Hiring an RCD tester

[Caecilius]

I've been experiencing occasional nuisance RCD tripping for a few years now. It's very occasional - sometimes going for up to six months without tripping. On average I'm getting a trip around once every two months.

I've done some equipment testing with a megger and basic RCD sensitivity testing with some home-made leakage plugs (plugs with resistors between live and earth to give a known leakage current). Based on this, I think my RCD is over sensitive.

My plan is to replace the RCD, which is fairly cheap and simple. But I'd like to test the old one and the new one with something better than leakage plugs to see if there's any change in sensitivity.

Does anyone know if it's possible to hire RCD testers for short periods at a reasonable rate? When I've looked, most hire shops direct me to a PAT tester which isn't what I'm looking for. I don't want to buy one for a single project.

[Harry Bloomfield]

Caecilius expressed precisely :

It would suggest the possibilities of hiring is doubtful, but DIY would be not really that difficult....

A suitable pot in series with a resistor to limit the maximum current between the L and E. Then just tweak the pot to the point where it just trips. Measure the overall resistance, you can check your mains voltage and from that work out the trip current. All you are missing is the time, but that might not be so important to your needs.

[John Rumm]

A RCD tester won't do much more than your improvised equipment will do in terms of things you are interested in (i.e. trip thresholds, rather than response times).

The price of swapping the RCD will be lower than any other option...

You could always grab one from ebay, use and re-sell.

[Caecilius]

I'll be changing the RCD anyway. As you say, it's a cheap option. I just wanted to see what difference it had made. In the past, I've made changes and thought the problem cured when months went by without a trip, only for the RCD to trip six months later.

Yes, that's a possibility.

[Andrew Gabriel]

I still use the one I made >20 years ago, with a 5mA, 10mA, 15ma, 30mA switch. If I was making it today, I would also include a couple of diodes and switches to do the pulsed DC test.

I've never found an RCD which failed by taking too long to trip. Only those that are too sensitive, or don't trip at all.

[Caecilius]

That's interesting. Perhaps the leakage plug method is not too crude after all.

My current 30ma RCD always trips at 16ma (15K resistor) and sometimes trips at 11ma (22K resistor), so it looks like it trips somewhere between these two values. I realise it should be somewhere between

15ma and 30ma, but I'd like to see something closer to 30 than 15.

[Andrew Gabriel]

Have you ruled out an additional 5-15mA leakage elsewhere, adding to your test leakage?

[Caecilius]

Sort of. I turned off the MCBs for all the RCD-protected circuits except for a small ring in the converted loft room, then unplugged everything else from that ring and tested from there.

There's going to be some capacitative leakage in the cables for the ring, but at 50Hz I guess that's negligable.

[Colin Stamp]

I wouldn't mind betting that the leakage that's causing your trips is coming from a single source. It may or may not be over 30ma, and it may be highly intermittent, but finding that slightly duff appliance would be a better bet than replacing the RCD for stopping the trips. A new RCD may well be very nearly as sensitive as the one you have already.

If it was my system, I'd be trying to measure the actual leakage current so I could go around plugging things in one-by-one, hoping to see a significant increase at some point.

Since you don't seem to mind fiddling about a bit, you could temporarily connect a multimeter to the current sensing coil and then characterise the reading you get against your leakage plugs.

On the other hand, you could just replace your kettle. It always turns out to be that in the end ;o)

Cheers,

Colin.

[harry]

The way to find out if the RCD is faulty is to swop two over. You can then see if the fault lies with the circuit or the breaker.

You are unlikely to discove ranything on an intermittent fault with instrumentation.

[John Rumm]

That can happen even if the RCD works perfectly... It could be for example that it is borderline "sensitized" - i.e. the normal leakage through it is close to tripping point. Hence it only takes an unexpected mains transient to create a brief moment of extra imbalance that pushes it over the edge.

[Caecilius]

It's possible, but after a few years of trying to find the problem (it only trips very occasionally) I'm willing to risk GBP 30 or so on a new RCD just to eliminate that.

One thing that tends to implicate the RCD is that the house is only around ten years old, and my next door neighbour also has problems with the RCD tripping. I expect the builders used the cheapest things they could get hold of.

I've done that. I've meggered (can you use that as a verb?) all the equipment between L+N (shorted together) and E. No problems found. But this only measures the insulation resistance, so...

