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Topic Title: RCBO tripping problem.
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Created On: 10 September 2013 12:40 PM
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 10 September 2013 12:40 PM
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Garya

Posts: 124
Joined: 08 April 2005

I have a little problem, and don't know if it's something you can advise on.

I have a customer where I have fitted 15 low energy, mains LED downlights, probably not even using 150 watts, and switched in 2 groups.
He also has 3 transformers which are to power strips of LED lights under his kitchen units, these are on the same circuit, a different switch (3 gang supplies all the lights), and further isolated through 3 fused spurs, 1 at each transformer, but all fed from the 3 gang lighting switch.

The 6Amp RCBO feeding the circuit is intermittently tripping, there is no earth fault, I have checked the live to earth insulation readings at 500mOhm +.
It almost always trips when the transformer fed LED lights are switched on, especially if the ceiling lights are switched on at the same time. The ceiling lights alone don't seem to trip it, unless you have switched on or off the LED strip transformers a fraction before.

These LED transformers take a few seconds to actually go off and on, seeming like there is some capacitor dischage going on with the LED strips.

I have a reading, Zs, of 1.54 Ohms, which was enough to make me fit a type C 6 Amp RCBO, to see if it would resolve the matter. It has made little difference, and the RCBO still occasionally trips with the transformers being turned on, especially with the ceiling lights at the same time.

Could this be a surge problem? The transformers are barely being loaded, and the load (tested with clamp on ammeter) being used when all the lights are on is 0.5 Amp.

I'm not sure where to go next. Unless one of ther transformers has a fault.

Have you come across this before?
 10 September 2013 12:44 PM
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daveparry1

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It certainly sounds like an "inrush" problem which i'd have thought the C type would have solved. Subject to Zs being suitable I reckon a 10 amp rcbo will be the answer,

Dave.
 10 September 2013 03:35 PM
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Garya

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Further to this. I have removed the type 'C' as I have some doubts as to the protective conductor size being sufficient, I have 1.5mm live/neutral and 1.0mm CPC cable. As it shows in table B7 of OS/Guide, the minimum is 1.5mm CPC with a 'C' type. I will get the manufacturer's (Protek) recommendations before putting it back in.
Gary
 10 September 2013 04:02 PM
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AJJewsbury

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Further to this. I have removed the type 'C' as I have some doubts as to the protective conductor size being sufficient, I have 1.5mm live/neutral and 1.0mm CPC cable.

You should be OK as long as the c.p.c.'s Iz >= In and the MCB's breaking capacity is OK for the PEFC - see reg 435.1. (As I see it, the thermal side of the MCB (6A) should protect a 6A or higher rated conductor, regardless of the magnetic side's energy let-though.) If that wasn't the case you'd be wiring 6A circuits in 4mm2 for a 10kA rated C type.
- Andy.
 10 September 2013 04:34 PM
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Garya

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Odd about the tripping though, and the three transformers. I have tested the insulation again, and still find no earth fault, which makes me think it is the inrush current. Surely a 'C' should be enough, but maybe it does need a 10Amp, and maybe still a C? 10A at C looks to be still within the max earth fault loop, as according to OSGuide it is OK up to 1.85 ohm.

The cable derate is 0.85, and seems OK. So maybe a 10A is the way to go. Just seems odd to me.

Edited: 10 September 2013 at 04:51 PM by Garya
 10 September 2013 04:49 PM
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AJJewsbury

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I have checked the live to earth insulation readings at 500mOhm +.

Silly question perhaps, but have you checked N (or L+N together) to earth? Just thinking that a (highish resistance) N-PE fault might produce the same symptoms (trips RCD side when v.d. takes the N voltage up a bit - highest during inrush).
- Andy.
 10 September 2013 04:54 PM
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Garya

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The last test of L and then N to earth showed over 600 mOhm, didn't test L-N (that was done originally with no lamps fitted and no transformers fitted, and was fine).

Was wondering whether replace the RCBO and have a standalone 2 pole RCD by the side of CU, and a normal MCB in CU.
 10 September 2013 05:10 PM
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AJJewsbury

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Was wondering whether replace the RCBO and have a standalone 2 pole RCD by the side of CU, and a normal MCB in CU.

I guess that would make it easier to see whether the problem is overcurrent or residual (leakage) current, but other than that (in theory at least) it shouldn't make any difference. Manufacturing tolerances might be different, but that might make things better or worse.

If it were mine, I'd be looking at replacing the transformers I think - for decent quality soft-start kind. Maybe switching the FCUs off one at a time to see if the problem can be narrowed down to a particular unit or number of units switched on at the same time.

- Andy.
 10 September 2013 05:17 PM
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Garya

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Yes, I've suggested that to the customer, isolating sections and see how it goes. He supplied the Transformers and LED strips, which can be cut to length, apparently they are also used in aquariums. Maybe I should get details of manufacturer from him. He got off internet I think.
 10 September 2013 05:52 PM
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daveparry1

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Why not try fitting a B or C type mcb? that will prove whether or not it's inrush or an earth fault. In fact, is there any need to have an rcbo there at all? I guess its because there are cables buried <50mm below the surface?

