PME break in neutral and RCDs

R

Roger Hayter 8 years ago

Following a discussion in another place I realise I am confused by this.

Consider premises with PME earthing which are currently suffering from a broken neutral wire with no adequate earth remaining connectd. Neutral swings to live potential. This is a well known theoretical problem if rare and unlikely in practice.

Consider a potential electric shock when touching live (or neutral) while being earthed by some extraneous earth, say in the garden for instance. AFAICS a double pole 30mA fast RCD will protect you but a single pole RCBO won't.

Further consider a potential electric shock from touching the case of an electrical appliance, connected to the protective earth of the premises, while connected to an extraneous earth. I don't think either of the RCD devices will protect you as they don't switch the protective earth connection. But will the RCD be triggered? And does it depend on whether the current comes predominantly from the index premises or a neighbouring one?

Am I over-thinking this?

Vote

T

tabbypurr 8 years ago

AIUI this is why new PME installs now require a local earth rod too

it would cut the power to the circuit you're touching, if it's RCDed/RCBOed . It would trip the main RCD if fitted. But if there is no main RCD, just a ll RCBOs, you're ok if getting a L/N shock, but I think you're out of luck if getting a shock from the CPC.

no such animal

Whichever RCD is supplying the shock current will trip. If one circuit lack s any RCD/RCBO, you may be frying tonight.

IRL there are much less safe wiring schemes in widespread use out there.

NT

Vote

J

John Rumm 8 years ago

Its questionable that even a double pole RCD would help - since its in effect not seeing any power to its electronics (i.e both sides of its supply are at the same 240V potential).

(although you might seem some fortuitous earthing from your main EQ bonding)

This is one of the reasons they take care to export the equipotential zone along with the PME supply when possible.

Probably not.

This is why the "multiple" bit of PME matters - there should be lots of parallel paths to true earth along the supply installation to try and mitigate these situations. There is also the suggestion that new PME installs may also require a local earth connection like a TT install.

Over thinking, no. Over worrying, perhaps.

Vote

M

Mike Humphrey 8 years ago

So this -

formatting link

- doesn't exist? Actually I can't find any double-pole RCBOs for domestic consumer units, so please let me know where you found them. Every one I've ever seen is single pole.

Mike

Vote

T

tabbypurr 8 years ago

To work it has to monitor current in both wires, not one. Both L&N pass through it.

NT

Vote

J

jrwalliker 8 years ago

Yes, but it only has a single pole switch so it won't disconnect you from a floating neutral that is being pulled live by other loads in the premises. John

Vote

T

tabbypurr 8 years ago

a floating neutral that is being pulled live by other loads in the premise s.

The current that goes through the user generally comes from an RCD somewher e, even if not the one on the circuit supplying the shock. That RCD will tr ip, so you might get a shock off a socket circuit yet something else trips. If all RCBOed circuits contribute to shock current you might find they all trip - albeit at a lot more than 30mA.

NT

Vote

R

Roger Hayter 8 years ago

(Not from our plastic oil tank and plastic water pipe.)

The point about needing power to actually operate the trip, which won't be available if the neutral is close to live voltage is a very good one I handn't thought about. Thanks for pointing it out.

Vote

A

ARW 8 years ago

Only just spotted this one thread.

formatting link

Whilst not answering your question is still well worth watching.

Vote

PME break in neutral and RCDs

Join the Discussion

Didn't find your answer?