RCD tripping in Consumer Unit

[Harry Bloomfield]

Fair enough - that's just a type of improvised 'power failure alarm' However, as said, that won't tell you about failure of a freezer which is still receiving power.

Our main freezer has a high temperature alarm, but that would give no warning if there was a complete failure. I bought separate battery powered unit, which displays the temperature and has settable temperature alarms. It is triggered even if the door is open for too long.

 

[JohnW2]

Our main freezer has a high temperature alarm, but that would give no warning if there was a complete failure. I bought separate battery powered unit, which displays the temperature and has settable temperature alarms. It is triggered even if the door is open for too long.

Oh sure, I certainly wouldn't rely on an alarm which was built into my freezer - all mine are, like yours, 'separate' and battery powered (but all connected to a central, battery backed-up, 'alarm system') - all being ones which I built very many years ago.

I have corresponding 'low temp' alarms on some of the bits of the plumbing/CH system in various dusty corners of my large house, connected to the same central alarm system, appropriate ones of which can be configured, if I wish, to turn on CH to relevant zones of the house if the temp of plumbing gets close to freezing.

Kind Regards, John

 

[crystal ball]

I had a customer whose freezer was on a tripped circuit for several weeks, you could smell the putrid food for weeks afterwards

 

[JohnW2]

I had a customer whose freezer was on a tripped circuit for several weeks, you could smell the putrid food for weeks afterwards

Was it a tripped 'dedicated' circuit ('dedicated' to the freezer), I wonder?

Kind Regards, John

 

[ericmark]

Defrost time depends on size of freezer, first I worked with was in 1980 in Algeria and they were the size of a container, and remember one going wrong and cooking the meat inside it.

But to have a spare freezer is rare, actually I do have one at the moment so could move food to flat under main house if a freezer failed, but most people if the freezer fails by time they have bought a replacement it is too late to save the food.

I tried to work out why my RCD's were tripping, they were old pre the electronic type we have today, I got them second hand in around 1992 and they were fitted as my son had decided at 14 to become a radio ham and I wanted to protect him. So two Wylex consumer units with two RCD's well before we got consumer units. They were 4 module wide, gives you an idea of the age. Tripping seemed to come in batches, they may trip 6 times in two weeks, the RCD tester found no fault, and the insulation tester found no fault, then it would do two years or more without tripping, I think spikes on the supply, possibly some one in the street welding?

In the main trips where when we were home, bit of a pain going out to garage to reset, but never did find out why.

As a result when I moved house, new house has all RCBO fitted, 14 in all. And only tripped for good reason, and mother house with two RCD's and 5 RCBO's was same, never tripped without good reason, so may have been due to age of RCD's?

 

[JohnW2]

.... As a result when I moved house, new house has all RCBO fitted, 14 in all. And only tripped for good reason, and mother house with two RCD's and 5 RCBO's was same, never tripped without good reason, so may have been due to age of RCD's?

I may have misunderstood you. You write a lot about 'nuisance trips' ('trips for no good reason') of RCDs/RCBOs being a common problem, and you appeared to be saying that such was your current experience. However, it now sounds as if you are saying that, in your current home, and like me, you experience virtually no such 'nuisance trips', and also that the same was the case in your mother's house. Is that correct?

Kind Regards, John

 

[crystal ball]

Was it a tripped 'dedicated' circuit ('dedicated' to the freezer), I wonder?

Kind Regards, John

No, kitchen ring, she was away

 

[ericmark]

I may have misunderstood you. You write a lot about 'nuisance trips' ('trips for no good reason') of RCDs/RCBOs being a common problem, and you appeared to be saying that such was your current experience. However, it now sounds as if you are saying that, in your current home, and like me, you experience virtually no such 'nuisance trips', and also that the same was the case in your mother's house. Is that correct?

Kind Regards, John

Yes it was just the house in Bryn-y-baal, Mold, however mother had 7 RCD/RCBO's and I have now in Llanfair Caereinion 14 RCBO's only the Byrn-y-baal house was limited to just two RCD's.