I broke the erath wire out of an extension lead, cut it and then ran all the equipment through that, using a multimeter on AC mA to measure earth leakage current. No problems found - nothing above around 100 uA IIRC.

I wonder about things like the fridge, which is on a thermostat and maybe has a defrost heater that comes on occasionally. That's difficult to measure.

I guess I could megger the fixed wiring, but it seems like clutching at straws.

That's possible. Once I've got the new RCD in, I might play about with the old one to see if I can convert it into an earth leakage measurement device.

Not as easy as that I'm afraid. I've checked the kettle earth leakage.

While this has been going on, I've had a few genuine earth leakage problems which have been found and fixed. One was the iron, and the other the oven element. But the underlying nusience trips remain.

[John Rumm]

Have you tested for neutral to earth shorts on each of the circuits?

[Caecilius]

No, I've not done any fixed wiring tests yet.

[Colin Stamp]

It might be interesting to see what happens, assuming you can get a significantly less sensitive RCD but, even if the trips stop, I reckon something somewhere will still be occasionally leaking abnormally. Still, as you say, it'll give you a spare RCD to play with.

I don't think you've got everything covered with those tests. The Megger doesn't allow the appliances to be powered, so many potential leaks won't have a chance to occur during the test. Also, leakage current doesn't always come back through the earth wire of the leaking appliance. The beauty of using an RCD to measure the leakage is that you'll end-up getting a proper RCD's-eye view of the imbalance between live and neutral currents with the appliance running.

Having said all that, I'd put money on the fault being highly intermittent and difficult create whilst you're watching for it. :o(

They can be tricky buggers. One gave me the runaround for over a year, tripping the RCD in the middle of the night once every few months and then playing all innocent the following day. It turned out the tiny water leak into the handle only became a problem if it was left nearly full for many hours.

I feel for you. These problems can be soooo frustrating. If you get

*really* desperate, perhaps you could arrange an independently RCDd supply that could be used to power strong suspects long-term whilst waiting for the next trip.

Cheers,

Colin.

[Dave Plowman (News)]

Unless they are double pole MCBs it won't help if the problem is in the wiring. The RCD will trip with a neutral to earth fault.

[Ron Lowe]

Depends how D-I-Y you want to get...

I faced a similar problem, and built a D-I-Y leakage meter.

I took an old 2-pole RCD ( Merlin Gerin, as it happened ), and carefully opened it.

I removed all the mechanical switching parts, leaving only the current transformer with it's 2 heavy windings passing L and N, and the sense coil which I wired to a pair of external jacks.

A 10k resistor was placed as a load across the sense coil, to provide a voltage output rather than a current output. This is important, otherwise the open-cct output voltage from a current transformer can posentially get hazardously high.

I then performed a calibration with a 12vAC supply, some test resistors, and a pair of flukes set up to measure mV out versus mA leakage. I calibrated in 1mA steps from 1 to 10mA, then in 5mA steps up to 50mA. The calibration was outstandingly linear, and I printed up a table of mV to mA. I got 45mV / mA.

Now, by wiring this in-circuit with the troublesome circuit ( both L and N, obviously ) I was able to make actual measurements and isolated several faults.

Finally, you may wish to consider replacing the single RCD with multiple RCBOs, if your CU can take them.

[meow2222]

Surely a new RCD would be cheaper.

As harry said, you can adjust the current trip threshold with just resistors. If you then set trip current with capacitors instead, you'd be closer to real world conditions and can see if it filters out such transients effectively, as it should.

The only thing that doesn't test is whether it trips within prescribed time limits, but that has not too much to do with your problem.

Or you can just replace the RCD.

NT

[Caecilius]

I'm going to replace the RCD anyway. I wanted a tester to see the difference between the old one and the new one; not to determine whether to replace or not.

But, from the responses received, it looks like my leakage resistor method is a fair test, so I'll probably continue with that.

[Caecilius]

That's very interesting. Once I've replaced my RCD, I may use the old one to build a similar device. Thanks for the information.

Not enough room in my CU unfortunately. But I 've seriously considered changing the CU for one with RCBOs on all RCD-protected circuits. If I'm still having the problem after swapping the RCD and checking the fixed wiring for N/E problems, I'll probably do that.