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?






--

Roger Hayter

On Thursday, 28 September 2017 01:09:03 UTC+1, Roger Hayter wrote:

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.

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

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

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 all 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.

single pole RCBO won't.

no such animal

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?

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

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


NT

On 28/09/2017 01:08, Roger Hayter wrote:
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.

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)

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.

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

But will the RCD be triggered? And does it depend on
whether the current comes predominantly from the index premises or a
neighbouring one?

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.

Am I over-thinking this?

Over thinking, no. Over worrying, perhaps.


--
Cheers,

John.

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| Internode Ltd - http://www.internode.co.uk |
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wrote:
On Thursday, 28 September 2017 01:09:03 UTC+1, Roger Hayter wrote:
single pole RCBO won't.

no such animal

So this - https://www.tlc-direct.co.uk/Products/WYNHXSBS6B.html -
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

On Thursday, 28 September 2017 11:36:08 UTC+1, Mike Humphrey wrote:
tabbypurr wrote:
On Thursday, 28 September 2017 01:09:03 UTC+1, Roger Hayter wrote:
single pole RCBO won't.

no such animal

So this - https://www.tlc-direct.co.uk/Products/WYNHXSBS6B.html -
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

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


NT

On Thursday, 28 September 2017 13:50:36 UTC+1, wrote:
On Thursday, 28 September 2017 11:36:08 UTC+1, Mike Humphrey wrote:
tabbypurr wrote:
On Thursday, 28 September 2017 01:09:03 UTC+1, Roger Hayter wrote:
single pole RCBO won't.

no such animal

So this - https://www.tlc-direct.co.uk/Products/WYNHXSBS6B.html -
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

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

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

On Thursday, 28 September 2017 14:18:59 UTC+1, wrote:
On Thursday, 28 September 2017 13:50:36 UTC+1, tabby wrote:
On Thursday, 28 September 2017 11:36:08 UTC+1, Mike Humphrey wrote:
tabbypurr wrote:
On Thursday, 28 September 2017 01:09:03 UTC+1, Roger Hayter wrote:
single pole RCBO won't.

no such animal

So this - https://www.tlc-direct.co.uk/Products/WYNHXSBS6B.html -
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.

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

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

The current that goes through the user generally comes from an RCD somewhere, even if not the one on the circuit supplying the shock. That RCD will trip, 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

John Rumm wrote:

On 28/09/2017 01:08, Roger Hayter wrote:
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.

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)


(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.

--

Roger Hayter

On 28/09/2017 01:08, Roger Hayter wrote:
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?

Only just spotted this one thread.

https://www.youtube.com/watch?v=RnriL5iEmZs

Whilst not answering your question is still well worth watching.


--
Adam