just a quick one

[switch10]

been to look at a job today, just need to confirm if im correct or wrong. this person recently had a new consumer unit fitted on a tt earth system that only has rcd protection on main incoming switch, am i wrong in saying that this is not correct and should have seperate rcd protection in the consumer unit?

im sure i am but would like confirmation.

The trouble is the customer wants his garage connecting up now for a few sockets and lights, but there is lack of room to fit another consumer unit in the meter cupboard, but im reluctant to do this if it requires rcd protection and the customer is arguing it doesnt. hopefully i got my facts right.

 

[NozSpark]

I probably wouldn't install it like that, but so long as all circuits have RCD protection then there isn't a problem.

Also if the CU is metal clad then the supply cables will need RCD protection

 

[m4tty]

Are you saying it's got one rcd before the cu? Covering the whole installation? If so will it comply as one fault will take the whole installation out. can't remember the exact reg but it's segregation of circuits or something like that. I stand to be corrected though. Not much help really ... sorry

 

[Lurch]

Erm, what?

I'm confused about where these RCD's are, and what they have to do with the garage?

 

[Zeespark]

The front end RCD is to protect the tails, further RCD's/RCBO's are needed for the outgoing circuits.

AndyGuinness

 

[tom1]

is the rcd 30ma or 100ma time delayed

 

[steptoe]

The front end RCD is to protect the tails, further RCD's/RCBO's are needed for the outgoing circuits.AndyGuinness

correct Andy,

almost,

on TT you MUST have frontend protection via (usually) a 100mA RCD,

this is protection for property, NOT life.

then in your CU you will either have 30mA RCDs or preferably RCBOs,

as protection for life.

 

[switch10]

sorry maybe i didnt explain things clearly.

the rcd is on the main incoming switch of the consumer unit, same as attached pic ,(this picture is not the consumer unit but displays the rcd on main switch)

the consumer unit is plastic, not metal, however im sure ive read somewhere that on a tt install the rcd can not be part of the main switch but should have a seperate rcd . however the consuemr unit is only 4 way.

 

[Zeespark]

Better put than Me steps, I apologise for the confusion caused.

AndyGuinness

 

[m4tty]

correct Andy,almost,

on TT you MUST have frontend protection via (usually) a 100mA RCD,

this is protection for property, NOT life.

then in your CU you will either have 30mA RCDs or preferably RCBOs,

as protection for life.

I'm not doubting you at all but I thought a split load dual rcd board complied. I put one in for assessment and passed and have put same in since. That must comply too. Can you explain you reasons more so I and others can learn. Cheers

 

[steptoe]

Better put than Me steps, I apologise for the confusion caused.AndyGuinness

you got it fine Andy,

switch10,

you NEED a 100mA on the incomer/main switch for a TT install,

there are other methods, but to keep it simple we will (for this instance) state you MUST have a 100mA RCD as a main switch on TT.

 

[Zeespark]

It doesnt comply for TT as the tails need protecting, this isnt required on TN based systems, and even a split RCD board is not technically fully regs compliant.

AndyGuinness

 

[Lurch]

the consumer unit is plastic, not metal, however im sure ive read somewhere that on a tt install the rcd can not be part of the main switch but should have a seperate rcd . however the consuemr unit is only 4 way.

Dunno where you've seen that but it is not entirely correct, or definitely not so for an all insulated unit. With a metalclad unit on a TT you either need to RCD protect the tails entering the metal enclosure or ensure the tails cannot come into contact with the enclosure if they come adrift. I have done this by shrouding the cables with PVC items, usually trunking or conduit.

It doesnt comply for TT as the tails need protecting, this isnt required on TN based systems

Really? This one has completely passed me by.

 

[steptoe]

I'm not doubting you at all but I thought a split load dual rcd board complied. I put one in for assessment and passed and have put same in since. That must comply too. Can you explain you reasons more so I and others can learn. Cheers

yes,

you can use a split load 30mA board for virtually any installation,

as long as it is plastic and is at the origin,

quite often with TT you may well find the CU is remote,

with TN systems a switch fuse is adequate, but with TT you need some sort of protection for the building until the tails reach the CU, hence the 100mA RCD front end, to ensure adequate disconnection times.

a front end RCD is also required if the CU is metal, otherwise you have NO protection at the point of entry to MC CU.

HTH.

 

[switch10]

ok cheers for replys. only second time since working for myself ive come across a tt installation which is why i ask, better to make sure things are correct.

so the property has a isolation switch there.

it also requires a 100ma rcd fitted in an enclosure before the C/U?

Is that what your saying? this whole 100ma rcd is whats getting me, othe one ive seen simply had isolation switch, and C/U with main switch and 1 x 80a 30ma rcd in.

 

[Zeespark]

100mA RCD should be installed instead of a 100A main switch in a metalclad TT Board.

AndyGuinness

 

[Chr!s]

No need for protection of tails providing the tails and and enclosures meet class II including the rcd housing.

If enclosure are class 1 then Rcd protection is required for the tails.

Discrimination of circuits needs applying so if there's only four ways, rcbos would have been the correct choice.

100 Ma rcd will not discriminate with a 30 Ma, best to fit a 100 Ma time delayed

 

[switch10]

the consumer unit is not metal clad though. its a mk insulated consumer unit

 

[Lurch]

100mA RCD should be installed instead of a 100A main switch in a metalclad TT Board.

That wouldn;t make any difference to the tails. In the past I have used flexible conduit and\or large PVC trunking to form a PVC enclosure from the main switch\RCD to the outside of the enclosure on a TT system. The basic principle is that if the tails become loose then there is no RCD to trip so you either need to eliminate the risk or install an RCD upstream.

 

[NozSpark]

Steps said it perfectly, but I'll try in my own words to see if it'll help you.

Under the 16th ed all circuits in a TT installation had to have 100mA RCD protection (no matter what cable construction or containment method) & downstairs sockets required 30mA protection.. If the CU was metal then the tails req'd RCD aswell.

Now the 17th ed has come in; usually all domestic circuits require 30mA RCD protection due to installation methods,, but bear in mind that all circuits require minimum 100mA protection even if they are MICC, SWA or Flexishield.

Short lengths of tails do not require RCD protection..

FYI.

100mA RCD is to protect livestock and against fire

30mA RCD is to protect life

HTH