Fused

[DaveOC]

Hi Quick Question, I have just replaced some cheap plug sockets with some new MK ones, straight forward enough, turned off the fuse to the socket ring and changed them, but while I was changing them the RCD tripped, reset it but it tripped again (not straight away), reset it and it was then OK.
Is there a problem or a reason this happened, I have two other socket rings which I didn't switch off.
Thanks

 

[mikeyd]

Most likely a neutral wire touched something which was earthed while it was disconnected. This could have been the backbox, the CPC (earth wire) your fingers etc. If so it's quite normal and nothing to be worried about. The MCB (fuse) only isolates the line conductor. All the neutrals are still connected together. If the neutral has a short to earth the RCD will see an imbalance between line and neutral currents and trip.

 

[ericmark]

The neutral is considered as a live wire, turning off a MCB does not switch the neutral so you have not isolated the supply before working, turning off most RCD's however will in the main isolate both live wires the line and neutral. With RCBO's you can get double pole even in a single width although they only monitor the line for over current flowing in circuit the neutral is only monitored for over the current permitted as unbalance so showing leakage to earth.

Neutral to earth current will vary according to what power is being used, and therefore the voltage difference between earth and neutral.

I know many electricians do change sockets without isolating the neutral, but since they are working on live sockets they have to work with that idea in mind and ensure the neutral and earth never touch.

537.1.2 Combined protective and neutral (PEN) conductors shall not be isolated or switched except as permitted by Regulation 543.3.4.
Except as required by Regulation 537.1.4, in a TN-S or TN-C-S system the neutral conductor need not be isolated or switched where it can be regarded as being reliably connected to Earth by a suitably low impedance. For supplies which are provided in accordance with the Electricity Safety, Quality and Continuity Regulations 2002, the supply neutral conductor (PEN or N) is considered to be connected to Earth by a suitably low impedance.

So your not breaking regulations by not switching the neutral. But you do need to take care, not unknown to find a borrowed neutral.

 

[DaveOC]

Thanks to both answers, put my mind at rest and learnt something.

 

[Risteard]

So your not breaking regulations by not switching the neutral. But you do need to take care, not unknown to find a borrowed neutral.

In a domestic installation isolation requires ALL live conductors to be broken as the installation cannot be considered to be under the supervision of a skilled person (electrically) or an instructed person (electrically), so I would not agree with that statement.

 

[eveares]

The neutral is considered as a live wire

All wires have the potential to become live.

I know many electricians do change sockets without isolating the neutral

In some situations, such as new builds where there are no borrowed neutrals, the risks of one getting a shock or casing a dangerous situation where Neutral and Protective earth are at near the same potential are low.

In a domestic installation isolation requires ALL live conductors to be broken as the installation cannot be considered to be under the supervision of a skilled person (electrically) or an instructed person (electrically), so I would not agree with that statement.

Sometimes that is not ideal so one takes risks at their and others own expense. i.e. turning of the RCD or main switch will turn off a circuit what needs to remain energised for what ever reason.

 

[ericmark]

So your not breaking regulations by not switching the neutral. But you do need to take care, not unknown to find a borrowed neutral.

In a domestic installation isolation requires ALL live conductors to be broken as the installation cannot be considered to be under the supervision of a skilled person (electrically) or an instructed person (electrically), so I would not agree with that statement.

Are you saying it is permitted for an "Ordinary Person" to renew sockets? I would have thought to renew sockets you would need some skill, historically "instructed" or "skilled", although maybe not "competent" in other words having enough knowledge to ensure your safety, but maybe not enough to ensure the safety of others.

 

[davelx]

In a domestic installation isolation requires ALL live conductors to be broken as the installation cannot be considered to be under the supervision of a skilled person (electrically) or an instructed person (electrically), so I would not agree with that statement.

That isolation is provided by the DP main switch, not the MCBs or RCBOs.

 

[Iggifer]

In a domestic installation isolation requires ALL live conductors to be broken as the installation cannot be considered to be under the supervision of a skilled person (electrically) or an instructed person (electrically), so I would not agree with that statement.

Sometimes that is not ideal so one takes risks at their and others own expense. i.e. turning of the RCD or main switch will turn off a circuit what needs to remain energised for what ever reason.

I cannot think of a single example of a domestic situation where a particular circuit NEEDS to remain energised. Unless you've got a relative on life support upstairs! Then things like computers and servers can be shut down and then turned on again afterwards.

 

[Risteard]

In a domestic installation isolation requires ALL live conductors to be broken as the installation cannot be considered to be under the supervision of a skilled person (electrically) or an instructed person (electrically), so I would not agree with that statement.

That isolation is provided by the DP main switch, not the MCBs or RCBOs.

I know that. Some were arguing that the neutral did not need to be broken to achieve isolation in a domestic installation - I pointed out that it does need to be broken. I never suggested that this wasn't the function of the main switch.

 

[JohnW2]

Some were arguing that the neutral did not need to be broken to achieve isolation in a domestic installation - I pointed out that it does need to be broken. I never suggested that this wasn't the function of the main switch.

AFAICS, the comments were about what was required by (by implication, BS7671) regulations. What reg requires that the neutral needs to be broken (in a TN system) to achieve isolation? 537.1.4 requires that that a switch/isolator should be present at the origin of an installation and that this switch should break the neutral if it is intended to be used by 'ordinary persons', but that's a different matter - as eric has quoted, 537.1.2 makes it clear that breaking of the neutral is not required to achieve isolation in a TN installation (even if one disagrees with that!).

Kind Regards, John

 

[eveares]

That isolation is provided by the DP main switch, not the MCBs or RCBOs.

Then why does one lock off the MCB's/RCBO's and not the main DP switch?

 

[JohnW2]

Then why does one lock off the MCB's/RCBO's and not the main DP switch?

I suppose because (in a TN installation) the regs (in their infinite wisdom) deem that to provide adequate isolation. In a TT installation, one theoretically (per regs) has to use DP isolation, and in a domestic installation that will usually be a Main Switch or RCD.

Of course, in the vast majority of situations, SP isolation will be 'safe' - it's only very rarely that one will come across circumstances that would make SP isolation 'unsafe', even with TT. In any event, despite what the regs say, even TN installations are not inevitably 'safe' with SP isolation - let's face it, the regs' explanation as to why SP isolation is adequate with TN relies on a singly bit of G/Y cable, usually with screwed connections at both ends. It could therefore be argued that one should at least measure EFLI before relying upon SP isolation in a TN installation.

Kind Regards, John

 

[EFLImpudence]

Table 53.4 would seem to be at odds with the definition of isolation/isolator in Part 2.

It's up to you when working on the circuit.

 

[davelx]

That isolation is provided by the DP main switch, not the MCBs or RCBOs.

Then why does one lock off the MCB's/RCBO's and not the main DP switch?

Because once one has de-energised the circuit one is working on, one does not wish it to be re-energised until one is ready! I don't consider that isolation, as AFAIC, isolation requires the disconnection of all live conductors.