2 Rcds - 1 On Cu, 1 On Appliance. Appliance Rcd Trips While Not In Use

[phil99]

Ok folks this is the situation

All circuits in the house are protected by an RCD from incoming supply. 

They have a whirpool type bath (its called a heritage) and was installed with an RCD socket under bath a spur off upstairs ring. I presume this set up is a standard fit by bath installation contractor.

No moisture or condensation evident under bath. There's no heating element, just a pump, hot water comes from combi.  

Problem:  The RCD socket trips after 3 or 4 bath uses. It never trips while bath is in use. The householder informs me that after 3/4 uses the tub start button doesn't work he then resets socket RCD & all works OK until next few uses

What I've done so far:

Pressed test switch on socket - trips RCD on Consumer Unit as I'd expect.

What I've not done:

IR tests, RCD test

1st question is - I don't get it. Ordinarily the local rcd shouldn't trip should it?

Please advise on how to proceed - given the above & the fact I can't get either RCD to trip I've a feeling the IR & RCD tests will draw a blank.

2nd question  What other avenues could I pursue?

cheers

Phil99

 

[OnOff]

Am I reading this right? You have an RCD protected socket for the bath coming off a ring circuit which itself is protected by an RCD? What are the respective mA ratings for the two(?) RCDs........is one a time delay type?

A tad confused.over the can/can't get them to trip!

..........could be a discrimination issue.

Any pictures?

 

[binky]

would look at removing under bath rcd - not rquired, and then see about rcbo for socket circuit. spa bath controls almost certainly pneumatic (or 12v), would suspect slight bath leak??? The fact it coincides with spa bath may just be coincidence with a build up of earth leakage from other appliances (hence RCBO). Worth isolating cct from CU and IR testing with all appliances unplugged, just to double check its not a latent cct fault.

 

[OnOff]

Steady there...he might have a 100mA TD up front, one might be faulty etc. I'd say we don't know enough to say to remove one surely. What earth type etc???

 

[binky]

nah I use ladders or scaffolding :slap

Steady there...he might have a 100mA TD up front, one might be faulty etc. I'd say we don't know enough to say to remove one surely. What earth type etc???

true, but unlikely chances are 2 off 30ma RCDs ie RCd and socket

 

[kme]

I would NOT remove the second RCD - given the area of use, its more "belt & braces".

As paddler - ramp test em (individually).

Could the under bath unit be going out on overcurrent, if the bath startup draws too much?

( I`ve a whirlpool bath - comes with an RCD in the control box - and, as TT, I`ve 30mA in the DB anyway - but never had any issue.)

Bottom line - if RCD trips, its either seeing something you haven`t seen (yet), or its faulty.

 

[steptoe]

:shakehead

would it not be wiser to actually do some testing first, then come back to us with the results if you are still stuck,

then we may be able to provide pointers,

bare, and I mean bare minimum for RCD faults is, [iMHO]

RCD rating and spec, ie, TD, type A etc

IR
Ze

Zs

trip times at x1 and x5

tripping current, ie RAMP test

 

[phil99]

Thanks for replies, just to clear up queries

I mentioned initially that I thought the reason for the install of the RCD socket for appliance RCD was probably a standard fit - something the bath installer would do at every site - sorta makes sense?

To my mind the appliance(socket) RCD is redundant in this set up. Which is why I don't understand why it's tripping - particulary only when not in use. 

Both RCDs 30mA, Not time delay, nothing fancy. Neither are for DC, just run of the mill 4 bed semi-detached stuff - one supply, no solar etc. 

With 2 RCDs of the same type, the upstream device always trips 1st - I thought.

What's confusing me is why the socket RCD ever trips. I've heard of RCDs; 

1. Melting open or closed - catastrophic failure so they're obviously goosed

2. Degrading so the tripping times are too long

but never developing intermittent fault 

I'll try the tests you mentioned, particularly the RAMP test - maybe the socket RCD has a lower threshold?.

I'll be back there next week - I'll keep you posted, but would appreciate any further ideas 

 

[steptoe]

sounds like the RCD at the bath has a HIGHER threshold,

,

having redundancy is not always a bad thing, just sometimes a waste of money, but not always.

the fault may NOT be at the bath, maybe thats why the bath RCD NEVER trips,  :C

 

[SPECIAL LOCATION]

With 2 RCDs of the same type, the upstream device always trips 1st - I thought.

Nope!...

it would all depend upon exactly how fast and at what current an individual RCD actually trips at??

(I am assuming during normal everyday testing you don't get exactly the same tripping times for all RCDs that you have ever tested?)

I'll try the tests you mentioned, particularly the RAMP test - maybe the socket RCD has a lower threshold?.

 Cant believe you haven't already done this as the basic starting point of your investigations??

You need to prove that each RCD is operating within its spec or not, before you can assume anything!!!

I'll be back there next week - I'll keep you posted, but would appreciate any further ideas 

A steptoe said...

Give us some info and we may be able to offer suggestions..

Otherwise its just guess work!

What I've not done:

IR tests, RCD test

1st question is - I don't get it. Ordinarily the local rcd shouldn't trip should it?

Please advise on how to proceed - given the above & the fact I can't get either RCD to trip I've a feeling the IR & RCD tests will draw a blank.

2nd question What other avenues could I pursue?

RCD's in series its pot luck as to which one or Both operate with any given fault or test!

If you are not going to do the BASIC electrical tests when investigating a fault...

Then other avenues could be.. Try being a plumber or Postman maybe???

:coat

 

[OnOff]

Intermittent................making sure all the connections are tight might be a good idea too!

 

[phil99]

I appreciate the comments food for thought, especially the careers advice from "trailer boy"

I'll post next week.....

cheers

Phil99