RCD tripping. Give me a clue!

[Piratepete]

Hi Guys
A Happy 2014 to you all.
I've recently had two calls to trace the cause of intermittent tripping in domestic installations, and failed miserably!

The latest - 4 way box feeding garage, utility room, garden shed. Customer reckons it sometimes trips when they switch the light on, but can trip without any human intervention.
RCD is fine. Also did a ramp test - 22mA Ok.
IR - 1.47 Meg for L+N of all circuits to Earth with everything plugged in. So it's a bit low but not anywhere near low enough to trip the RCD.
Two fridge freezers - I've been told that these are prone to high earth currents - measured under 1 mA.
Washing machine and tumble drier are usually turned off at socket. Doesn't trip while they're running.

Any bright ideas to track this fault, please?

Cheers!
Pete

 

[telectrix]

once you have eliminated the impossible, whatever remains, however improbable, must be the truth.

 

[Marvo]

1.4 Megs is a bit on the lowside and might not be a stable reading. It would probably at least warrant localising then make a decision if it's worth remedial work.

If you can't see a sufficiently low IR with a Mega tester then use an earth leakage clamp meter around the L&N simultaneously and monitor over time and under different load profiles.

 

[guest119]

22ma?
Did you ramp it with all outgoing disconnected to get the ramp reading for the RCD alone?

Boydy

 

[Octopus]

To properly investigate low IR you need to disconnect EVERTHING and IR all 3 cores.

 

[MDJ]

Could be a collective earth fault. Add up all the leakage from the appliances with a clamp meter and see just how many milliamps are leaking. You may have to split the circuits over different RCBOs/ RCDs

 

[Piratepete]

22ma?
Did you ramp it with all outgoing disconnected to get the ramp reading for the RCD alone?

Boydy

I don't know what you're getting at here. How will disconnecting everything affect the ramp test? The only thing switched on was a light.

 

[Piratepete]

To properly investigate low IR you need to disconnect EVERTHING and IR all 3 cores.

I would've thought that a general IR was adequate. Seperating the circuits will only result in increased IR readings - worst case will be one circuit at 1.47Mohm -This is not low enough to trip an RCD.

 

[Piratepete]

Could be a collective earth fault. Add up all the leakage from the appliances with a clamp meter and see just how many milliamps are leaking. You may have to split the circuits over different RCBOs/ RCDs

Good idea along with Marvo's suggestion,above. I don't currently possess a clampmeter that's sesitive enough. Still saving up for that torque driver that I need to do up CU screws correctly. Could use my inline meter to check overall earth leakage. All depends whether my customer wants to spend the money!

 

[Deleted member 72666]

Why do an IR with everything plugged in?

 

[guest119]

Why do an IR with everything plugged in?

Ditto ramp test!

Boydy

 

[Silly Sausage]

I don't know what you're getting at here. How will disconnecting everything affect the ramp test? The only thing switched on was a light.

You are testing the RCD only.
Testing with circuits connected could result in false readings due to leakage on the circuits.
At the very least, turn all the MCBs fed by the RCD off.

It's GN3 somewhere...I think!

 

[Piratepete]

Why do an IR with everything plugged in?

Because that is the situation when the RCD trips, which could be caused by an appliance. If I'd found an exceptionally low IR, I'd then unplug everything and retest, to see if an appliance was at fault. The fallacy with this is that some faults don't show up unless the appliance is actually running - washing m/c pump motors comes to mind.

 

[Marvo]

Good idea along with Marvo's suggestion,above. I don't currently possess a clampmeter that's sesitive enough. Still saving up for that torque driver that I need to do up CU screws correctly. Could use my inline meter to check overall earth leakage. All depends whether my customer wants to spend the money!

Forget the torque driver, an earth leakage clamp meter would be more important.

You can't use an inline ammeter to check leakage current because it will only tell you what's going down the CPC, not what's being lost down parallel paths. The RCD doesn't monitor the CPC current so the test would be meaningless.

 

[Piratepete]

You are testing the RCD only.
Testing with circuits connected could result in false readings due to leakage on the circuits.
At the very least, turn all the MCBs fed by the RCD off.

It's GN3 somewhere...I think!

Ok. Point taken, but if there is existing leakage, say 5 mA, and the ramp test shows 22 mA, then the tripping current for the RCD is 27 mA, I think.