Dave.
 10 September 2013 05:58 PM
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Garya

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Yes, buried cables, new installation, kitchen, the works.
I suppose we'll try the isolating sections route. See what manufacturer (if I can find them) recommend for their transformers, maybe they are known for this problem, maybe it is something to do with an imbalance between L&N & Earth when transformers fire up, who knows.
 10 September 2013 06:06 PM
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daveparry1

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I've never had this problem with electronic transformers, only wound and toroidal types,

Dave.
 10 September 2013 06:28 PM
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Garya

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Oddly, one transformer stopped working for a bit, as I was switching on and off to see if the type C had solved it. I went up and tested the supply, checked the fuse in FCU, there was power going to the transformer, it has like a kettle lead affair going into it, tested at the ends. After a bit, it started working again, reset itself. Like it had to cool down, or didn't like going on and off, but it wasn't particularly warm.
 10 September 2013 06:38 PM
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iomtt

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Had a similar problem


30 ma main trip and fitted 10 led robus tri down lights( led built in )
Ever since we fitted them , and they used the induction hob this did trip the main trip
Obviously checked everything and as it happens changed the db later anyway to rcbos (hager)
To be fair it was a square d board and goes down in history as perhaps the devils own invention. I'm sure he would be using one down there
All these electronics and leakage currents
You need to measure your leakage to earth

Ramp test the rcbo and see what she trips at first as this is an easy test
Thinking of easy things first
 10 September 2013 06:51 PM
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michaelbrett

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Originally posted by: Garya

I have a little problem, and don't know if it's something you can advise on.



I have a customer where I have fitted 15 low energy, mains LED downlights, probably not even using 150 watts, and switched in 2 groups.

He also has 3 transformers which are to power strips of LED lights under his kitchen units, these are on the same circuit, a different switch (3 gang supplies all the lights), and further isolated through 3 fused spurs, 1 at each transformer, but all fed from the 3 gang lighting switch.



The 6Amp RCBO feeding the circuit is intermittently tripping, ..........

It almost always trips when the transformer fed LED lights are switched on, especially if the ceiling lights are switched on at the same time. The ceiling lights alone don't seem to trip it, unless you have switched on or off the LED strip transformers a fraction before.



These LED transformers take a few seconds to actually go off and on, seeming like there is some capacitor dischage going on with the LED strips.



Have you come across this before?


A number of manufacturers (can't remember whom at the moment) recommend a max of 10 LEDs per circuit breaker because of inrush issues.

You might want to contact the LED manufacturers and get their view before you do anything else.

Regards

Mike
 10 September 2013 07:07 PM
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Garya

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Ramp test. Yes, I hadn't done that. I thought it odd that two new RCBO's a B and a C both have the same problem.

This is a new CU, all RCBO's as the customer requested, as he was having a lot of main switch rcd tripping with an ancient poorly organised CU, it was all done sort of upside down (busbar at top, rcd fed at bottom, bit of 16mm bent across from top of RCD to first MCB and busbar), a right bodge up, someone had adapted it. But it would trip regularly, and yet there seemed to be no fault to find.

So the kitchen lights are isolated from the rest of the circuits now, and are new anyway (there's one bit of original cable used in the wall, but no fault there).
I believe when it was tripping previously on the old CU, he did change some lamps in some other downlights, and it temporarily cured it.

This customer is very techie, he likes his gadgets. 15 LED style downlights, good energy saving.

Edited: 10 September 2013 at 07:33 PM by Garya
 10 September 2013 07:27 PM
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Garya

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"A number of manufacturers (can't remember whom at the moment) recommend a max of 10 LEDs per circuit breaker because of inrush issues.

You might want to contact the LED manufacturers and get their view before you do anything else." (couldn't do quote...it's not like on other forums I post on!)


Thanks.
He supplied the LED lamps to fit a standad GU10 fitting, I'll see if I can find out. However, them on their own don't seem to trip it, just when the 3 transformers are added to the mix.
 10 September 2013 10:11 PM
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michaelbrett

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What type of transformers are they? Electronic or magnetic wire wound? If the later, i would expect tripping when added to thelighting circuit. Magnetic wire wound have a much higher in-rush current characteristic

Regards

Mike
 11 September 2013 06:46 AM
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Garya

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TBH as he had supplied them, I didn't look too closely at the details, just that the current/voltage etc was OK, however, I'm almost certain they are electronic (to the point that I hadn't even considered they might be wire), as they have little weight and are for simple LED strips of lights (come as a kit), like a tape almost for going into aquariums. They have a kettle style push in lead, which I might suggest the customer pulls out on each one at different points to see if it resolves the problem. I was thinking of capacitors charging/discharging simultaneously, wondering whether it might upset an RCBO.
The intermittent tripping happens when these are turned on, and usually if the centre LED downlights are already on.

Will suggest the customer does a process of elimination, if these prove to be causing, or to be contributing to, the tripping, he might have to rethink.
 11 September 2013 09:36 AM
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AJJewsbury

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They have a kettle style push in lead

Do they have an earth connection? It's just that I've just had problems with a HF fluorescent ballast - (i.e. another case of electronics driving lighting) that has been OK for years, but just recently decided to trip RCDs somewhat randomly. Insulation test read about 0.3M L+N to E (which isn't right but doesn't of itself account for tripping a 30mA device) - so clearly is behaving differently (leaking to earth) when powered up.
- Andy.
IET » Wiring and the regulations » RCBO tripping problem.

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