Electrical maintenance - tripping main breaker with earth/neutral contact

[David WE Roberts]

Modern (and ancient) circuit breakers are on the live wire.
So when they trip (or are turned off) they break the circuit but leave
neutral and earth connected.
I can't be the only one to have knocked the whole house off when working on
a circuit with the breaker out by touching live and neutral.
Which is a pain if you need light to see where you are working, and also
mfor anyone else who wants to do something in the house whilst you are
working.

Split consumer units allow you to have only half the house turned off, but
still impose limitations.

So why not pass all circuits through a double pole isolating switch straight
after they leave the consumer unit?
For spurs such as lighting, cooker, immersion, electric shower etc. this
would be straightforward.
For a ring, you would either need a 5 pole (2 live, 2 neutral, earth)
switch - (do they even make these?) - or a short uprated spur before the
switch with the ring commencing after the switch.

Anything in the regs to prevent this?
Is it worth considering to provide better circuit isolation for the rare
cases of maintenance, or just a waste of money and space?

Cheers

Dave R

--
No plan survives contact with the enemy.
[Not even bunny]

Helmuth von Moltke the Elder

(\__/)
(='.'=)
(")_(")

 

[pcb1962]

Modern (and ancient) circuit breakers are on the live wire.
So when they trip (or are turned off) they break the circuit but leave
neutral and earth connected.
I can't be the only one to have knocked the whole house off when working
on a circuit with the breaker out by touching live and neutral.
Which is a pain if you need light to see where you are working, and also
mfor anyone else who wants to do something in the house whilst you are
working.

Split consumer units allow you to have only half the house turned off,
but still impose limitations.

So why not pass all circuits through a double pole isolating switch
straight after they leave the consumer unit?
For spurs such as lighting, cooker, immersion, electric shower etc. this
would be straightforward.
For a ring, you would either need a 5 pole (2 live, 2 neutral, earth)
switch - (do they even make these?) - or a short uprated spur before the
switch with the ring commencing after the switch.

Anything in the regs to prevent this?
Is it worth considering to provide better circuit isolation for the rare
cases of maintenance, or just a waste of money and space?

Just put an RCBO on each circuit - neater solution.

 

[Andrew Gabriel]

Just put an RCBO on each circuit - neater solution.

Yep - that's what I've done with all consumer unit refits since 1995.
It costs more (although nowhere near as much more as it did in 1995).
However, it probably costs less than an MCB and double pole isolator,
particularly for higher current circuits.

BTW, an isolator never switches the earth.
Also, if you use a single uprated branch before the ring, I would
make provision for a second earth conductor in parallel so you still
have one of the ring's key safety advantages of multiple earthing paths.

http://www.cucumber.demon.co.uk/cu1.jpg shows use of multiple RBCOs
and a branch before the ring circuit (with multiple earths), although
the branch in this case is only about 5" long, because the original
wires were too short to reach the terminals in the new CU.

 

[David WE Roberts]

Just put an RCBO on each circuit - neater solution.

Are you saying that an RCBO doesn't trip on an earth/neutral fault?

--
No plan survives contact with the enemy.
[Not even bunny]

Helmuth von Moltke the Elder

(\__/)
(='.'=)
(")_(")

 

[David WE Roberts]

Are you saying that an RCBO doesn't trip on an earth/neutral fault?

Gah! Forget that.
Are you saying that an RCBO will isolate both live and neutral?
Presumably needs a slightly different consumer unit?
On my current (old) unit all the neutrals are connected to a single common
rail at the bottom.

--
No plan survives contact with the enemy.
[Not even bunny]

Helmuth von Moltke the Elder

(\__/)
(='.'=)
(")_(")

 

[pcb1962]

Are you saying that an RCBO will isolate both live and neutral?
Yes

Presumably needs a slightly different consumer unit?

On my MK Sentry CU, RCBOs and MCBs are interchangeable

On my current (old) unit all the neutrals are connected to a single
common rail at the bottom.

An RCBO has a neutral wire which you take to the common rail, neutral
for the circuit connects to a terminal on the RCBO.

 

[pcb1962]

You need to take care here specifying exactly what type of RCBO. Many of
the modern tall single module wide ones don't actually switch the
neutral (even though it passes though it for current sensing)

Thanks for that correction John, as you say the MK Sentry RCBOs I have
don't switch the neutral. Sorry David for misleading you, but using an
individual RCBO per circuit rather than one big RCD with an MCB on each
circuit is still the solution to your problem.

 

[ARWadsworth]

Yep - that's what I've done with all consumer unit refits since 1995.
It costs more (although nowhere near as much more as it did in 1995).
However, it probably costs less than an MCB and double pole isolator,
particularly for higher current circuits.

Ever tried a 20 way RCBO CU?

There needs to be a big change in CU layouts.

 

[Andrew Gabriel]

Ever tried a 20 way RCBO CU?

