RCD incomer. (Does it have a test button on it, yes?) What is the rating of the RCD, is it 30mA or 100mA?

There will be a N-E fault on the circuit (not main) affected. The tools you mention have electric motors in them, these draw quite a large current on startup (a kettle doesn't), and this current spike is enough to cause the RCD to trip due to the N-E fault.

You need to do an Insulation Resistance test on the circuit.

 

[techbiscuit]

Thanks for the info; yes, there's an RCD on the main incomer, although I can't remember the rating. Not there at the moment, but will post back when I've checked.

I'll also check the replaced sockets for obvious neutral-earth problems first and hopefully I'll bump into something.

 

[LoveRocket]

Good luck marra.

Even a cheapo basic multimeter will show you if there's a dead-short across N-E. If you're confident to do so - with the main switch off, remove the N & E wires from both legs of the ring at the consumer unit and check the resistance between them. They should be open circuit.

 

[Taylortwocities]

If there was a hard fault between earth and neutral then the RCD would trip even with no load connected.

You'll need to look somewhere else for the fault.
Are the planer and drill 230v or 110v (supplied by a transformer?) if so the tranny may be the problem.

As it is the main incomer RCD that is tripping then it will not be something like a neutral on the wrong busbar, or a borrowed neutral.

When did this start happening?

 

[techbiscuit]

The planer and drill were standard 230V - no transformer in between.

This is the first time the problem has been experienced, perhaps because it's the first time any work has used heavyweight tools on the upstairs ring. Impossible that the drill could have interfered with any electrics, as it's not had chance to make a hole yet!

 

[LoveRocket]

If there was a hard fault between earth and neutral then the RCD would trip even with no load connected.

Not on TNC-S it wouldn't.

Although I agree that in this case there couldn't be a hard fault as the kettle would trip it - so my advice about checking with a multimeter was a bit pointless.

TB - Are you sure there's nothing else connected to the upstairs ring? A boiler or extractor fan maybe? Completely disconnect these and try again. Although it looks like you are going to need someone with an IR tester buddy!

 

[techbiscuit]

What is the rating of the RCD, is it 30mA or 100mA?

Checked RCD trip rating yesterday - it's 30mA. Not had chance to move anywhere else on this yet, but will scour around and see if there's anything I've not taken into account that might be connected. Boiler is on downstairs ring, so it's not that...

 

[ColJack]

N - E swapped on the 2 replaced plugs..

check them..

 

[LoveRocket]

N - E swapped on the 2 replaced plugs..

check them..

It only does it on the upstairs RFC, Col.

 

[ColJack]

and the upstairs RFC is the one with the recently changed sockets on it, hence my suggestion..

the drill and planer are both DI most likely, whereas the kettle isn't usually