Tracking RCCB nuisance tripping

[James]

I know this is an old chestnut, I've been googling !

This 70s house (live wires are red - not brown) has always been sensitive to tripping.

The old washer made it happen a lot. It was replaced with a great improvement. (I guessed there were leaky radio interference suppression capacitors from live and netral to ground causing this ???) An old iron made it happen a lot. It was sent to another house with no problem there.

Well, the problem seems to be back.

I've come up with an idea to help locate it and would be grateful for any comments.

The house uses a 63A / 30mA Crabtree RCCB with everything except lighting and immersion heater on the protected side.

This assumes an appliance problem. (I am reasonably electrically savvy.)

1. I made up a 2m 13A extension cable with the earth lead disconnected in the plug.
2. I *VERY CAREFULLY* connect appliances to the extension cable.
3. I measure current using a multimeter between the opened earth lead and the earth pin in the plug.

Checking the washing machine, I measure 2mA. I'm going to work my way around everything. Obviously it will be more difficult on hard wired units, eg the cooker. I'll try to post measured values.

I understand that a 30mA breaker could trip at 50% x 30mA = 15mA, so several times 2mA can add up quickly to 15mA.

I hope it's an appliance fault as wiring leakage sounds a very painful and expensive fix.

Any comments or pointers to other tracing techniques.

I don't have a megger, but could easily do resistance checks with the multimeter.

I seem to recall while googling seeing a technique of (power off situation) strapping together the live and neutral, then measuring resistance to earth. This sounds promising and not too difficult.

Is there any way to test to see if the RCCB is too sensitive ?

Thanks in advance.

James

[Owain]

And for things like the WM you will need to measure throughout the cycle, and the cooker with elements hot and cold.

I suppose there are places that would hire a megger/RCD tester?

Owain

[John White]

You will find that RCCBs (Residual Current Circuit Breakers) are now called RCDs (Residual Current Devices).

Or it never really went away.

This is a really, really bad idea.

If you want to test appliances get hold of a PAT tester and do the job properly.

Stop doing this NOW - you could kill yourself if the appliance is seriously faulty.

You should also either dismantle completely, or repair the broken extension lead NOW as it is potentially lethal.

Sorry but your understanding is wrong.

With a leakage current flowing equivalent to 50% of the rated tripping current of the RCD, the device should not open.

With a leakage current flowing equivalent to 100% of the rated tripping current of the RCD, the device should open in less than 200ms if manufactured to BS 4293 or BS 7288, or less than 300ms if manufactured to BS EN 60108 or BS EN 60109.

In the case of a RCD rated at not more than 30mA, it should also open in not more than 40ms when tested at five times the rated tripping current.

It's usually means replacing rather than repairing the faulty components.

Not really. A resistance tester carries out the test at 500 VAC on domestic circuits and most multimeters are not up to this.

That's the "alternative" method for checking insulation when there is sensitive equipment that cannot be isolated. You cannot check for Live to Neutral, Live to Earth or Neutral to Earth leakage in this way.

You need a RCD tester to do this properly.

John

[James]

Agreed, the system seems to be marginal, and those appliances were pushing it over the edge.

OK, if I rephrased that and simply said "I've put a milliammeter in the earth lead and measured very carefully" would you be so concerned ? I appreciate your concern, but I am fully aware of the dangers of

240VAC and not touching the appliance/casing - only looking at the meter. (CEng, MIEE)

Agreed, but I only have a multimeter.

James

[The Natural Philosopher]

Well thats what I had...and with the house loaded up the bloody thing would trip on just about anything, from a bulb blowing to a thermostat coming on.

Maybe there is an earth neutral short somewhere apart from the actual incoming where they are bonded together.

I suppose I should have gone round every circuit and lifted the earth and measured neutral to earth.

Instead I shoved in a 100mA trip. One day I will put 30mA RCBO's in the outside and downstairs rings, bit when it tripped after I left and extension out on the rain, I figured it was doing the job I wanted it to.

My advice. Scarp the 30mA trip. Its too sensitive for the total supply on a big house, and go for a 100mA one, then if you feel paranoid, put

30mA RCBO's on the necessary circuits.

