Rcd tripping

[aebersold]

Silly question, but have you had anybody test the RCD?

Any electrician with a testing ticket and an MFT can test to make sure the RCD is not faulty, and causing nuisance tripping

Good point and no I haven’t, thanks .

 

[aebersold]

Not required, the supply cable to each outbuilding does not need an RCD only short circuit protection in the form of a fuse / MCB and maybe you do not have a problem.

Thanks, that’s another dimension to look into. The 3 outbuildings have just the one supply armoured cable running from one building to the other. I think that’s referred to as ‘in series’ ?

 

[aebersold]

That wiring is OK.
30mA rcd's at the sub boards to protect people.
100mA rcd at the main board to protect the distribution cables to those sub boards, provided the cables are installed in a way that there's no risk of somone putting a screw into them. They should be in conduit or run in steel wire armored cable. (But agree with Roy, an RCD is not required on that circuit, but it is reasonably sized and if it's not defective, it is pointing to an issue).

Still, that is a lot of cumulative leakage setting off the 100mA RCD.
The obvious next steps to my mind are to have the 100mA RCD tested with an electrician's multifunction tester (and do the 30mA ones while he's at it)
Then to check the AC earth leakage at numerous points with a high sensitivity, earth leakage clamp meter.
Depending on what you find, you may need to look for DC leakage current after that.

Some clear guidance, thank you. The main supply cable is armoured and underground. Would fitting a type A Rcd in the house make a difference or is it best to just get an electrician to make an assessment ?

 

[Sideways]

I think I'd turn it over to your spark.
If the 100mA RCD is tripping too easily, you have the option of replacing it or removing it. Use a type A if you replace it.
I think a little investigation with an earth leakage clamp meter may unearth the problem. Not all electricians have one but they are a very useful tool now RCD'Ss are widespread.
All clamp meters look basically like this - they can measure a current by clipping around a wire without cutting it - but the earth leakage ones have a fatter, not bigger, detecting jaw than ordinary clamps and are much more sensitive - 1mA or less.

In use, the jaws are closed around both live and neutral wires, the currents in these two are in opposite directions and their magnetic fields should cancel out, but if there is leakage, they aren't equal and the meter will read the difference. This is similar in principle to what the RCD does.

 

[aebersold]

I think I'd turn it over to your spark.
If the 100mA RCD is tripping too easily, you have the option of replacing it or removing it. Use a type A if you replace it.
I think a little investigation with an earth leakage clamp meter may unearth the problem. Not all electricians have one but they are a very useful tool now RCD'Ss are widespread.
All clamp meters look basically like this - they can measure a current by clipping around a wire without cutting it - but the earth leakage ones have a fatter, not bigger, detecting jaw than ordinary clamps and are much more sensitive - 1mA or less.
View attachment 187357
In use, the jaws are closed around both live and neutral wires, the currents in these two are in opposite directions and their magnetic fields should cancel out, but if there is leakage, they aren't equal and the meter will read the difference. This is similar in principle to what the RCD does

Thanks Sideways, I’ll try the type A, but as you and others have mentioned, there may be an underlying issue somewhere which needs sorting. I want it to be safe and will engage an electrician.

 

[aebersold]

Thanks all for your most helpful posts and steering me in the right direction,
much appreciated.

 

[Spectric]

I think that’s referred to as ‘in series’ ?

I would suggest it is looped in / out rather than in series. Also good advice about getting a qualified electrician in to take a look, all to easy to make something safe into something that is not.

 

[Paul Hannaby]

The problem might be caused by the multiple earth stakes. That could cause issues with each one being at a different potential in the event of current flowing to earth and possibly earth loops. Are all the separate earths connected back to the main earth or is each outbuilding just fed with live & neutral?

 

[Dabop]

You are so lucky in not having ring mains, they can be a right pita when it comes to testing and people bodging.


Yes we are going that way, we call them RCBO's (Residual Current Breaker with Over-Current) but many still do not use them as they are / were more expensive.


Yes you can have just a protective device for the cable to an outbuilding providing in that outbuilding all the circuits are protected by an RCD, ideally an RCBO on each radial circuit. A problem you see is people hook into a ring main to feed the outbuilding because it might be easier and then problems begin rather than a dedicated outbuilding supply.

They can also be dangerous...

A broken ring can lead to currents in excess of the cables rating (and broken rings are far too common in the UK, as the EICR testing program for rental properties found out...)

A ring is usually 2.5mm on a 32A MCB, as long as it is unbroken, thats fine...
But if it breaks (and depending on the loads and where the break is) that can leave that 2.5mm cable seriously overloaded...

Worst case is a ring running right near the breakers capacity (electric heaters going in winter in several rooms say), and a ring broken right at the end ie one leg is literally unloaded, the other is carrying the entire ring load...
This worst case could see the final leg link (between the consumer unit and the first heavily loaded outlet) carrying the full up to 32A, which depending on the cables install (say the feeds buried in a plaster wall ie direct contact and fully enclosed) would leave the cable running at almost twice its rated capacity!!!!

Well, at least the heaters would have a easier job- the houses wiring would be contributing to the job lol

 

[Spectric]

But if it breaks

I have found that is a less common fault than someone actually doing some DIY work where they have broken the ring to add an extension to somewhere, removed a socket and just taken each cable to some new sockets so in effect you now have two overloaded radials on a common 32 amp protective device.

 

[Dabop]

I have found that is a less common fault than someone actually doing some DIY work where they have broken the ring to add an extension to somewhere, removed a socket and just taken each cable to some new sockets so in effect you now have two overloaded radials on a common 32 amp protective device.

Still counts as a 'broken ring', and if that happens- bad things can happen


(for those unaware, the breaker on a radial on 2.5mm should only be 20A maximum (in certain cases only a 16A should be fitted), so by breaking the ring (either by accident or intentionally) the 32A breaker as fitted to a ring is well above the maximum safe working load for a single 2.5mm)
Its one of the reasons I am not a fan of them...

 

[aebersold]

The VFD for my mill required disconnection of the EMC filter if the power source had asymmetrical grounding. This is what we have for single phase in the UK (bottom left in the pic), so I did.

It means the voltage between each live conductor and earth is not the same.

Symmetrical earthing is where the centre of the transformer is tapped to earth, and the power is taken from the outer taps, like in the US (eg phases S and T in top right).

Of course, if the VFD is fed with UK 3 phase, it will also have symmetrical earthing.

View attachment 187347

I did speak to the vfd supplier who informed me of the emc filter screw which can be removed. I’ll look into that, thanks.

 

[aebersold]

The problem might be caused by the multiple earth stakes. That could cause issues with each one being at a different potential in the event of current flowing to earth and possibly earth loops. Are all the separate earths connected back to the main earth or is each outbuilding just fed with live & neutral?

The armoured cable from the house travels from one building to the other. Because of the distance involved, separate earth spikes were installed. Thanks