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

[pcb1962]

Just put an RCBO on each circuit - neater solution.

[Andrew Gabriel]

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.

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]

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

[David WE Roberts]

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.

[pcb1962]

Yes

On my MK Sentry CU, RCBOs and MCBs are interchangeable

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

[John Rumm]

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)

The significant point about all RCBO installations however is not whether they disconnect neutral or not, its the fact that each circuit has its own RCD rather than sharing one between circuits. Hence tripping the RCD on your circuit, does not affect those on others.

[John Rumm]

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.

[pcb1962]

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.

[John Rumm]

Your point was valid though - most of the older "wide" RCBOs did switch the neutral, and there may be some slim ones that do also.

[ARWadsworth]

Ever tried a 20 way RCBO CU?

There needs to be a big change in CU layouts.

[Andrew Gabriel]

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.

[ARWadsworth]

Page 18 of this for anyone interested

[David WE Roberts]

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.

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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]

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.

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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

[John Rumm]

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

If you take a conventional modern CU layout such as:

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:

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:

[John Rumm]

Indeed, and that would be a very wise move!

Some double pole switched RCBOs would solve it, as would have all circuits protected by single pole RCBOs. See my other post on this.

[David WE Roberts]

Most useful - thanks.