Two parallel RCDs flick off at the same time - earth capacitance - any pointers?

[WilcoxElec]

I need to rectify a fault where an outside SPA hot tub is supplied through a length of 3 or 4 meters of buried armoured (shielded) cable and – when the MCB is closed – after 10 seconds the 30mA type A RCD above this MCB opens – and so does another in parallel with this on a separate UK style CU. It is a TT schema installation here in France. I was wondering that I might have some sort of earth capacitance problem, rather than a straight leak to earth – ( because both RCDs flicking off – after the same pause). Could it possibly be caused through the sheathing on the armoured cable being connected to the earth?

 

[Grants]

I would put the spa on its own cu with a RCD fitted, like a shower unit.
then fit a time delayed RCD for the main cu. That should stop the upstream RCD tripping but you still need to rectify the fault on the spa circuit. Did it test ok

 

[WilcoxElec]

The second RCD is not "upstream" it is actually at the head of a separate unit in parallel to the one which is above the MCB for the hot tub supply. Going out there tomorrow to have aa look. I can test the RCDs separately at 30 mA then, is that what you meant? :smiley2:

 

[Grants]

What were the results of the initial tests done, ins. Resistance, earth loop. Do you have a meter that will show how many mA it takes to trip the RCD.
So by your reply the SPA is fed from a separate unit, is this connected directly to the mains or is it from an MCB. Sorry if this is of no help, sometimes you need to be there to see for yourself.

 

[WilcoxElec]

Both units are fed directly from the mains the supply goes to the French unit first, and the second UK CU is supplied from above the headr RCD on this, the french board. The original DIY fitted UK CU (with monopolar MCBs )- headed by the second of the two RCDs - the hot tub was not connected to that - and the client wanted it supplied. Since here it needs its own bipolar MCB and we can't connect anything new to the UK CU this was put in a separate French board under a new 30A MCB. I can test that the RCDs cut off with a 30mA leak to earth, but haven't got anything to specifically test at what precise value they each open.

SHould I do a circuit diagram?

I just need to establish whether there is a problem in what I have done - in connecting the sheilding of the armoured cable to earth say -- or is it the SPA unit itself?

 

[vernam616]

Call an electrician perhaps?

 

[Grants]

No you have done it correctly the swa of the cable should be connected to the earth bar. Do you have any paper work that came with the spa, it may shed some light on the problem ie. do not connect to an RCD.

 

[Grants]

Call an electrician perhaps?

Who you gonna call ghost.......never mind

 

[RogertheBodger]

Both units are fed directly from the mains the supply goes to the French unit first, and the second UK CU is supplied from above the headr RCD on this, the french board. The original DIY fitted UK CU (with monopolar MCBs )- headed by the second of the two RCDs - the hot tub was not connected to that - and the client wanted it supplied. Since here it needs its own bipolar MCB and we can't connect anything new to the UK CU this was put in a separate French board under a new 30A MCB. I can test that the RCDs cut off with a 30mA leak to earth, but haven't got anything to specifically test at what precise value they each open.

SHould I do a circuit diagram?

I just need to establish whether there is a problem in what I have done - in connecting the sheilding of the armoured cable to earth say -- or is it the SPA unit itself?

Are you sure that the rcd's are in parallel? If they are in fact in series, a fault to earth on the spa could cause both rcd's to trip at the same time.

 

[WilcoxElec]

Definitely in parallel - by which I mean, one is neither upstream nor downstream from the other...

 

[Richard Burns]

If you truly have two RCDs in parallel then there will be a choice of pathways for the current to flow and the differential could well be over 30mA and so both will trip irrespective of the load applied.

 

[WilcoxElec]

I don't know where the Hot tub came from, it is not French, must have been brought across from the UK.... a notice inside says that it should be connected *by a qualified eklectrician" - which I am, behind a 30mA type "A" RCD - which it is. I have been given a few things to look for in the case of - "déclenchements par sympathie" of two RCDs and I expect I shall get to the bottom of it tomorrow...... Thanks. :vanish:

 

[Grants]

Hope you get to the bottom of this problem. Let me know your findings it will be interesting to see what the problem was. Good luck.

 

[WilcoxElec]

It was a fault on the "circulation motor", easily isolated, except that is was really hot and sunny and I had forgot my hat! :rolleyes2:. Looks like moisture had got it to it - and it was grilled - resistance of 200 ohm live - earth - and 300 ohm - Neutral connection - earth --. So no real mystery why both RCDs opened as soon as the motor was energised by the control card .... The second RCD actually only opened in sympathy when it was under a little load, otherwise, it remained closed ....... However I found some evidence that the whole thing had been dismantled and put back together - not entirely correctly - - or at least not completely according to the circuit diagram I managed to get. .... Anyway, the supply circuit I put in all checks out fine. It was nothing complicated - . Thanks for the support anyway. That is another post down towards the next level. I shall try to find something more interesting for my next........ :sunny:

 

[Grants]

Good to hear you got it sorted. They all seem easy after you find the problem.