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!

 

[pc1966]

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?

Quite a lot of meters have peak-hold and similar. But they often have an underlying response time in the tens or hundreds of ms as part of the usual noise filtering (i.e. rejection of AC ripple on DC, etc). So if it is a surge like motor starting over several AC cycles they would pick up on most of it, but I'm not so sure about a half-cycle charging of filter caps, etc, where they might underestimate by a significant degree.

I am sure you get kit for that job, but it is easier to had a sales feature to an existing measurement system than to engineer in fast capture capabilities!
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Another thought I had to reduce the switch-on surge was to use a set of three solid state relays (on an adequate heat sink!) that have zero-crossing switching.

The first SSR is switched by the incoming single phase supply (so L1 load only comes on at the next zero-crossing of the supply), L2 is switched by L1 load coming on, and L3 is switched by L2 load coming on.

Unfortunately I don't have any to play with, but my hope is you would get the three-phase load coming on as:

L1 - (10ms delay) - L2 - (10ms delay) - L3

Also as they are not coming on at a voltage peak the dV/dt is less so hopefully reduced surge on any filter capacitance L-E.

 

[static zap]

Am I interpreting this right ?
a) 1 Phase present
b) Tripping happens if RCBOS on non present phases are left Active. (ie turned on).
c).. Feels like leakeage at some notional Unbalanced wye point ..
d) Is the correct action , to leave the the two non present breakers off.
e) (an automatic fix - may be possible - But it will basically achieve the same as leaving 2 RCBOs turned off)

 

[marconi]

-MAtt: Good morning Sir. This was an interesting problem. I just wondered if there was any more you could say on it since you last posted? I think we would all like to know what racing machine it is for too? Pictures are always good

M