Strange RCD fault II

[securespark]

Went to an RCD fault yesterday.

The RCD was tripping intermittently.

On testing the RCD from a socket outlet, it refused to trip on the 1 x test.

The Zs was ok (0.9), Ze 60.9 (subterranean TT).

When testing the unit in isolation, it performed perfectly.

Disconnected each circuit in turn. On disconnecting the lighting, the RCD again tested perfectly.

Found a dead N/E short on the lighting which, when rectified & lighting reconnected, sorted the problem.

Now here's an extra twist.

One socket in the lounge is used for the vacuum. Every time it was switched on using this socket, the RCD used to trip (before we rectified the lighting fault). In any other socket, it would work OK. The socket had a good Zs (0.92), polarity OK & connections good & tight.

What's that all about

 

[Spark123]

Is it the MEBs causing such a low Zs?
With the dead N-E short in the lighting circuit may have been enough current present with the start up surge from the vacuum cleaner to cause enough current to flow through the N-E fault bypassing the RCD causing the trip.
What sort of setup 100mA(S) / 30mA split load? Which ones were tripping?

 

[securespark]

Yes. Disconnect MEB's & EFLI rises to 60ish.

Possibly, but why only from one outlet?

30mA G type incomer tripping. No other RCD's fitted.

 

[Spark123]

Wasn't just the kick of the vacuum cleaner motor kicking in causing enough current to flow though the fault causing it to trip? How did you check this socket?

 

[fumb]

Could it be that socket is worn and arcing extending the switch on surge from the cleaner past the trip time ?

 

[dingbat]

Have you tried testing the actual socket, in isolation? The other week I tracked down an RCD trip to a single socket with low-ish resistance between its terminals. It only came to light when several computers were running at once; not enough earth current themselves to cause the trip, but added to the continual earth leakage via this socket it did the trick. It may be that a similar fault on your guilty socket diverts some neutral current to earth via the lighting circuit short?

Just a thought....

 

[bernardgreen]

One socket in the lounge is used for the vacuum. Every time it was switched on using this socket, the RCD used to trip (before we rectified the lighting fault). In any other socket, it would work OK. The socket had a good Zs (0.92), polarity OK & connections good & tight.

What's that all about

Was there any equipotential bonding taken from that socket ? Creating a link to the lighting earth via pipe work may have made that socket "special"

 

[securespark]

123, fumb, ding & bernard

Thanks for your input.

No doubt these are all potential scenarios - I'll give them all a bit of thought.

Bernard - no there was no supp bond to make this socket special...

 

[viewer]

Doubt this helps, but...I'm curious.
Had something similar last year. Old stone cottage. Kitchen sockets tripped RCD, no others. Found no problems with circuit or unexpected difference between kitchen and other rings. Traced problem to fault in a ceiling rose upstairs (intermittent short due to L/E copper whisker across terminals). Once lighting isolated no problems. Never did work out why the kitchen was implicated, but as above, fridge, washing machine have high switch-on currents - but so what, they are completely different circuits. Bonding questionable. It was TT would this have any bearing?
V

 

[bernardgreen]

One socket in the lounge is used for the vacuum. Every time it was switched on using this socket, the RCD used to trip (before we rectified the lighting fault). In any other socket, it would work OK. The socket had a good Zs (0.92), polarity OK & connections good & tight.

This may be a result of a static electrical charge between vacuum and carpet being discharged along the live and neutral and passing through the sensor of the RCD. As the discharge currents on live and neutral are not anti-phase and self cancelling they will be seen as fault currents.

It would take quite a large static discharge to trip a 30mA RCD but if there were pre-existing fault currents from another near fault then the static discharge could take fault current high enough to trip the RCD.

 

[securespark]

Cheers, Bernard!

 

[sm1thson]

but why just that socket? -it could be just that carpet?

 

[bernardgreen]

but why just that socket? -it could be just that carpet?

Well yes, so spray it with either expensive anti-static treatment or a dilute solution of Comfort Fabric softener which used to give about 2 to 3 weeks of static free carpet. If tjey haven't changed the formula it should still work.