BY all means test appliances, but my experience is that when faulty they ALWAYS trip it.

The only fault that cause random tripping seems to be a neutral earth short somewhere.

LOTS of RFI filters can add up to 30mA pretty quickly tho.

10nf is about 200K ohms at 50hz, a lot less if there is any transient HF noise - sparking contacts and the like. at 230vAC thats a mA per 10nF capacitor - which is sort of what RFI filters have. 30 bits of electronic kit and there's your 30mA. Consider that you are as a manufacturer allowed up to 3.5mA leakage (5mA for heating devices) and its not hard to see that 30mA is a totally inadequate trip current to use in anything but a small house, or a single ring.

[Mike Tomlinson]

In article , James writes

Ditto, except Protek in my case, installed by me 6 years ago. The RCCD started randomly tripping earlier this year, at frustratingly intermittent intervals. Careful unplugging of selected devices on the three rings (one downstairs, one upstairs, one kitchen) and disconnecting the cooker and shed circuits made no difference. All the circuits megged out OK when disconnected from their respective breakers.

Not sure. In the end I gave in and replaced the Protek 63A 30mA RCCD with a Crabtree one of the same spec; the random trips stopped and haven't happened since.

The supplier from whom I bought the Protek CU complete with 100A isolator, 9 MCBs and the RCD didn't stock a compatible Protek RCCD to replace the suspected faulty one. The closest equivalent they could offer was a Crabtree unit. This installed fine with just a minor alteration to the busbar to allow it to fit the connectors in the Crabtree unit, which are sited slightly farther back in the module than the Protek ones.

The replacement Crabtree RCCD alone cost me 75% of the cost of the complete Protek CU with isolator, breakers and RCCD. I've learnt my lesson: quality branded kit from now on.

[John White]

That's how I would read it. Either that or the RCD has deteriorated and is now tripping at a lower current.

Much better. :-)

(That should read 500 V DC not AC)

You may miss something.

Good luck

John

[Andy Wade]

RCD is a generic term. An RCCB is one instance of an RCD. (An RCBO is another, combined with an OPD.)

[meow2222]

Typical poor design I'm afraid. If it were mine I'd change the rcd/cu arrangement. If youre not up for that, use a multimeter to measure resistance from L+N to E on all plug in appliances is the best first step. There should be no conduction. Although a multimeter does not test at 500v, in practice it will still pick up almost every case of leakage, and most likely will track down any leaky appliance.

Checking rcd sensitivity is easy, with a resistor L to E passing 15mA, no trip should occur. You'd have to remove all other loads for this to work, as other loads would add their leakage curren.t

NT

[James]

Thanks for the various inputs. I particularly liked Meow's method of checking the RCCB (or RCD :-) ) using resistors from from L to E.

15mA = 16K 30mA = 8K As this problem seems so common, I'll post back my findings when a get a "result".

So in summary, now I'm looking for:

1. Earth current on various appliances.
2. Resistance to E with L to N strapped on various circuits. (Power off, of course) - unfortunately only using multimeter.
3. RCCB sensitivity as Meow.

Does that cover it?

If all the above fails, is there a Crabtree 100mA RCCB that will drop into the position of my 30mA unit ?

Cheers,

James

[meow2222]

just bear in mind little common Rs are 200v rated and only half or third of a watt, so you cant just grab a little 16k and use it.

15mA @ 240v = 3.6 watts so you need a 5w resistor rated at 400v, or 2x 2w 200v rated Rs in series.

you could but really that will be found out from the R test below, on each appliance, quicker easier and safer.

I'd do that on the appliances rather than the fixed wiring. Those are many times more likely to be the culprit, and you dont have the added issues you will with doing that on fixed wiring.

Pretty well I think. Its most likely not a fauilty rcd, I'd test appliances first.

NT

[Pete C]

If the wiring is in good condition, it may be there's a left over connection from the lighting/immersion neutral to a ring or other neutral. Check for any continuity between, as this would pass the usual N-E checks.