We are told we should work in times three when selecting RCD's, so for a 30 mA RCD no more than 9 mA drain to earth without fault, and a 30 mA would be protected by 100 mA and 100 mA protected by 300 mA and 300 mA protected by 1 amp etc. Not that this happens in the home, with domestic 30 mA supply is still protected by a further 30 mA with caravans and boats. However they do have battery backed lighting.

However we should check the leakage does not exceed 9 mA under normal running conditions, but lowest my clamp-on will measure is 100 mA all I can do is check the insulation resistance which uses DC so not really much good, and test with RCD tester that it does not trip at ½ rated current, which means could have 14 mA leaking and be unaware. Unlikely the RCD is bang on 30 mA but even 4 mA leeway is sailing a little close to the wind.

We are allowed equipment with up to 3.5 mA leakage, mainly due to EMC filters, and since through capacitors it will not be picked up with a DC insulation tester, and this leakage is only there when the appliance is plugged in. And we have traditionally tested the RCD with everything unplugged, in fact I watched a video talking about type AC and type A RCD's where the experts were talking about dropping tails to test, which to me seems crazy and in earth days main reason for RCD failure I found was due to strain on the casing from the cables in the terminals, more than once dressing cables was all that was needed to get RCD to pass.

There is also the question about DC desensitising the RCD, if it makes it more sensitive as claimed with high frequency noise that means it fails safe, but if we have something which injects DC for example a three port motorised valve in a Y plan central heating system, we really want to test with it all running to be sure not desensitised.

So we look at

314.1 Every installation shall be divided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Section 537)
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation
(v) mitigate the effects of electromagnetic interferences (EMI)
(vi) prevent the indirect energizing of a circuit intended to be isolated.

which seems to say the RCD is considered as forming a circuit, OK BS7671:2018 may say something slightly different, but the point is if there is a possibility of unwanted tripping of RCDs then it should be split into more circuits. Before the person responsible for design selects a CU with just 2 RCD's he needs to be satisfied it is unlikely to cause unwanted tripping of RCDs, and if you can't measure the leakage current so are sure under 9 mA then best option is all RCBO's.

But when I did my inspection and testing exam, I was not told to check leakage, and I was told to test with all appliances disconnected. To some extent this makes sense, switching off PC's while in use can cause problems, and the switch mode power supply used in older PC's did cause problems, so it is the equipment we would normally switch off before testing which is the equipment most likely to cause problems.

So with mothers house moving the kitchen off the general RCD's and having a bank of 5 RCBO's was likely enough to ensure leakage was within limits, if we look at kitchen equipment, the induction hob, inverter washing machine, inverter fridge/freezer, and inverter freezer are all items likely to both freeze and cause RCD's to trip, and the oven with mineral insulated elements is also likely to cause a RCD to trip. So late mothers house could have 100 mA leakage and not trip as spread across 7 RCD devices, this house 200 mA as 14 RCD devices. OK unlikely to have maximum of 15 mA which should not trip a 30 mA trip on all circuits, but in the days of using type S 100 mA RCD's I do not remember problems with tripping, only when we went to 30 mA did we start having problems.

Never worked out why they are called high integrity consumer units, but having some key circuits on RCBO's seems to be enough to tip the balance and stop 'nuisance trips' as to if lights or sockets, or cooker is put on RCBO it seems this is often enough.

But since I could not measure the leakage, best idea seemed to be all RCBO. The other point is sockets split side to side, and lights split up/down, so could not arrange so when a RCD is tripped due to a fault not also plunged into darkness in some rooms. OK general power cut can plunge us into darkness, but that is not at same time as getting an electric shock, so sockets and lights should not be on the same RCD in any room, so in my house at least 3 RCD's are required.

 

[BS3036]

Unlikely the RCD is bang on 30 mA

As has been mentioned many times before, a 30mA RCD is designed to a specification which says it must trip somewhere between 15mA and 30mA.

