Hiring an RCD tester

[Peter Parry]

Not quite, neutral to earth faults in fixed wiring are not that uncommon and quite often go unnoticed for years. One of the most common causes is a nail through a cable which shorts neutral to earth. Some years ago I helped a friend with such a fault. It was discovered when plugging in a drum type extension lead (with nothing attached to it). Plugging it in to some sockets would cause the RCD to trip. Plugging in to others (on the same ring or an alternative) wouldn't. The extension lead was perfectly OK when tested. An alternative extension lead did the same thing - but on different sockets!

Both leads were fine if his wife was putting clothes into the airing cupboard (noticed only by chance rather than skilled detective work). It turned out that the upstairs ring had a floorboard nail neatly through it on the landing near the airing cupboard. When standing near it it moved just enough to break the E-N link.

[John Rumm]

Out of interest, how many circuits are being protected by the single RCD?

[John Rumm]

You can buy high sensitivity clamp meters designed to assist with this kind of test - looping it round L & N it measures the imbalance.

(although I have not tried it, you may be able to get a similar effect with a "normal" clamp meter, clamped round test leads with multiple turns in them to magnify the current sensed. I use a similar arrangement to measure loadings on IT kit for UPS sizing, with and extension lead with ten turn coils made up and broken out)

[Caecilius]

There's a total of five circuits, all on B32 MCBs. They are three rings for sockets, one unused radial for the cooker (changed from elec to gas hob, so not needed now), and one radial for a shower steam unit.

The things not on the RCD are the lights (thank goodness), smoke detectors, and a radial for the immersion heater.

[meow2222]

[John Rumm]

Having all the house sockets on one RCD can often be enough to get close to the trip limits. Especially if you have a house full of electronic gadgets.

Shifting stuff like kitchen circuits off to a separate RCBO can sometimes make all the difference to nuisance trip problems.

[Caecilius]

checking insulation resistance, but it doesn't allow me to measure the earth leakage under operating conditions like the device Ron describes would.

[ARW]

What is the make and model no of the RCD? I might have a spare and I if I have it's all your's for P&P (or just post a picture of it).

I'll even test it and give you the test results.

I am a little too busy to read all the thread ATM but is this a whole house RCD?

[Fredxx]

You can buy affordable clamp meters where if you can pass just the live and neutral through the clamp together you can measure residual current directly. Depending on supplementary bonding you can also get an indication by measuring the current flowing in the protective conductor. You might then be able to find the offending circuit.

As others have suggested, each appliance does have an allowable leakage and combined it may be close to the RCD limit.

Moving some circuits on the non-RCD portion whilst using RCBOs on the most likely offending circuits like the kitchen circuit may solve the problem.

[Caecilius]

The RCD is a Tenby unit, in a tenby C series CU. Installed around year

2000 I think.

really want to replace it with a different model though, because I suspect it's the cheapest the builders could find.

One thing I would appreciate is advice on good quality RCDs. I see screwfix have BG (British General), Wylex, Volex, and MK for between GBP 20 and 35. I'd probably go for MK, as it's a brand I know, but a view from someone in the trade would be useful. I don't really care about a few pounds if it's likely to improve the situation.

I'm assuming that any 80A 30ma RCD will fit in my CU. They all look pretty similar to me from the images on the screwfix website. If there's some compatability issues that I should be aware of, please let me know.

It's a split load CU with lighting and immersion without RCD protection, and all the power circuits protected by a single RCD.

[Caecilius]

That's interesting. I thought that sensitive clamp meters were fairly expensive. I'll have a look around. Another toy for the collection if nothing else :-)

Yes, I suspect this is part of the problem, together with a fairly sensitive RCD.

Yes, RCBO is doubtless the way to go, and they look quite affordable now. But I don't think my CU is big enough for them, as they look much taller than a normal MCB, and my CU is about ten years old.

Here's some photos of my CU:

located less than an inch from the ceiling, and right behind the garage door rail.

