RCD nuisance trips

[HandyPete]

I was working in the garage at a friends house and plugged in my Bosch Cordless drill charger. The main circuit breaker in the house fuseboard tripped!?! Charger must have a problem. Right. So I ran out my extension lead and plugged in my Bosch hammer drill. Same thing! The householder kept re-instating the power, but both my tools kept tripping the main board. I tried my corded drill in a different part of the house (different circuit) and same thing. The householder insisted it had never happened before and must be my tools. We plugged his kettle in, and no problem.
So back at home I carefully checked both my tools, and they both work fine. My nextdoor neighbour has a newer RCD consumer unit than me and my tools work fine there too. Whats going on? It seemed also that when re-instating that power supply, the main RCD was reluctant to re-set, needing several attempts, or turning off all the individual circuits first before resetting the main.
My tools are in good nick and never been a problem before. And both simultaneously? What can I say to the householder? Whats going on? Advice or ideas gratefully received.

 

[pc1966]

Probably that house already had enough leakage to be close to the RCD tripping point, and your charger just happened to push it over the edge.

Was it a CU with just a single RCD? Or maybe a dual RCD board with a lot of stuff off one RCD?

 

[HandyPete]

Probably that house already had enough leakage to be close to the RCD tripping point, and your charger just happened to push it over the edge.

I thought that, until he plugged his kettle in, and no problem!

Was it a CU with just a single RCD? Or maybe a dual RCD board with a lot of stuff off one RCD?

There was 8 individual RCD's and a main circuit breaker. It was the main one that was tripping.

 

[HandyPete]

Kinda want to go back there and pursue it in depth, but in his eyes no problem, just my tools!

 

[Ziggie]

I had the problem of installed an RCD socket in the garage and put in a tool and the rcd and main RCD tripped.
put extension cable in the rcd socket and the RCD didn’t trip putting the tool in the extension.

my problem, I think, was the freezer in the same circuit.
Removed the RCD socket and put in a RCD on the extension cable - never trips.

rcd’s vary in sensitivity and tools like drills will have inrush current when starting motor.

 

[MFS Electrical]

It sounds like what your mate had was an RCD main switch and MCBs for each circuit.

My best guess without looking/testing is that your tools have a little earth leakage going on in an Installation which is already teetering on the edge of the limit of earth leakage the RCD will tolerate.
the kettle may well have less leakage than your tools or perhaps the extension lead you were using?
Reverse polarity could potentially also be an issue which wouldn't show up with something like a kettle as they are simply a heating element but may show up in tools/chargers as they generally have electronic circuits inside which by design leak a little current to earth in some cases.

According to the latest version of the regs (the 18th) RCDs should not operate at more than 30% of their rated trip current (usually 30milliAmp max so 30% would be 9mA which is 9 millionths of an amp, for reference the smallest fuse in said house would be 3 Amp and the smallest MCB is likely to be 6 Amp)

18th Edition: Earth leakage explained

Do you need to know if your cables are leaky? According to the 18th Edition, the answer is ‘yes’, as Dave Sweetman, Marketing & Business Development Director at Di-LOG Group explains. How often ha

professional-electrician.com

Attached is typical expected leakage currents.

 

[westward10]

Be aware this was moved from the DIY forum to attract more answers so word your advice accordingly.

 

[James]

Please also note that 9mA (miliAmpss)
Is NOT 9 millionths of an amp, it is 9 thousandths of an amp
0.009A

 

[HandyPete]

It sounds like what your mate had was an RCD main switch and MCBs for each circuit.

My best guess without looking/testing is that your tools have a little earth leakage going on in an Installation which is already teetering on the edge of the limit of earth leakage the RCD will tolerate.
the kettle may well have less leakage than your tools or perhaps the extension lead you were using?
Reverse polarity could potentially also be an issue which wouldn't show up with something like a kettle as they are simply a heating element but may show up in tools/chargers as they generally have electronic circuits inside which by design leak a little current to earth in some cases.

According to the latest version of the regs (the 18th) RCDs should not operate at more than 30% of their rated trip current (usually 30milliAmp max so 30% would be 9mA which is 9 millionths of an amp, for reference the smallest fuse in said house would be 3 Amp and the smallest MCB is likely to be 6 Amp)

18th Edition: Earth leakage explained

Do you need to know if your cables are leaky? According to the 18th Edition, the answer is ‘yes’, as Dave Sweetman, Marketing & Business Development Director at Di-LOG Group explains. How often ha

professional-electrician.com

Attached is typical expected leakage currents.

View attachment 64452
View attachment 64453

Thanks for this. Very interesting, although a little advanced for me, but I do understand. I didn't realise that some earth leakage is 'normal'. If my big drill for example had more earth leakage than it should, is that something that can be tested? Although I only have regular multimeter.
I will also double check the polarity, although I don't know how, beyond the colour of wiring in the plug?
As for my battery charger, which also popped the circuits, my friend produced a similar one and plugged his in the same socket and it seemed ok.
I can't imagine a low voltage plastic charger could have much earth leakage, but I don't know.

 

[pc1966]

It is not always the "normal" AC leakage you get from the likes of PC power supplies, etc, where you see the odd 0.5mA or so, that can trip the RCD. Sometimes it is switching noise (or just the act of plugging in something live) that is sufficiently unbalanced between L & N that the RCD sees it as a fault.

 

[HandyPete]

It is not always the "normal" AC leakage you get from the likes of PC power supplies, etc, where you see the odd 0.5mA or so, that can trip the RCD. Sometimes it is switching noise (or just the act of plugging in something live) that is sufficiently unbalanced between L & N that the RCD sees it as a fault.

Overall, it sounds to me as if the RCD is slightly dodgy, the fact that it won't always re-engage without several tries, might mean it could trip too readily. I have replaced RCD's before when the little 'hair trigger' inside gets so it won't hold in the on position. Does that sound reasonable?
Although until my friend has a problem himself, he'll continue to believe it's my tools.?

 

[pc1966]

Overall, it sounds to me as if the RCD is slightly dodgy, the fact that it won't always re-engage without several tries, might mean it could trip too readily. I have replaced RCD's before when the little 'hair trigger' inside gets so it won't hold in the on position. Does that sound reasonable?
Although until my friend has a problem himself, he'll continue to believe it's my tools.?

That does suggest either a defective RCD, or a system whose nominal leakage is quite close to the trip threshold already.

If you read this forum you will see the advice is often to install a board (CU) that is fitted with RCBO, instead of 1 or 2 RCD and MCBs. The RCBO combine the overload protection of a MCB (e.g. 6A for lights, 32A for sockets or small cooker, etc) with the shock protection of the RCD in to a single device.

By doing so you have less risk of spurious trips as the each circuit's leakage is separately checked, and so is unlikely to add up to the 15-30mA threshold on its own. Also if you do have a fault or spurious trip it impacts on just the one circuit, causes less grief overall.

 

[pc1966]

However, it could also be some emerging fault (loss of insulation resistance). So if they do start to see problems then getting a sparky over to check using a 500V tester before replacing the RCD would make sense.

 

[Lucien Nunes]

Surely a cordless tool charger will be Class II, and a normal 230V power tool likewise with a maximum permitted leakage of 0.25mA being handheld. In which case, the idea of just those particular items pushing a system with marginal existing leakage over the RCD trip threshold is unlikely.

Was the kettle tested in the same socket-outlet as the tool and charger? If not, I am inclined to think there is a neutral-earth fault on the garage circuit near or at that point, that is causing leakage due to load, not due to leakage in the appliance itself. That would be easy to discover by an IR test, however if the kettle works fine on the same outlet, it's also not a likely scenario.