Question Tt system

[m top]

hi all wondered what your views on using split load board on Tt,explained to customer best option would be rcbos to cover 8 cuircuits only.
gave her price oh that's to much !!!
as long as ze below 200 ohms all good
reg state all final circuits to be protected by rcd 30ma they will be.
reg 411.5.3 the disconnection time shall be that required by 411.3.2.2 or 411.3.2.4
2 secondDisconnection times will be met by above arrangement
Have I missed something here thanks.

 

[D Skelton]

`additional`?..

come on Jamie...TT...so its fault protection..isn`t it but as eng says a 100mA s type doesn`t comply...its so if used as a mainswitch you get the discrimination...

I would argue that if a type-s is used as a main switch then it is protecting a distribution circuit (albeit a very small one within the housing of the DB), therefore it would comply with the 1s max disconnection time.

Even if my argument doesn't stack up, who the hell cares! lol. It would be stupid not to use a type-s

You're quite right though, additional protection can only be provided by a 30mA RCD or supplementary bonding.

 

[Knobhead]

Assume you use SP RCBO’s with a S type RCD as back up.

So a N→E fault occurs:
1/ the RCBO picks up the fault and operates, breaking the Line side only. The fault still exists.
2/ the RCD finally picks up the fault and blacks the entire board out.

Is this a step forward?

 

[Engineer54]

That's gonna have to be a hell of a sustained N-E fault to take out a 100/300mA S type RCD. I'm all in favour of using SP+N RCBO'S and MCB's alike, and they should be a lot more freely available and promoted, but unfortunately they aren't. I'd hazard a guess and say most sparks in the UK have never even seen one, apart from photo's!!

In Cyprus my TT system home is now completely protected by standard 30mA RCBO's across all 3 CU's (2 X 3Ph + 1 X 1 Ph) with an incorporated 500mA non S type TP+N 60A MCCB at the Head end. I've had several N-E faults when mucking around with the garden electric's, which has always taken out the 30mA RCBO protecting the circuit, but has never tripped the 500mA MCCB.

 

[Knobhead]

Worse if there’s no E/F front end protection at all. I’ve had a fault in a house down the street exporting a N→E fault to my house. At one point I’d got 35A flowing through the water pipe where it entered the house and out through the earth nest and gas pipe.
I’d got RCD’s fitted he hadn’t. He was blithely unaware of the fault and so was I until I started doing a bit of plumbing. It took the then NWEB 5 hours to sort the fault out.
OK this was a combination of two faults. A burnt neutral line tap on the OH line and the N→E fault.
A SP RCBO would have tripped but the fault would have still been there, hence my dislike of them. The DP ones are ridiculously expensive, SP+N aren’t much better.

Shows it can happen.

 

[GLENNSPARK]

You are right Glenn, its bin a long day and two bottles of red wine but theres no excuse for stupidity.lol

who said owt about being stupid?

 

[shanky887614]

Assume you use SP RCBO’s with a S type RCD as back up.

So a N→E fault occurs:
1/ the RCBO picks up the fault and operates, breaking the Line side only. The fault still exists.
2/ the RCD finally picks up the fault and blacks the entire board out.

Is this a step forward?

Never seen them in domestic but wouldn't you want to use dp rcbo's here

 

[Knobhead]

You paying for them?

 

[shanky887614]

You paying for them?

not me but nice to know you have the option