Nightly Tripping RCD

MarkTT · Posted: Tue Dec 20, 2005 9:29 am Post Subject: Nightly Tripping RCD

The last two nights in a row the RCD in my house is blowing around 4:00 - 4:30 am and all the electric in the house goes out. I got up this morning at 4:15 and switched it back on but it blew again around 4:30. On both days i got up again at 6:00 and it worked fine. The electric is on all day and all evening with no problem but trips out around 4:00 am ish

I have checked all bulbs and appliences but as everything works perfectly during the day i am now a bit stuck.

anyone have any ideas

johnny_t · Posted: Tue Dec 20, 2005 9:45 am Post Subject:

Economy 7 ?

Central Heating ?

Anything else timed to come on about that time ?

Qedelec · Posted: Tue Dec 20, 2005 11:09 am Post Subject:

You may have mice!!!
They love PVC.
From your original post I assume you have one RCD controlling the entire installation.
If this is so you could try isolating circuits by switching them off before retiring.

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thomasjeans · Posted: Fri Dec 23, 2005 9:56 pm Post Subject:

I had this problem in my house, the electricity (just plugs in certain zones)would randomly go off and not switch back on it took months to fix. It would be ok for a week or two but then just go off (usually at night) and then not switch back on for days or weeks. Anyway it was fixed and the electrician said it was earth leakage, so try that, Ihope you get it fixed soon though, it was hell for me even though i want really living in that house full time.

JohnD · Posted: Fri Dec 23, 2005 10:35 pm Post Subject:

I have a 1970's house, before I fitted an RCD i measured the earth leakage, it was very slight, but measurable. There was a slight background leakage from the immersion heater, and some from the ground flooor ring (which serves the garage and unheated utiity room). I found that most of the leakage was coming from the washing machine, but even with it unplugged, that ring had more background leakage than any other circuit. I felt it was probably to do with cold and damp areas.

From what i found (I am a householder and part time lighting operator, not a professional electrician) i would expect that there is slight background leakage from "watery" things like the washer, steam iron, immersion heater, ketttle, central heating pump), and maybe something switching on during the night is giving just enough extra leakage to trip the breaker. I do periodic PAT testing in my part-time job, and find that things like kettles and water boilers have more leakage than anything dry, even when not faulty and well inside tolerance.

If you have any supply to an unheated garage or shed, they might be getting cold and damp during the night. Try unplugging things for a few nights, and, if you can, try switching the MCBs off on the more likely circuits.

mapj1 · Posted: Mon Dec 26, 2005 10:24 pm Post Subject:

It is sometimes possible to debug intermittent RCD faults by measuring the headroom, by adding controlled amounts of L-E current until the trip just fires, and noting this down. A suspect circuit or device is then isolated, and the procedure repeated. If the headroom is noticably reduced when a particular device is connected, it is a candidate for trouble, and should be investigated.
I have a box with some switches and suitably watty resistors for just this, that can add combinations of 5mA (47K)/10mA(22K)/20mA (10K).
For reliable operation, an absolute minimum of at least 10mA headroom (22 K resistance, no trip) is a good start for anything protected by a 30mA RCD.

One other possibilty, is the neighbours economy 7 or other time switch , if you have a shared neutral to the two houses, and you have a neutral-earth fault in your wiring. - in that case pulling fuses or flipping single pole MCBs, may not help isolate the problem.
I have been deliberately terse, and I'm afraid that if this explanation makes no sense to you, then I respectfully suggest you should not be debugging this without further asstance from someone who understands the potential pitfalls - without seeing your installation, it can be quite hard to advise safely to the right level.

regards Mike.

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Big_Spark · Posted: Tue Dec 27, 2005 12:01 pm Post Subject:

If it is occuring at the same time every night, or whenever it occurs then there is also an outside possibility it could be a supply issue.

Most RCD's are NO VOLT RELEASE, so if they loose suply, they switch off.

Now the RCD's do this with the same circuit used to detect leakage, so if the voltage drops below a certain threshold they will operate. Generally this threshold is around the 190-210V range.

Should there be a loading near to you that comes on at that time, then it could cause a drop in voltage momentarilly..causing your RCD to operate.

As I said, this is an outside possibility, but I have come across this regularly, about once a year, and is always in areas with lots of industrial units or even heavy industry.

Lectrician · Posted: Tue Dec 27, 2005 12:59 pm Post Subject:

VERY rare to find RCD's that have undervoltage release! The only ones TMK are sockets incorporating RCD's

Never seen an RCD in a CU trip with no volts - some will trip with a lost neutral on the supply.

Big_Spark · Posted: Tue Dec 27, 2005 2:13 pm Post Subject:

Lectrician wrote:

VERY rare to find RCD's that have undervoltage release! The only ones TMK are sockets incorporating RCD's

Never seen an RCD in a CU trip with no volts - some will trip with a lost neutral on the supply.

Try MEM MEMshieldII and Merlin Gerin, plus some Hagar models do this as do some of the older MK units.

I agree that more will trip with a Neutral fault on the supply, but try it Lec..you will get quite a few to trip with a Voltage problem too..Especially if you get a momentary loss that causes the magnetic field on the detection circuit to be out of sych with the phase and neutral.