Supply Isolator required to be metal on TT

[oscar21]

Found it, I think.

RCDs, metal consumer units and TT installations | NAPIT

NAPIT's Bill Allan answers another common contractor query. Contractors sometimes ask whether it's necessary to install an RCD in a TT installation before a metal consumer unit (i.e. a 'front end' RCD

professional-electrician.com

you can avoid 10 minutes reading it if you want and skip to the last line, which is what NAPIT could have done.

Do you need to install an RCD on a TT installation before a metal consumer unit if all the outgoing sub-main and final circuits are RCD/RCBO protected? The answer is no

 

[SparkySy]

Found it, I think.

RCDs, metal consumer units and TT installations | NAPIT

NAPIT's Bill Allan answers another common contractor query. Contractors sometimes ask whether it's necessary to install an RCD in a TT installation before a metal consumer unit (i.e. a 'front end' RCD

professional-electrician.com

you can avoid 10 minutes reading it if you want and skip to the last line, which is what NAPIT could have done.

Do you need to install an RCD on a TT installation before a metal consumer unit if all the outgoing sub-main and final circuits are RCD/RCBO protected? The answer is no

It is indeed no, but I still go for one just in case the 30ma protection fails, at least there is still some protection as a last resort back up!

 

[grumpyjohn01]

It is indeed no, but I still go for one just in case the 30ma protection fails, at least there is still some protection as a last resort back up!

I totally agree, on a new TT install or a replacement CU I pretty much always install a Wylex 'S' type RCD upfront in a Wylex Isolator enclosure (plastic) which fits perfectly and has restraint for the tails built in, has the bonus of fault protection for submains or other circuits you might want to have on MCBs or fuses downstream.

 

[Simon47]

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.

Being pedantic, only ferrous metal mets the requirements.
There are probably many materials that would meet the requirement if they had turned on some brain cells and referenced a standard for what "combustible" means. Pity someone didn't tell the fire brigade something like "tell you what, leave us to do the electrical engineering, and we'll leave you to the fire fighting"

 

[Yellowhammer]

I've read that article before, it does make some sense, though I'm not sure, personally that fitting the upstream S has downsides to be avoiding.
Often on a TT it is the provided a means of isolation before the board which is always handy.

It's also handy that it helps you meet the disconnection times for a submain on the install, so you can fit say a garage supply from an MCB in the main board (assuming it's not buried <50mm in a wall, or any other means of installation requiring 30ma RCD protection). You can then have a couple of upstream RCBO circuits in the garage and still maintain selectivity. I'm a fan, personally of local RCBOs in the garage as I want to lights to stay on ideally if the sockets were to go while I have a welder or grinder or similar in my hands. (Of course the downside being any fault on the submain cable itself will take out the entire house, given the MCB is as likely as good as useless).

 

[mainline]

I've read that article before, it does make some sense, though I'm not sure, personally that fitting the upstream S has downsides to be avoiding.
Often on a TT it is the provided a means of isolation before the board which is always handy.

It's also handy that it helps you meet the disconnection times for a submain on the install, so you can fit say a garage supply from an MCB in the main board (assuming it's not buried <50mm in a wall, or any other means of installation requiring 30ma RCD protection).
You can then have a couple of upstream RCBO circuits in the garage and still maintain selectivity.

I'm a fan, personally of local RCBOs in the garage as I want to lights to stay on ideally if the sockets were to go while I have a welder or grinder or similar in my hands. (Of course the downside being any fault on the submain cable itself will take out the entire house, given the MCB is as likely as good as useless).

An upfront rcd whether it be delayed or not will only give partial selectivity.

 

[Simon47]

An upfront rcd whether it be delayed or not will only give partial selectivity.

Absent faults or incorrect design, it should give full selectivity.
I.e., regardless of the fault current the RCDs are exposed to, the downstream unit should have opened the circuit and removed the fault long before the delayed unit will trigger.

 

[Risteard]

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.

That presumes that an isolator is a "similar switchgear assembly" to a "consumer unit" (single phase type tested DBO) which is doubtful.

 

[mainline]

Absent faults or incorrect design, it should give full selectivity.
I.e., regardless of the fault current the RCDs are exposed to, the downstream unit should have opened the circuit and removed the fault long before the delayed unit will trigger.

A 30 mA device would require a minimum of a 100 mA Type S RCD upstream to provide selectivity.

An rcbo board could have accumulated leakage and not provide full selectivity.

 

[SparkySy]

A 30 mA device would require a minimum of a 100 mA Type S RCD upstream to provide selectivity.

An rcbo board could have accumulated leakage and not provide full selectivity.

I have installed this arrangement many times over the years and never had an issue with accumulated leakage on a domestic installation to that level.
I have however come across this issue on dairy installations where the vacuum pumps have been upgraded and changed to vfd driven. The vfd's were causing the milking shed to trip and shut the whole lot down!

 

[mainline]

I have installed this arrangement many times over the years and never had an issue with accumulated leakage on a domestic installation to that level.
I have however come across this issue on dairy installations where the vacuum pumps have been upgraded and changed to vfd driven. The vfd's were causing the milking shed to trip and shut the whole lot down!

Single pole rcbos with a N+E fault can take out the upfront S type.
I also came across where next doors' induction hob was tripping the up front s type in the other house.

 

[SparkySy]

Single pole rcbos with a N+E fault can take out the upfront S type.
I also came across where next doors' induction hob was tripping the up front s type in the other house.

That they can but that is not accumulated leakage, that is a fault.
Having double pole RCD's downstream of the main switch RCD limits that to a few circuits, now with double pole RCBO's this is much less of a problem.

 

[mainline]

That they can but that is not accumulated leakage, that is a fault.
Having double pole RCD's downstream of the main switch RCD limits that to a few circuits, now with double pole RCBO's this is much less of a problem.

The point being whether it’s a fault or accumulated leakage you can’t guarantee total selectivity using a up front rcd on a tt supply.

 

[nicebutdim]

I've seen an upfront 30mA/100ms upfront RCD taken out when testing downstream RCBOs, although I'd consider that a fairly unusual situation. In the majority of domestic TT installations, any upfront Type S RCD would generally be 100mA - I doubt most wholesalers would stock any other single phase Type S RCD.

 

[mainline]

I've seen an upfront 30mA/100ms upfront RCD taken out when testing downstream RCBOs, although I'd consider that a fairly unusual situation. In the majority of domestic TT installations, any upfront Type S RCD would generally be 100mA - I doubt most wholesalers would stock any other single phase Type S RCD.

I agree but in most domestic situations they wouldn't be on a TT and if they are then they probably wouldn't have vfd driven borehole pumps, dual EV charging points, Solar, Wind, and battery storage