new meter has no off-switch

[securespark]

A lot of isolators I see re only connected to the live conductor. THe neutral is blocked through.

I can't see what all the SP/DP debate is about.

When we lock off to work on a circuit, it is acceptable (unless TT) to isolate just the live.

 

[JohnW2]

I can't see what all the SP/DP debate is about. When we lock off to work on a circuit, it is acceptable (unless TT) to isolate just the live.

That's the bit I've never understand the logic of ...

537.2.1.1 of the regs do indeed say that neutral isolation is not required for TN-S or TN-C-S systems - but then goes on to explain that this is because, in such systems "the neutral conductor is regarded as being reliably connected to Earth by a suitably low impedance". However, is it not the very situation of a supply-side fault which results in neutral of a TN system NOT being "reliably connected to Earth by a suitably low impedance" which is one of the few situations in which there is an argument for DP isolation (i.e. isolating the neutral)?

What am I missing?

Kind Regards, John.

 

[bernardgreen]

However, is it not the very situation of a supply-side fault which results in neutral of a TN system NOT being "reliably connected to Earth by a suitably low impedance" which is one of the few situations in which there is an argument for DP isolation (i.e. isolating the neutral)?

What am I missing?.

The need to isolate the earth supplied by the DNO as well to ensure the CPC does not become a hazard to anyone when the neutral becomes live in a network fault. There is a lot to be said for a TT supply where an effective earth rod is possible. PME and its variants solved some problems but have created others Gas meter readers getting shocks from meters, the tingles from the outside tap on the wall of the house, the tingle and dangers of using an earthed appliance in the garden, the hazards of connecting the CPCs of two buildings fed from different network neutrals ( data cables between two PCs on CPC derived from different neutral being one way to connect them ).

30 years ago I supported PME but now after "surviving" two serious neutral bounces and probably a few minor ones that went un-noticed I am not so sure about PME

 

[JohnW2]

However, is it not the very situation of a supply-side fault which results in neutral of a TN system NOT being "reliably connected to Earth by a suitably low impedance" which is one of the few situations in which there is an argument for DP isolation (i.e. isolating the neutral)?

The need to isolate the earth supplied by the DNO as well to ensure the CPC does not become a hazard to anyone when the neutral becomes live in a network fault.

Perhaps I'm going through a dim phase, but I don't see how that answers my question as to why the regs call for DP isolation in TT systems but say that it is not necessary (i.e. SP isolation is adequate) for TN systems. Particularly with TN-S, I would have throught that the need was at least as great (probably greater) than with TT.

Kind Regards, John.

 

[Wontdothatagain]

Thanks. It occurs to me, BTW, that the meters with isolators I've seen are not Smart Meters, which may have confused the discussion a bit.

I have never seen a Smart Meter with an external isolation switch but several Smart Meter manufactures have a inbuilt isolation swith that allows them to phone into the meter and disconnect the supply.

This function is not designed for safe isolation. It was built in on the request of suppliers so that they can disconnect a customer without having to force entry into a property. At the present time they are not allowed to use the function in this way.

Sorry to dig up an old post but some here may be interested to know that this function is now being actively used by all suppliers who requested this type of meter to be fitted.