If an RCBO will fit in that, then I'd probably take that route. I guess I could put a couple of circuits on RCBOs and move the RCD over so it protects less circuits. If I could do that for the cost of a couple of RCBOs plus a busbar then I'd be a happy bunny.

[meow2222]

All brands need to meet the same electrical spec.

NT

[John Rumm]

To be fair I have not noticed much difference between them. I have a mixture of MK, Hager, and possibly some Contactum ones here - they all do what they are supposed to.

In broad terms, yes - although there is some variation - things like the distance front to back that the live terminal exits the enclosure which can make lining up the busbar with other devices difficult if they don't match well, or a very slight difference in the height they "ride" the din rail - making the protruding sections not line up etc.

(there is also the point that a mix and match set won't have type approval - so you may be taking pot luck on device cooling characteristics etc)

We had a stab a while ago at doing a grid of interchangeability - its not up to date but may give you something to go on. Bottom of this page:

[tony sayer]

In article , Peter Parry scribeth thus

So just the extension lead with Nothing at all plugged into that to load the circuit?..

That wasn't a proper ring main was it?..

So if that was on a ring main how come some sockets made it trip and others didn't?..

Think of what's going on here!...

[ARW]

I can probably find something to fit and replace your RCD. However I really do not think that this is the answer. Your solution is to divide and conquer - could you lose the RCD and install RCBOs on the RCD side? I suspect that the CU will not accomodate them.

In another post it was mentioned that you could get an all RCBO CU from Denmans for £100 (Mr Roberts was not specific on how many RCBOs he got but I suspect that it is 8). Your best solution is to do what Mr Roberts did and install a full RCBO CU. All RCBO installations are the new Peter Kaye Garlic Bread.

The offer still stands if you want me to send you a RCD (if I can find one)

Cheers

[js.b1]

I presume there is no rubber cable in use - and this is a 1980+ installatio= n? I presume you have checked any outdoor sockets & lights for water penetr= ation, isolated any external shed circuit?

Hiring an RCD tester is going to be =A380, perhaps =A380+VAT, with delivery= & collection. So replacing with RCBO is indeed the better solution and may= pin the fault down to one circuit, well worth doing.

Buying an RCD tester can work - if you get an end of line electrical factor= clone like Newey & Eyre or Alto Electrical "Own Brand" which whilst new ca= n be a fraction of the Megger branded RCDT320 etc.

[Caecilius]

Yes, I think I probably need to bite the bullet and replace the CU with one that can take RCBOs.

That's a bit beyone my DIY skills though, so I'll need to get someone in to do it. Do you have any idea of the labour charge for a CU replacement like this? I'm in medway towns, kent if it makes any difference.

Thanks for the offer, but I think I'll go down the RCBO route as you suggested.

[ARW]

Get a couple of quotes and I'll beat them if you like.

I was going to be down your way between Xmas and New Year but I ended up with a rewire in Edinburgh instead. Depending on your timescale I could do it in January when I finish off a job in Lambeth (hopefully finish - she keeps changing her mind). And in Feb/March I should be starting a first fix of 6 apartments in Deptford so I could do it then (they were supposed to start in August). It will have to be a Saturday or Sunday depending on when I leave London.

You have to be aware that installing RCDs onto circuits that currently have no RCD could cause problems. However your installation is of the age where that is unlikely to be a problem.

Of course I would actually use the job as an excuse to meet up with TMH again for a massive breakfast in a cafe.

[Andy Dingley]

No, the method of testing them is to measure their tripping _time_ at rated current, over and under (shouldn't trip). Measuring the current that causes tripping (possibly slowly) instead isn't anything more than a basic test for the RCD's total failure.

[Andy Wade]

They are, but... I bought a Fluke 360 a couple of years ago and wouldn't be without it now.

And to answer your original question: (e.g.)

ring round your local electrical wholesalers, many of them do test equipment hire. What you really need is a leakage data logger though...