RCBO tripping - any thoughts

[polo1]

Hi guys,
am I missing something obvious? Hager 40amp type B RCBO supplying a 9kw shower (actually pulling 36 amps). Whilst shower is on, no matter for how long/short, everything is ok. However after it is switched off the RCBO trips - this is not immediately, it can happen an hour or two, or even longer after. Cable insulation tests fine, all connections (shower, switch, board) good, Zs fine. Only the shower on the circuit. RCBO ramps at 27ma.

I appreciate that the rating of the RCBO is technically too low and a 45 amp would be better, but I can't see this being an overload issue given when the tripping occurs.

As I started, what if anything am I missing?

Regards.

 

[nickblake]

i would check and re check the insulation if you have a intermitant N to E fault it could well cause the RCBO to trip, especially if there is supplimentary bonding going back from the lighting circuit to the shower switch ,ensure the switch polarity is right neons not on all the time , also check the Ze as supply faults can also trip an RCBO just a thought as for the breaker size you probably will not have a problem with it tripping my 10.8 kw shower is on a 40 amp MCB its never tripped in the 4 years its been in

 

[joe j]

I'm wondering if it has anything to do with the neon in the switch. Is that left on?

 

[Richard Burns]

Have you checked the current it is pulling and the earth leakage to see which is tripping.

Is there a high resistance fault in the isolator switch that is pulling 50 amps or so when off?

Is there water leaking into the shower body and slowly reaching the power supply?

Is it switched off at the isolator or just at the shower?

 

[Spazz]

Have you tried IR testing the shower when its on and off?

It does not sound like a over current unless something is drawing large current when it switches off - doubt it.

How new/old is the shower? could a little bit of water of got into it which only crosses contacts when it is being turned off?

other option if you think it might be over current (or to eliminate it) change the RCBO to a MCB of same rating and try that - again I doubt it is an over current

 

[a-z electrics]

Have you tried replacing the RCBO - could be faulty? Also, is the system TT - could be "unstable"?

 

[polo1]

All, thanks for the ideas and feedback. Sorry for the delay in responding, kept loosing my internet connection last night.
Shower is a Mira, not sure how old but looks to be in good nick.
No evidence of water pen or internal leaking.
Earthing is a good TNS, no supp bonding.
Interestingly, the isolator (MK 50 amp pull cord) had been getting left on as "it was stiff to operate". I've since changed this like for like and I don't actually know if the tripping has occurred since then - I'll check, 'cos you've got me thinking about the neon!
Have not tried changing the RCBO yet (don't carry these as spares due to the cost - I know, I'm a tight arse!).
Did not check the earth leakage given when the tripping occurs, but I will do if the problem persists.
Thanks again for the feedback - as always, food for thought!