Any electrical gurus, appreciate thoughts

Zeeflyboy

Zeeflyboy

Zeeflyboy

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So just recently been having a seemingly random tripping RCD. Tripped yesterday morning several times, I thought I had tracked down the culprit to a surge connector in the man cave as it then behaved all day, before tripping again last night.

Behaved again until it tripped again this morning.

This time it wouldn’t re-engage until I flicked off the the MCB for the man cave.

So this is where it starts to confuse my simple brain. I have 99% tracked it down to my new motion rig control box, but the socket it was connected to wasn’t turned on. Put simply I managed to get the power to trip off twice immediately after plugging that in again (turned off at socket).

That suggests a neutral to Earth leak tripping the RCD, but if that’s the case then why does tripping the MCD allow the RCD to be re-engaged since it is only breaking Live?!

There is a lot of EMI filtering in the box, which is probably most likely to blame. The real head scratcher though is that this morning if I plugged it in directly to the turned off socket using a 3m IEC cable then pop goes RCD. If I plugged it in through a different 1m IEC cable and a 15m extension lead then it didn’t pop…

So that leaves me confused.

Basically the head scratching revolves around the fact that if it trips with power switch off, it should be N-E fault, however flipping MCB allowed me to reset which only cuts Live.

If it’s the EMI filters causing it, why is it not consistent and only randomly cutting out seemingly in the morning and evening? Why does it not fault when plugged it through a different IEC cable and extension?

If it is the IEC cable, why doesn’t it fault with the cable plugged in to the wall socket, but only when the cable is connected to the motion control box?

I am confuzzled.
 
Mmm ok, thanks.

still slightly confusing why the MCB breaking live to the cave allowed the RCD to reset with a N-E fault… not really any load to speak of in the cave with everything turned off so flicking the breaker shouldn’t have had much effect on the entire system’s load.

However that’s a working theory, perhaps a crap high resistance Earth in the 5m IEC cable which is enough to cause a problem with the EMI filters which only showed when other stuff in the house kicks in, hence the randomness.

Perhaps the answer may be to wire my own IEC from higher gauge wire.
 
Actually 3m IEC, but zero current when it’s turned off at the plug which is when it’s tripping. It’s fused at 13A and should see at most around 10A peaks in use, but again it’s not while in use (or even switched on at the wall) that I’ve had any issues.

After trying the 3m IEC cable and getting an immediate trip on the RCD again, I then used a shorter 1m IEC cable and a 15m extension lead I have lying around and it didn’t trip.

I’ve ordered a rewireable IEC C15 (15A) and will wire with 1.5mm flex. See if it continues to have any issues after that.
 
I don't think you understood my suggestion. It's likely that higher resistance in some of you cables is reducing the fault current and masking the fault.

I would talk to whoever supplied the motion rig and see what they say.
 
K.C. Leblanc said:
I don't think you understood my suggestion. It's likely that higher resistance in some of you cables is reducing the fault current and masking the fault.

I would talk to whoever supplied the motion rig and see what they say.
Click to expand...

It's DIY so that would be me!

From what I have been able to research it is not as simple as that however, leakage currents from such as EMI filters have a higher capacitive reactance vs resistive component.

I think to get to the bottom of it I need to do some measurements with the neutral and earth connected. With mains disconnected I cannot find any measurable resistance value between earth and neutral on the control box, on all resistance modes my multimeter just can't find a connection between the two, so there is no obvious direct short between them.

Spec for leakage current of each of the filters is less than 0.5ma which x5 is 2.5ma at a max. There is a further filter on the IEC inlet, but that has utterly negligible leakage at 373uA. All well below spec on the thirds rule for the RCD which is a 30ma trip current... the RCD is shared between half the house circuits and the man cave, so it's possible something/multiple other things in the house coming on in the same bank manage to combine with the 2.5ma from the control box to trip the RCD.

Ideally I would give the Cave it's own RCD, but that would require either an additional small consumer unit (not ideal as they are visible in the downstairs bathroom) or maybe a high integrity consumer unit which would allow me to have two banks protected by two RCDS as normal but place things like the man cave on it's own RCBO. Might get expensive though! An interim option might be moving the Cave over to the other bank on the consumer unit so it is sitting on the other RCD, potentially avoiding whatever summation of leakage loads is causing the trip.
 
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Invest in an earth leakage clamp or as cookie says run an extension from a socket on the other rcd.

even with the clamp, it will need some investigating and poss splitting onto another rcd.
 
Sounds more like poor socket installation rather then an extension cable or IEC being at fault. I would also doubt it's a neutral to earth fault. A large portion of UK homes are PME (Protective Multiple Earthing) meaning neutral is earth and earth is neutral. That's most likely why you are getting no resistance between Neutral and Earth.
 
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Just to update, friend popped over with his PAT tester and there were no faults or issues we could find.

I had narrowed it down to only occurring when the boiler pump fired up, so we moved the man cave supply over to the other RCD bank and fingers crossed everything has behaved since. Seems the combination of leakage from all the filters and the boiler pump was enough to push the RCD over the limit.

I think longer term solution will be to install a high integrity consumer box and have the cave on its own RCBO.
 
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