interesting fault

[studentspark]

Only had a telephone description of a fault. Would appreciate your thoughts.

A small cu has been installed, fed from the incoming tails of main cu, via Henley blocks (before main cu RCD)

This small cu is to power a EV charging point
Small Cu has a 30ma RCD feeding the charge point which has a built in RCD.

Now when the charge point is used, it trips the RCD, not in the small cu, but in the main cu.

Switching off the shed MCB on the main board stops the tripping.

 

[ban-all-sheds]

A small cu has been installed, fed from the incoming tails of main cu, via Henley blocks (before main cu RCD)

Right now my instinct would be to doubt that until you've seen it.

 

[Taylortwocities]

I agree, doubt the phone call. Who actually installed it? You, the householder, 5DW? Has it ever worked properly?
I would look carefully at the neutral wiring, everywhere….

 

[bernardgreen]

I appreciate you only have telephone contact so details not available.

My thoughts are a fault in the shed creating a path through ground to the charging point which probably has a local ground rod to its metal work. (if not the body of the vehicle being charged will have a reasonable path to ground via its anti static tyres ). If the charging point is double insulated then the potential of the metal work ( charger and / or car ) would be pulled to around 115 volts ( mid point Neutral to Live via capacitive coupling ). This could be dissapted by a current going via ground to the fault in the shed and thus back to the RCD in the main CU.

 

[deadshort]

Mm, what's plugged in in the shed then ?

DS

 

[securespark]

Yeah, check the shed appliances and bell out the circuit.

 

[JohnW2]

Now when the charge point is used, it trips the RCD, not in the small cu, but in the main cu. Switching off the shed MCB on the main board stops the tripping.

Doesn't that suggest the possibility of a connection between neutrals of the 'shed circuit' and the EV charging one? In that situation, one might expect both RCDs to trip, but funny things happen. Given that the current in the charging circuit may be quite large, I suppose that a relatively high impedance connection between neutrals could do it.

Kind Regards, John

 

[ban-all-sheds]

My money is still on a gross wiring mistake rather than an obscure fault.

 

[deadshort]

"Bell out" -ah that takes me back to the good old days!

DS

 

[ban-all-sheds]

No - you misheard - it wasn't "bell out" they were calling you behind your back.......

 

[JohnW2]

My money is still on a gross wiring mistake rather than an obscure fault.

You're very probably right ("common things are common") but, if your comment was in response to my last post, I would have thought that a connection between the neutrals of two circuits might qualify as a "gross wiring mistake", mightn't it?

Kind Regards, John

 

[ban-all-sheds]

It was just a reply to the seemingly increasing number of hypotheses.

Best thing is to wait for the results of the Mk 1 Eyeball Test.

 

[deadshort]

"Bell out" -ah that takes me back to the good old days!

DS

My, we are on form to-day BAS , have they changed your medication ? Long may it continue, baby steps , by christmas who knows ? you may not require to follow ALL eleven steps

Kind regards AH,

DS

 

[studentspark]

Having a look tomorrow will post findings
Thanks all

 

[studentspark]

Turns out to have nothing to do with the EV charging circuit.
Neutral earth fault on shed circuit - simple fix.
I think it was one of those situations where the home owner wanted to blame the last person in.
'All was fine until the charger was put in.....'
Turns out charger circuit had been working fine for a month anyway before fault developed.