Weird RCD Issue...

[-Matt]

We have a race trailer in which runs off both single phase and 3 phase.
A contactor for each, which activates when plugged in, with the single phase all phases linked.

The DB has a 100ma upfront RCD incomer, then 30ma Europa RCBO's.
In 99% of scenarios there will also be a 30ma RCD on the supply to the trailer, as this is what most track side supplies have, or off generator. So there is no selectivity, but this is normal for this industry.

All works perfectly fine on 3 phase.

However on single phase it will trip the rcd in the workshop on startup.
However if i leave 2 B20 RCBO's turned off, all works fine.

Have tested all final circuits, all good.

I have also tried swapping circuits around, by putting different circuits onto these 2 rcbo's, and it is still these 2 RCBO's that trip! If there is no loading on these 2 all starts up fine.

So the problem seems to be linked to these 2 RCBO's when loaded on startup.



Is it possible for there to be a fault with these 2 RCBO's that is leaking current to earth (they have earth leads), or some imbalance?

But then why are they fine on 3 phase.... (the 3 phase in workshop has 30ma RCD).


Can anyone think of anything?, as its driving me nuts trying to diagnose!

 

[DefyG]

The obvious fault usually the neutral lead in the wrong neutral bar?

 

[pc1966]

Are they the RCBO types with the earth flying lead (as well as neutral)?

 

[-Matt]

The obvious fault usually the neutral lead in the wrong neutral bar?

Both Neutral bars are linked

Are they the RCBO types with the earth flying lead (as well as neutral)?

Yes they have the earth leads, which are terminated.

 

[pc1966]

I suspect when they are powered on the initial surge powering the electronics is tripping the 30mA RCD. With 3 phase each RCBO set would spike at a different time, on single phase all power simultaneously.

You might find a different style (no flying earth) avoids the issue, but your board might only take that style.

 

[-Matt]

I suspect when they are powered on the initial surge powering the electronics is tripping the 30mA RCD. With 3 phase each RCBO set would spike at a different time, on single phase all power simultaneously.

You might find a different style (no flying earth) avoids the issue, but your board might only take that style.

There is a lot of LED lighting (around 700-800W), and they never use the light switches, so it all comes on at once, along with 4 fridges, A/C, Air compressor, plus anything plugged into sockets.

So there is a fair bit of power when plugged in.

 

[pc1966]

If it trips with the down-stream stuff switched off then it is the RCBO doing that, and it might be they have a surge to Earth.

What you could try (with care!) is to wire the RCBO earth flying leads to neutral (obviously having checked all is well with polarity, etc) and test it to see if it still happens. If not you have the culprit!

Don't forget to change back afterwards. I know many RCBO don't have the E lead but I don't remember off hand how important it really is to those that do.

 

[Lucien Nunes]

That's a fair chunk of load to switch on at once, bearing in mind that many of those items will have suppressor capacitors connected L-E and N-E which cause an impulse of capcitive leakage when switched on or off due to the fast rate of change of voltage. The size of that impulse will vary according to which order the poles of the supply make contact as the switch operates, typically being greater if the line closes first as a momentary line-voltage pulse then also appears across the N-E capacitors. Switching both poles of a circuit that has borderline high leakage, using a switch with non-sequenced contacts, is thus a potential cause of random tripping without any actual fault being present. If the single-phase source selection contactor happens to make first on the line pole, that could push the leakage impulse over the tripping threshold.

On 3-phase, as PC1966 notes, the effect will be different and the leakage typically much smaller, since the loads on the different phases have their lines at different instantaneous voltages and the currents tend to cancel out instead of adding.

None of this explains the strange relationship of those two RCBOs to the problem, but I am suspicious that they may be a red herring; otherwise they are presumably the source of at least some of the overall imbalance.

It would be good to get some readings. You might be sailing close to tripping at the best of times, in which case an isolating transformer for the worst offenders could be a useful insurance policy.

 

[marconi]

It would be illuminating to see what happens when you swap over the incoming line contactor for one of the tripping RCBOs with that for the non-tripping RCBO. See red swap in my diagram attached.

Then restore back to 'as was' and do the same thing for the other tripping RCBO. See blue swap in diagram.

I wonder if it is the difference in dynamics of the incoming contactors causing the problem on single phase. This little test might discover that.

In my diagram I assumed R2 and R3 are the ones that always trip on power up.

Next, having restored to 'as was' what I would then do is supply all three RCBOs from the output of each contactor in turn to see what happened. There maybe something about the contacts on one or two of the contactors causing the problem.

Noting all the equipment is single phase what you may have to do is introduce some delay on relays to sequence the closure of line contactors upon power up - easy to do - so that the electrical transients are separated in time. Doing this would be kinder to the electricity supply whether mains or from generator.

Buy delay on relay online from RS Components - https://uk.rs-online.com/web/c/?sra=oss&r=t&searchTerm=delay+on+relay
[automerge]1589954268[/automerge]
What csa and length is the cable between the van and the source of supply?
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The residual current in the neutral when powered by 3 phase supply will be smaller than when powered by single phase supply.

 

[-Matt]

Thanks, Will have a play with when i get to work.

The landline is 6mm HO7, around 15M long, its a 32A commando.

I did order 2 new Europa RCBO's from RS (at £40 each!), which may turn up today, so will also try swapping these over.

 

[marconi]

6mm 4 core flex has a vd of 6.7mV/A/m. So 15m of it has a neutral conductor vd of 0.1V/A or 1 Volt for every 10A. For 20A through each RCBO and SP supply vd neutral is 3 x 20/10 x 1 = 6V

It has a CCA of 32ishAmps. Might the neutral be undersized when operating from a single phase supply? 3 x 20A = 60A?

http://www.aeicables.co.uk/literature/CurrentRatings.pdf

Do you have a DVM with capture mode to examine the N-E voltage at power up. Some RCBOs use a rise in N-E voltage to detect loss of neutral and initiate a trip. That is why they have a functional earth.

 

[pc1966]

Do you have a DVM with capture mode to examine the N-E voltage at power up. Some RCBOs use a rise in N-E voltage to detect loss of neutral and initiate a trip. That is why they have a functional earth.

Most DVM would not see that, unless they are fancy scope-style models.

But your observation about the 1/3 size neutral in the single phase case, coupled with the big inrush from the loads listed above, could well be the explanation!

Temporarily putting the RCBO earths on neutral as well ought to answer that point, and you suggestion for a timed start-up of the phases might also save the day longer term. As an aside, some computer rack supplies offer sequenced start up for similar reasons.

 

[Lucien Nunes]

Marconi and I seem to have slightly different interpretations of the description, perhaps the OP can clarify: Notwithstanding the comment:

still these two RCBOs that trip

I understood the problem to be that the overall RCD feeding the shoreline in the workshop trips, if these RCBOs are connected to loads. Do the RCBOs also trip?

 

[marconi]

Seriously - I like a bit of ambiguity in a problem description - adds to the fun

PC1996 - I don't have one - but don't some of the Fluke DMM have the peak or min capture modes for this type of investigation?

 

[Strima]

What happens when you bypass the contactors and link the phases directly for single phase?