Does the Supply Isolator require to be metal on TT, is so metal could become live because before RCD
Discuss Supply Isolator required to be metal on TT in the UK Electrical Forum area at ElectriciansForums.net
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.I assume you are talking about a separate domestic double pole isolator if so, no.
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.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.
In that case how do yo feel about a "S" RCD upfront in a plastic unit before the main CU.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.
Rcds are often installed ahead of a consumer unit on a TT supply.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.In that case how do yo feel about a "S" RCD upfront in a plastic unit before the main CU.
In that case how do yo feel about a "S" RCD upfront in a plastic unit before the main CU.
That is not non combustibleIf 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!
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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.That is not non combustible
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.How would you feel about a metal enclosure when PSCC/PFC is likely insufficient to take out a main fuse?
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!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' RCDprofessional-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
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.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!
Being pedantic, only ferrous metal mets the requirements.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.
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.
An upfront rcd whether it be delayed or not will only give partial 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).
Absent faults or incorrect design, it should give full selectivity.An upfront rcd whether it be delayed or not will only give partial selectivity.
That presumes that an isolator is a "similar switchgear assembly" to a "consumer unit" (single phase type tested DBO) which is doubtful.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.
A 30 mA device would require a minimum of a 100 mA Type S RCD upstream to provide 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.
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.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.
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