Supply Isolator required to be metal on TT

[DaveOLW]

Does the Supply Isolator require to be metal on TT, is so metal could become live because before RCD

 

[westward10]

I assume you are talking about a separate domestic double pole isolator if so, no.

 

[DaveOLW]

I assume you are talking about a separate domestic double pole isolator if so, no.

Regulation 421.1.20 of BS 7671:2018 states, for consumer units and similar switchgear assemblies in domestic premises, to be manufactured from non-combustible materials or be enclosed in a cabinet constructed from non-combustible materials.

 

[oscar21]

Regulation 421.1.20 of BS 7671:2018 states, for consumer units and similar switchgear assemblies in domestic premises, to be manufactured from non-combustible materials or be enclosed in a cabinet constructed from non-combustible materials.

It does but that doesn't seem to bother the DNO or meter installers, I see lots of plastic DP isolators in plastic enclosures just after the meter.

 

[westward10]

The fact they are still being produced unlike plastic consumer units suggests they are not the same thing.

 

[James]

I look at it as a poor way of saying consumer unit or similar i.e. a collection of switches, fuses, circuit breakers or similar protective devices as an assembly.

I do not personally consider a single isolator as falling into the regulation listed above.

 

[oscar21]

I look at it as a poor way of saying consumer unit or similar i.e. a collection of switches, fuses, circuit breakers or similar protective devices as an assembly.

I do not personally consider a single isolator as falling into the regulation listed above.

In that case how do yo feel about a "S" RCD upfront in a plastic unit before the main CU.

 

[mainline]

In that case how do yo feel about a "S" RCD upfront in a plastic unit before the main CU.

Rcds are often installed ahead of a consumer unit on a TT supply.
Requiring a metal enclosure would introduce new risks.

 

[James]

In that case how do yo feel about a "S" RCD upfront in a plastic unit before the main CU.

Depending upon all the other circumstances then I may well be ok with it.

 

[nicebutdim]

In that case how do yo feel about a "S" RCD upfront in a plastic unit before the main CU.

How would you feel about a metal enclosure when PSCC/PFC is likely insufficient to take out a main fuse?

 

[DefyG]

If incorporated inside a fire retardant meter box, plastic enclosure would be compliant to Regulation 421.1.20 .
Not sure all meter boxes are fire retardant though!
Wording like 'similar switchgear' only helps with various interpretations!

Mitras Fire Retardant Recessed Electricity Meter Box

The standard "Fire Retardant" recessed electric meter box is designed to house any domestic electric meter. This electric meter box is manufactured from DMC material and complies with BS476 part 7 class 2. This product is available nationally through Builders' Merchants. Click the 'Find Your...

www.meterboxes.co.uk

 

[James]

If incorporated inside a fire retardant meter box, plastic enclosure would be compliant to Regulation 421.1.20 .
Not sure all meter boxes are fire retardant though!
Wording like 'similar switchgear' only helps with various interpretations!

Mitras Fire Retardant Recessed Electricity Meter Box

The standard "Fire Retardant" recessed electric meter box is designed to house any domestic electric meter. This electric meter box is manufactured from DMC material and complies with BS476 part 7 class 2. This product is available nationally through Builders' Merchants. Click the 'Find Your...

www.meterboxes.co.uk

That is not non combustible

 

[brianmoooore]

That is not non combustible

Indeed. IIRC, when this ridiculous regulation first made the light of day, there were several questions and proposals regarding the use of non combustible plastics, but it turned out that the regulation was framed such that only metal would meet the requirements, without resorting to exotic and expensive materials.

 

[Yellowhammer]

I would/have put a plastic enclosure up front with an S RCD in it on a TT install. Tails torqued correctly and ideally checked again the following day, suitably glanded in and out with a tails gland, with the earth hole blanked. I think this is a preferable/less risky scenario than having the metal enclosure with only the main 100A fuse protecting against something shorting to a metal enclosure with rather likely a PEFC of under 10A..

 

[oscar21]

How would you feel about a metal enclosure when PSCC/PFC is likely insufficient to take out a main fuse?

Is there not some sort of guidance or similar issued by NAPIT or the NIC that states tails that are securely clamped and glanded in a metal enclosure aren't classed as a risk in a metal TT enclosure.