A 16A bridge rectifier can be put in the earth connection so as to limit the open earth voltage to 1.2V, then connect the meter across this (as long as leakage x meter resistance Checking the washing machine, I measure 2mA. I'm going to work my way

If you borrow a suitable isolation transformer and try it on each appliance in turn, this would eliminate intermittent earth faults, bearing in mind any reduced leakage.

I'd work out what resistor leaks 1mA to earth, then add one at a time until it trips.

cheers, Pete.

[The Natural Philosopher]

Try 250K ohms

[James]

Guys,

I've built a little tester loosely based on the ideas of Meow and PeteC.

It consists of six 47K resistors (2W, though 1W is about OK for a quick test) which are connected one at a time from protected the side to earth.

A 47K resistor passes 5mA at 240V, so 2 pass 10mA, 3 pass 15mA......6 pass 30mA.

The results are interesting:

All breakers off, RCD trips at 25mA (=5 resistors). Conclusion: the RCD is OK (25mA is within spec for a 30mA breaker).

All breakers on, RCD trips at 20mA additional (=4 resistors).

COOK-1 breaker only, trips at 25mA additional COOK-2 breaker only, trips at 25mA additional RING-1 breaker only, trips at 20mA additional SHOWER breaker only, trips at 25mA additional RING-2 breaker only, trips at 25mA additional

Conclusion: There is some leakage in RING-1 circuit, none anywhere else. However 5mA is not much (and would not be causing tripping), but it does include the washer where I've already measured 2mA in an earth lead. It also includes some other suspect areas such as dryer, PC, kitchen outlets, garage.

Sod's Law: Of course it's all very well behaved at the moment with no nuisance trips since I went on the hunt.

I do recall a tripping fit several months ago when the local supply seemed to be suffering from surges and brown outs (lights were going bright and dim). Possibly an external problem (???).

Thanks for all your help.

James

[robgraham]

What a usefully timely thread. I'm suffering from the same problem after replacing an old fuse CU with a modern split one. I have just been along and seen that it is a 30mA rcd so that's the first thing to address.

I do suspect that the problem is a bit of what can ONLY be described as botching - I seem to remember that I had to frig a double switched lighting in the new kitchen after her-indoors initially specified a single switch and then changed her mind about it !! Should really not have done it but I had to 'borrow' a neutral for that light circuit from one of the rings; it does seem to be when that light circuit is switched on or off that the rcd trips - would that make sense ?

I'll now need to try and undo my 15 years ago botching.

Rob

[meow2222]

You cant run a borrowed neutral off an rcd - unless both circuits involved are on the same rcd.

NT

[meow2222]

connecting it from input live to output neutral also works, and avoids any real earth current.

exactly, youve not found enough leakage to explain the problem. So either its intermittent, from an appliance not currently on or plugged in, or else the rcd has faulty filtering, which is not something you can realistically test at home.

Howeer the prime suspect remains the same, an appliance, so you need to insulation test em all with your multimeter.

NT

[John Rumm]

A minor point to bear in mind when testing RCDs like this. You can verify if the trip threshold is out of spec, but unlike with the correct tool, you can't as easily establish if it meets the disconnect timing requirements. From a "saving your neck" point of view, the latter is equally important as the former!

[The Natural Philosopher]

That's the problem with the 30mA trips. Live to earth capacitance (and there is a surprising amount juts in the cabling on a big install, before you take RFI capacitors into account) is enough that a nice high frequency spike - like a lightning strike on a power line, or the neighbors doing arc welding...or even a hesitant thermostat...can take the whole thing over the edge.

[Tony Williams]

A check for low N-E resistance in the cabling?

First go round the house, measuring the AC voltage between N and E along the various rings, etc. They should all be the same, be suspicious of one that is lower than all the others.

Then. Incoming power Off.

Lift and separate all Neutrals off the commoning block in the consumer unit.

Check that each Neutral wire has a reasonably high resistance to Earth. ie, If the N-E voltage is say 1Vrms then an N-E resistance of 50 ohms will cause an unbalance current in the RCCB of 20mArms.