nuisance tripping of RCD

[brian.tabernor]

Any suggestions greatfully received.

A friend has just asked me to sort out why their RCD keeps tripping.

It generally trips in the hight or early morning, although it did trip once when they turned a computer on. Apart from the once with the PC, the only loads running are fridge, freezer and heating. The house is approx 17 years old, PME supply and 80A FCD as main switch, first time I,ve seen a RCD with a small lever as the test switch instead of a button.

When I went tonight, we turned everything on, opened the fridge & freezer doors to get the pumps cycling and flicked the heating on & off to get that switching and nothing tripped.

They also stated that the supply is on and off fairly regularly with them waking up to flashing clocks etc.

Is there anything you can suggest for me to check?

 

[Lectrician]

If the supply is on/off during the night, this suggests a loose connection, probably at the CU, if the neighbours are not also affected.

I think you are saying you have an RCD as a main switch?? What rating?? 30mA or 100mA are typical.

On a PME install, an RCD as a main switch is not required, and not recommended.

You need to isolate the CU, turn all MCB's off/remove fuses, remove all neutrals from the neutral bar and then test from the earth bar to all lives and neutrals with a insulation tester/megohm meter. You will no doubt find a circuit with a low insulation resistance from L to E or N to E. This will be the problem, and may seem to be remote from the problem appliances.

 

[JohnD]

I'd start by unplugging the washing machine, dishwasher, kettle, electric iron and any other appliances that have water in them - these generally have a bit of earth leakage, and it can add together to cause a trip. Youi also get a bit of leakage from immersion heaters, especially if old. You can't easily unplug that, but if it has a DP switch you can turn it off. I've heard the motors in fridges and freezers can give leakage with wear, though I haven't experienced it (I think domestic models are all brushless).

From what you're saying, is it a whole-house RCD, and is it 30mA sensitivity? (this is generally considered too sensitive, thhough correct e.g. on the downstairs ring. I have 100mA on my house and never get nuisance trips). You say PME so presumably not an overhead supply. If it isn't 30mA, lay your hands on one of those portable RCD adaptors used with lawnmowers machines and put suspect appliances through that.

I've got a really good flowchart from MEM (no doubt there are others) if I feel strong enough I'll put it up later.

If you're not a pro:

Unplugging all the appliances and plugging them back in one at a time is the easist way to identify a (single) appliance.

Start by turning off all the MCBs on protected circuits, and turn them back on one at a time. If you have a live/earth fault this will isolate it, to find the circuit with the problem, and is easier than tracking a neutral/earth fault as you have to disconnect cables to find it.

 

[JohnD]

p.s. it's usual for a PC to have a very slight leakage, but not much unless faulty. I'm wondering if they just have a combination of slight leakages around the house, which just need a little extra to trip the RCD? When I tested my house (some years ago) I found a constant fluctuating leak in the region of 10mA. The washing machine was the largest measurable cause.

 

[Lectrician]

Unplugging all the appliances and plugging them back in one at a time is the easist way to identify a (single) appliance

This is never full proof, and rarely works. You simply increase loading, to a point where sufficent current can flow through the alternative path, either direct from N to E or from L to N to E via an appliance ellement or similar.




I must also just add that:

You say PME so presumably not an overhead supply

This is again very misleading - underground services can easily be connected as TT, and occasionally for certain reasons will be. Overhead supplies are also very likely to be PME these days, and very occasionally TN-S (more likely in remote areas where a pole tranny supplies a souple buildings only).

 

[brian.tabernor]

Thanks Lectrician

Sorry, not sure of RCD rating. I admit that I made an assumption of PME supply as its only 17 years old in west midlands (Walsall).

My friend guesses that the neighbours supplies go of as well as the alarm lights are flashing in the morning on their houses when she's noticed the flashing clocks.

I checked connections in the CU and all was tight, but as it was dark I couldn't go ulling the MCB's.

Thanks

JohnD

Thanks for the reply.
I went with the intention of plugging things on untill something tripped, but as with everything on nothing tripped, I was scuppered.
I like the idea with the plug in RCD.

 

[JohnD]

Hi, Leccy, do you think the power cuts are significant? In my area they are rare, but I have once or twice had an RCD trip after or during a power cut.

 

[Qedelec]

Replace that RCD incommer with a 100mA RCD or a double pole 100A switch. Add 30mA RCBO's for downstairs socket circuits.

 

[Steve]

My friend guesses that the neighbours supplies go of as well as the alarm lights are flashing in the morning on their houses when she's noticed the flashing clocks.

erm . . . did you know power cuts can take out an RCD with little trouble? This is the only thing to explain the above statement. When the power goes off, the RCD trips. The RCD may not be detecting any leakage from appliances, just the imbalance when the power goes off from further up the chain (substation). You can get "sticky" RCDs that stay on, think MK have them in some of their sockets, cant remember the real name.

 

[JohnD]

Latching?

 

[Lectrician]

Umm, yes, not un-heard off.

Personally, my split loader has never tripped during a power fail....did at xmas - fecking lights!

Most RCD's will trip when there is a lost neutral on the supply side.

The oldschool BS5490 RCD's trip when they loose Live on the supply side (forget the new BS EN number).

I expect that when the power is restored, it can be quiet 'dirty', and this may affect the correct operation of an RCD.

 

[div69]

Not sure if applicable in this case, but my experience may help someone else.
I was having problems with my electric shower RCD, it was tripping off for no apparent reason and not to mention at the wrong times.
It turned out that someone had connected the door bell transformer to that same RCD. Not sure how that affected it, but since the wires were coming from outside...it shouldn't really affect it, but there you go. Keep in mind another possible culprit if you have one.
I got rid of the transformer and all is fine now with wireless switch.

 

[brian.tabernor]

Thanks for all of your replies.

I'll check out the raing of RCD at next visit and suggest changing to a latching RCD if everything in the installation checks out ok.

 

[JohnD]

When I said Latching, I think I meant "passive"

 

[andy]

When I said Latching, I think I meant "passive"

you did.

passive trips on power fail (use for stuff you dont want to start without warning on power resume)

active doesnt trip on power fail(ideal for fridges)