What type of earthing system is this?

[JohnW2]

What are your thoughts with the TT earthing and the metal CU unit, there is the potential for the CU to become live and the DNO fuse not to blow due to the relativly high impedance of the TT earth

This has been discussed many times here.

My personal 'thought', for what it's worth, is that it is one of the additional hazards created by the (I personally believe not-well-thought-through!) introduction of the requirement for metal (at least, non-combustible' - whatever that might mean) CUs in domestic premises, inspired (insisted upon?) by the LFB!

Even in the absence of any fault, I think it is only a matter of time before someone dies as a result of 'fiddling around' without inadequate care in a CU which has an earthed metal enclosure, when they would not have died had it been plastic!

Is there any point installing a separate 100mA S type RCD on the feed to the CU as I presume this would also need to be in a metal enclosure and simply moves the problem to that box?

It doesn't move the problem to the RCD's enclosure if that enclosure in not metal. It's not really clear as to whether the regulation intends that the regulation requiring non-combustible CUs "and similar switchgear" is intended to include such enclosures, but electrical safety/common sense says that it should not in the situation being discussed!

I do have up-front Type S RCDs in my TT installation (even though the CUs are all non-metal), but that is because they are needed to protect long distribution circuits. They are in plastic enclosures and will remain so for as long as I have any control over the situation!

Will there be a requirement to replace the main MCB on the CU to a 100mA 80A RCD?

Even that would not deal with the hazard which concerns you if the fault was in conductors upstream of that RCD.

Kind Regards, John

 

[JohnW2]

A single RCD in an enclosure is not, by definition, a consumer unit so may still be plastic.

As I've just written, although it is not a 'CU', the uncertainly (one of the many uncertainties about this reg!) is whether it was intended to qualify as "similar switchgear".

As I wrote, considerations of electrical safety and common sense dictate that the reg should not extend to the RCD we are talking about that - but there is little common sense in this regulation.

Mind you, as I also wrote, electrical (rather than 'fire') safety considerations probably should logically result in metal CUs being banned (not 'demanded'!) in domestic installations - so there's a paucity of logic in all this!

Kind Regards, John

 

[JohnW2]

The information given to me in 1980 by the then supply company ( DNO ? ) was that failed Neutrals in 4 wire overheads was frequent enough to make PME undesirable in such cases.

Fair enough - as I said, it's not something I've noticed in LV overhead cables.

This was odd as they offered a PME system from the 4 wire in the street.

Yes, rather odd - but I presume that it must imply that they were not particularly concerned about the theoretical risk

ABC cable ( 4 wires spiraled in a bunch ) is far "safer" thn open 4 wire systems.

As I said, that has got to be the case.

Kind Regards, John

 

[JohnD]

As I've just written, although it is not a 'CU', the uncertainly (one of the many uncertainties about this reg!) is whether it was intended to qualify as "similar switchgear".

Distribution Boards are "similar."

 

[JohnD]

As I've just written, although it is not a 'CU', the uncertainly (one of the many uncertainties about this reg!) is whether it was intended to qualify as "similar switchgear".

Distribution Boards are "similar."

Useful article

https://electrical.theiet.org/wirin...ble-enclosure-requirement-for-consumer-units/

 

[securespark]

I came across a cottage with a VOELCB which had been converted to PME, so it is not a given

Yes, they were often left in as an isolator.

 

[JohnW2]

Distribution Boards are "similar."

Well yes, of course - a 'CU' is merely a type of DB.

However, that doesn't answer the question as to whether (in their minds) an enclosure just containing an RCD is also considered to be 'similar'. If it contained just an RCD and one MCB, I think that most people would probably say that it was not only 'similar' but in fact was a CU (or, at least, a DB).

Kind Regards, John

 

[JohnD]

In the past, a "consumer unit" was defined as a thing that contained both a main switch and one or more fuses or breakers.

I don't see that a main switch, on its own in an enclosure, or an RCD, is similar.

 

[bernardgreen]

I don't see that a main switch, on its own in an enclosure, or an RCD, is similar.

It is a switch, it is therefor an item of switch gear.

 

[JohnD]

and is it similar?

 

[JohnW2]

In the past, a "consumer unit" was defined as a thing that contained both a main switch and one or more fuses or breakers.

That obviously makes sense. As yoiu know, the current definition of a CU is that it is a 'type tested' DB, intended for use in domestic installations - but I agree that a commonsense view requires it to have some sort of switch (even if an RCD) and at least one OPD.

I don't see that a main switch, on its own in an enclosure, or an RCD, is similar.

"Similar" is obviously a word that really should not appear in any regulation, since it is essentially 'a matter of opinion'. It actually says "similar switchgear" which perhaps makes it more likely that they were thinking (I would say dangerously, in the context we are discussing) that it could include an RCD in an enclosure.

I think what we can probably all agree is that to have an up-front RCD in a TT installation in a metal enclosure is simply dangerous - so if that is what they 'intended', then that regulation is also potentially dangerous!

As I've said, the up-front Type S RCDs in my TT installation are in insulated enclosures, and that's how they are going to remain so long as I have any control of the situation - regardless of what any regulation might say. I regard my safety as more important than compliance with regulations!

Kind Regards, John

 

[JohnD]

It is a switch, it is therefor an item of switch gear.

it says "assembly" as well as "similar"

Being a "switch" is not enough.

even if it has a fuse with it

 

[securespark]

What we used to do with TT and metal boards in the good old days was fit an insulating gland for the tails (some were made out of wood!) and fit an S type RCD on the tails.

 

[JohnW2]

it says "assembly" as well as "similar" ... Being a "switch" is not enough. ... even if it has a fuse with it

Who knows. Is a switch in an enclosure not an 'assembly'?

Kind Regards, John

 

[JohnW2]

What we used to do with TT and metal boards in the good old days was fit an insulating gland for the tails (some were made out of wood!) and fit an S type RCD on the tails.

... but what was the RCD enclosed in (I suspect not a metal enclosure)?

Kind Regards, John

 

[securespark]

You suspect right, otherwise you'd need an RCD to protect that RCD...........

 

[bernardgreen]

I regard my safety as more important than compliance with regulations!

Well said.......