Neutral-earth fault on lighting circuit with RCBO

[rattlehead85]

Located a fault between neutral & earth on a lighting circuit consisting of pvc singles run through steel conduit run between two bedrooms. The point i don't get is the device was an RCBO which apparently has never tripped. I checked the device applying an RCD test which passed and the trip mechanism works fine. Ze value of the installation was good and also the circuit earth continuity was integral.
Any theories as to why this RCBO has not tripped under fault condition present?

 

[Brightspark2]

What was the fault?...

Not 30mA flowing??...

 

[onions1066]

Might sound stupid but why was you testing it if the rcbo hadn't tripped or was it found on an eicr ?

 

[rattlehead85]

Might sound stupid but why was you testing it if the rcbo hadn't tripped or was it found on an eicr ?

Was found during a minor works light and fan replacement in a shower room.
Tested circuit before carrying out the work and found it.

 

[rattlehead85]

What was the fault?...

Not 30mA flowing??...

0 Megohms N-E and i did disconnect from the RCBO before carrying out the insulation testing so i know i wasn't reading through the device or service PME.

 

[Brightspark2]

But what or where was the fault?...where did you find the short?...

 

[MDJ]

Is the earth lead from the RCBO connected to the earth bar, purely out of interest?

 

[Guest55]

Is the earth lead from the RCBO connected to the earth bar, purely out of interest?

not all brands use a functional earth fly lead.

;-)

 

[MDJ]

not all brands use a functional earth fly lead.

;-)

I know that Bud, but the older versions for most boards do, Just a thought really.

 

[polo1]

You say "0 Mohms" - did you try to measure the actual resistance?
Regards

 

[rattlehead85]

But what or where was the fault?...where did you find the short?...

Turned out believe it or not that the fault was a twisted bare neutral somebody had joined and pulled through the conduit when the lighting was originally wired!!! The fuseboard had been upgraded last year. Goes to show how much testing was done either pre or post board change that this hadn't been flagged up when it was done.

 

[NickD]

Although that doesn't explain the failure of the RCBO to trip presumably?

Was the conduit definitely earthed?

- - - Updated - - -

Oh sorry you said earth continuity was integral. Ignore me.

 

[telectrix]

if the leakage resistance was >8kohms, the RCD would not trip, but your IR test would read 0.00 Mohms.

 

[Deleted member 9648]

I've known plenty of instances where RCD's dont trip on a N-E fault....seems random/pot luck sometimes,I just accept thats the way it is!