Highest I've done is 10-way, but those both predated the requirement
for pretty well all circuits to be RCD protected.

There needs to be a big change in CU layouts.

I liked the Memshield2 range because they had lots of space and were
solidly built, unlike many domestic CU's, and RCD pods were available
to attach to any MCB if they didn't do the exact combo you wanted,
but I think that range has gone now.

Not looked at what might have replaced it yet.

Incidently, I noticed in the trade press that the safety body is
concerned about the number of fires which spread from domestic
plastic consumer units, and is reviewing if plastic consumer
units should continue to be allowed.

 

[ARWadsworth]

Highest I've done is 10-way, but those both predated the requirement
for pretty well all circuits to be RCD protected.


I liked the Memshield2 range because they had lots of space and were
solidly built, unlike many domestic CU's, and RCD pods were available
to attach to any MCB if they didn't do the exact combo you wanted,
but I think that range has gone now.

Not looked at what might have replaced it yet.

Incidently, I noticed in the trade press that the safety body is
concerned about the number of fires which spread from domestic
plastic consumer units, and is reviewing if plastic consumer
units should continue to be allowed.

Page 18 of this for anyone interested

http://www.esc.org.uk/fileadmin/user_upload/documents/industry/SwitchedOn-25-Locked_2.pdf

 

[David WE Roberts]

[Posting from Windows Live Mail which sucks donkey nuts as a news reader]

"John Rumm" wrote in message

Modern (and ancient) circuit breakers are on the live wire.
So when they trip (or are turned off) they break the circuit but leave
neutral and earth connected.
I can't be the only one to have knocked the whole house off when working
on a circuit with the breaker out by touching live and neutral.

Indeed not - even if actively attempting to avoid this situation its
still very easy to do!

The "proper" solution is to open the CU and disconnect the neutral(s) of
the circuit you are working on.


***************************

Being a coward I would have to knock the whole CU off before I started
poking around inside to disconnect the neutral from one circuit.
This does not meet my requirement to be able to completely isolate one
circuit for maintenance (live and neutral) without affecting the rest of the
circuits.
It sounds as though some RCBOs will do this but not all.

Cheers

Dave R

 

[David WE Roberts]

"pcb1962" wrote in message

You need to take care here specifying exactly what type of RCBO. Many of
the modern tall single module wide ones don't actually switch the
neutral (even though it passes though it for current sensing)

Thanks for that correction John, as you say the MK Sentry RCBOs I have
don't switch the neutral. Sorry David for misleading you, but using an
individual RCBO per circuit rather than one big RCD with an MCB on each
circuit is still the solution to your problem.

************************grrrrrrrrrrrrrrr****************************

Slightly confused here.
In the case where an RCBO does not isolate neutral, and I'm trying to avoid
the whole box tripping on an earth/neutral short, how does the RCBO
(non-neutral-isolating) prevent this?

Cheers

Dave R

 

[David WE Roberts]

"pcb1962" wrote in message



Thanks for that correction John, as you say the MK Sentry RCBOs I have
don't switch the neutral. Sorry David for misleading you, but using an
individual RCBO per circuit rather than one big RCD with an MCB on each
circuit is still the solution to your problem.

************************grrrrrrrrrrrrrrr****************************

Slightly confused here.
In the case where an RCBO does not isolate neutral, and I'm trying to
avoid the whole box tripping on an earth/neutral short, how does the
RCBO (non-neutral-isolating) prevent this?

Basically by having no mechanism for the "whole box" to trip...

If you take a conventional modern CU layout such as:

http://wiki.diyfaq.org.uk/index.php?title=File:17th5-5Fixed.JPG

Since the cooker shares a RCD with a lighting and a socket circuit, a
neutral earth short on the cooker circuit, which causes RCD1 to trip,
would also de-energise the upstairs lights and downstairs socket circuits
at the same time. Creating the unwanted effect you describe.

If the CU was an older arrangement with a "whole house" RCD protecting all
the circuits, then you would lose all the circuits.

With an ALL[1] RCBO arrangement such as:

http://wiki.diyfaq.org.uk/index.php?title=File:17thAllIndependent.JPG

A fault that trips for example the cooker RCBO will now only affect that
RCBO. You will still get some current flow from the neutral to the earth
since the neutral bus bar is shared with other circuits that are still
energised - so the neutral potential can still be slightly elevated above
earth. However current drawn from the neutral of the now disconnected
circuit will be drawn from all the other circuits in parallel *after* the
point that each RCBO makes its current balance measurement. So they won't
see an imbalance.

[1] Its the face that there are no shared RCDs that makes the difference.

above diagrams from:

http://wiki.diyfaq.org.uk/index.php?title=17th_Edition_Consumer_Units

Most useful - thanks.

--
No plan survives contact with the enemy.
[Not even bunny]

Helmuth von Moltke the Elder

(\__/)
(='.'=)
(")_(")