 

[ericmark]

As has been mentioned many times before, a 30mA RCD is designed to a specification which says it must trip somewhere between 15mA and 30mA.

Agreed but what I am saying between 9 mA to 15 mA = 6 mA and 9 mA to 30 mA = 21 mA so the RCD will trip some where between 6 mA and 21 mA. Where with old 100 mA trip it was 32 to 50 mA = 18 mA and 32 to 100 mA = 68 mA so would trip between 18 mA to 68 mA so there is a massive difference between 6 mA leeway and 18 mA leeway, specially as one item can leak up to 3.5 mA and be acceptable.

So with the 30 mA trip plugging in two items could cause it to trip with no fault, with 100 mA it would need 5 items, the chance of 5 items all on the edge of permitted leakage is slim, but 2 items very possible. Today we tend not to find EMC filters which leak anywhere near the limit, but older equipment it was quite common, I have returned goods due to leakage and not passing the inspection and testing of in service electrical equipment, (PAT testing) I knew it was OK, it was brand new, however I also knew in a years time it would be failed, seems the EMC filter was removed and then returned to us.

But domestic appliances are not with owner occupied houses inspected and tested, so they would be unaware it was near the limit. Early PC's often took out RCD's when turned on, clearly some thing has changed as today they don't seem to take the RCD out.

 

[JohnW2]

... We are told we should work in times three when selecting RCD's, so for a 30 mA RCD no more than 9 mA drain to earth without fault ....

I wonder when a third of 30 became 9 (rather than 10)?

However, it's surely a totally ridiculous guideline for a sockets circuit (which is where any problem is most likely to arise), since there is no way that a designer can have a clue as to what might be 'plugged in' to the circuit.

In any event, if an RCD/RCBO trips because the total of 'in normal use' earth leakage currents of protected items adds up to more than 15mA, then I don't think I would call it a 'nuisance trip' (in the usual sense), because the RCD/RCBO would then merely be doing what it was meant to do. Furthermore, if such a trip were to occur whilst all the connected items were 'running' (rather than 'at switch on'), then it would presumably often not be possible to reset the RCD/RCO without disconnecting some of the loads (which, as I understand it, is not the usual experience with 'nuisance trips')?

I doubt that, in most cases, it will ever be possible to determine the cause of true ('for no good reason') RCD/RCBO trips. Given that virtually all of these devices are now 'electronic', my suspicion would be that the most likely mechanism would probably be 'interference' (by 'noise', particular EMC) with the electronics, despite there being little or no actual L-N current imbalance.

There is also the question about DC desensitising the RCD ...

It's definitely "a question" since I continue to fail to find any really definitive/useful information about that topic!

So we look at .... which seems to say the RCD is considered as forming a circuit...

We've been there before. As so often, it's imperfectly worded, and probably should say something like "...Every installation shall be divided into circuits protected by different overcurrent and residual current devices as appropriate, as necessary, to: ...", but 'we know what it means' and, as you go on to say ......

...the point is if there is a possibility of unwanted tripping of RCDs then it should be split into more circuits.

Indeed, provided that one means "....split into more circuits protected, where appropriate, by different residual current devices" (rather than just more 'circuits' per BS7671 definition {same OPD}).

Having said that, whilst the inconvenience etc. caused by trips will reduce, I presume that the more residual current devices one has, the more common would it be that one would suffer 'nuisance trips' due to the mechanism I suggested above (interference with electronics) or due to a device becoming faulty.

... OK general power cut can plunge us into darkness, but that is not at same time as getting an electric shock, so sockets and lights should not be on the same RCD in any room, so in my house at least 3 RCD's are required.

That certainly helps but, even if one has "just had an electric shock", one will often still have to hunt around in the dark to switch on a sockets-circuits-supplied light (unless one is already on) - and, of course, there's no guarantee that every room will have such a light 'plugged in and ready to go'.

