RCD's in parallel

[dmc]

Hi,
Working at a house (TT system) and a new build office/garage is to be built.
House has split load RCD board.
1. Should i use 1 of the spare ways and feed the office with a sub-main. or
2. Use henley blocks and take the supply from meter tails.

Option 1, will mean if the house trips then the office supply will turn off.

Option 2, the mains cable to the office will not be RCD protected and it is a TT system.

Thanks dan

 

[Mark.W]

Henley Block the tails into a switch fuse and then out to office/garage into a sub main with 30mA protection.

 

[Richard]

Henley Block the tails into a switch fuse and then out to office/garage into a sub main with 30mA protection.

Being TT, would the cable leaving the switch fuse to the outside office not need protecting by 30mA RCD?

 

[Mark.W]

The office/garage will be 30mA protected, you'd have to stick a rod in at the office/garage side.

 

[Richard]

Sorry, I'm not good on regs so please excuse my ignorence!

If the installation is TT, then the Zs would not be enough to get the required disconnection time should a fault occour on the cable feeding that submain?

 

[Engineer54]

If you don't totally rely on an RCD device, and get a decent Ra level of around 20ohms on the TTsystem then surprise, surprise, the earthing system will protect the sub-main circuit cable...

Nothing to stop you installing a 100mA S type RCD after the switch fuse. That will discriminate with any 30mA protected circuits down stream...

 

[Mark.W]

Again if you stick a rod in at the garage would that not constitute an Ra at the submain.

 

[Engineer54]

If you get a decent Ra value at the garage, ...YES!!!

 

[Mark.W]

If you get a decent Ra value at the garage, ...YES!!!

Soz that was meant for Richard.

Richard, you would not be taking a Zs reading as you are not exporting the earth, if you stick a rod in at the garage and get a good Ra reading and disconnection times are met then that cable going from the switch fuse to the office garage will be protected by the 30mA RCD in the garage.

 

[Engineer54]

Sorry, Thought you were answering my post!! ...lol!!

Nothing to stop Richard taking the house earthing with his cable, nothing to stop him combining his rod Ra value with the house Ra value either. Just needs to check if combining, betters or worsens either of the separate TT systems values. ...usually it will better.

 

[Mark.W]

I agree but as you stated earlier I would only do this if I protect it by a time delayed RCD at the house.

Mark

 

[dmc]

Thank for all you answers.
Just a quick question out of interest are Rcd's allowed in parallel.

 

[i=p/u]

if have discrimination between them

 

[Engineer54]

Thank for all you answers.
Just a quick question out of interest are Rcd's allowed in parallel.

Yes..... In this and most other situations upstream needs an ''S Type'' RCD. Time delay is the only real discriminating means between 2 RCDs in parallel.... They say you need 5x the rating of the downstream RCD to achieve discrimination, ...in reality it more often doesn't work that way at all!!! I've seen a 30mA RCD take with it a 300mA and 500mA upstream RCDs more times than i care to remember ...lol!!!

 

[dmc]

Thanks engineer54 i understand now

 

[Andyrc]

Richard W, HOW? will the RCD in the GARAGE!!!! protect the "supply" cable? from the house, as it's UPSTREAM of the RCD, on the SUPPLY side NOT down stream on the "load" side, please Explain!! ANDY, 20+ YRS ,JIB, 15th/16th/17th EDITION (it needs a 100ma time delayed) supplying the garage/office if? TT at Origin of Mains I:E house side)

 

[Richard]

Richard W, HOW? will the RCD in the GARAGE!!!! protect the "supply" cable? from the house, as it's UPSTREAM of the RCD, on the SUPPLY side NOT down stream on the "load" side, please Explain!! ANDY, 20+ YRS ,JIB, 15th/16th/17th EDITION (it needs a 100ma time delayed) supplying the garage/office if? TT at Origin of Mains I:E house side)

I think you want to check who you addresssed that too! That was exactly the point I was trying to establish!

 

[Mark.W]

Ooops

That's what you get for trying to answer 2 threads at once. There was another thread similar to this on the go at the same time.

Soz Chaps