I can but repeat what I have written countless times about my personal experience over 35+ years of living with RCDs (initially single up-front ones protecting the entire installation) - in that 'being 'plunged into darkness' in my house is far far more commonly due to a power cut than to operation of any protective device - so that 'emergency lighting' (adequate to provide at least some lighting in ever room and area) is the only solution. Given the way the regs have been evolving, I remain rather surprised that a requirement for that has not yet shown its face.

Kind Regards, John

 

[ericmark]

I wonder when a third of 30 became 9 (rather than 10)?

Less than 1/3 and 10 is 1/3 so could be 9.9999 etc. But meters often can only measure 9 or 10 and 10 is over limit and 9 under limit. However all you say is correct. What we should do with an existing installation is measure leakage, but I have not got any meter which can do that. I have looked at a new clamp-on, cheapest I can find is £40 which can measure down to 0.001 amps and DC. And since retired have not bothered.

But unless you have one, you have no idea how close to wind one is sailing, we simply guess. What impedance twin and earth is I don't know, not that much difference to 300Ω ribbon cable, but there must be some leakage, it is after all AC.

But we both know many electricians never measure it, which includes me, maybe you have a meter? But a twin RCD CU is fitted as that is what they always fit, not because they have done a risk assessment and know with this installation it will be good enough. The put the insulation tester on the cables and if over 1MΩ good enough, but the insulation tester uses DC, so means nothing.

 

[Harry Bloomfield]

Less than 1/3 and 10 is 1/3 so could be 9.9999 etc. But meters often can only measure 9 or 10 and 10 is over limit and 9 under limit. However all you say is correct. What we should do with an existing installation is measure leakage, but I have not got any meter which can do that. I have looked at a new clamp-on, cheapest I can find is £40 which can measure down to 0.001 amps and DC. And since retired have not bothered.

Not advisable for the average DIY'er, but as you seem desperate to know - why not put a meter in series with the earth for a quick test?

 

[JohnW2]

... What we should do with an existing installation is measure leakage, but I have not got any meter which can do that. I have looked at a new clamp-on, cheapest I can find is £40 which can measure down to 0.001 amps and DC. And since retired have not bothered. ... But unless you have one, you have no idea how close to wind one is sailing, we simply guess.

As I said, if one is talking about a sockets circuit (which is generally only where the issue arises) the designer has no choice but to 'guess', no matter what test equipment he/she may have - since they won't have a clue as to what loads,, with what 'normal' earth leakage, will be 'plugged into' the circuit.

What impedance twin and earth is I don't know, not that much difference to 300Ω ribbon cable, but there must be some leakage, it is after all AC.

T+E cable is physically symmetrical, so the 'leakage impedance' (capacative reactance') should be the same between L & E as it is between N & E.

I don't think that I have ever really thought of this before, but, although the 'leakage impedances' to earth are the same for both live conductors, the L & N 'leakage currents' (due to capacitance) will be very different for L & N, since the L-E potential difference is very much greater than the N-E pd. There is therefore seemingly scope for a cable to result in a degree of L-N imbalance.

However, if one does the sums this seems to be a fairly negligible effect. As far as I can make out, the between-cores capacitance of smallish T+E is of the order of approximately 100 pF/m. Hence, for, say, 50m of cable that would be 5,000 pF (5 nF) - so, about 640 kΩ at 50 Hz, resulting in an L-E 'leakage' current of about 0.36mA at 230V. Since the N-E potential difference will usually be fairly negligible, the N-E leakage current (which will reduce the L-N imbalance) is also likely to be fairly negligible but, in any event, the maximum L-N imbalance would be about 0.36 mA for 50m of T+E.

Since L and N currents will be virtually (if not literally) zero, inductive coupling between live conductors and the CPC should presumably not result in any imbalance of L-N currents.

Kind Regards, John

 

[JohnW2]

Not advisable for the average DIY'er, but as you seem desperate to know - why not put a meter in series with the earth for a quick test?

I obviously can't speak for others but, probably because all of my meters (including the 'fairly expensive' ones) are pretty (in some cases very!) old, I don't have any which can measure AC currents in the mA range.

Kind Regards, John