A liitle guidance please!!!!!!!

[hoppy]

Hi

I have a job supplying a garage cu. House is a TT system with 17th ed dual rcd board. From house CU to garage is about 35m. Ra is 28 ohms at house.
Customer wants small dual board in garage suppling 2 flourescent lights, alarm and a double socket for a compressor.

Unsure what to do for the best regarding the earthing. was thinking of running a 2 core 6mm swa to feed gargae cu and putting a rod in for the garage earth. But my question is, the house is a TT system and the garage supply cable will be on 32A MCB on and 30ma RCD protected at board, thus earthing the supply cable, but giving the garage its own earth (rod) and obviously rcd protection through an rcd main switch in the small cu. Will there be discrimination between the garage RCD and the house rcd as the earths are seperate, just cant get my head around this.

What would you do for the best???

thanks

 

[jaydee81]

would make no difference house rcd would still pick up the inbalance between live and neutral and trip only way to get discrimination would be to protect garage supply with 100ma s type rcd

 

[hoppy]

sorry should have made myself a bit more clear. what i mean't is in theory i have two seperate earths TT rod at house and one at garage, the sheath of 2 core swa will be earthed at house end but not at garage end, so the earths are kept seperate.

Is this the best way to do it or should i connect the earthed sheath to the earth at the garage end, just thinking of any house faults going to earth through the garage rod.
Also if i keep the garage earthed seperately from the house, is there a nedd for an RCD at the garge if the supply cable is rcd protected.
All i am thinking is the only link from the house to the garage is the Line and neutral of the two core swa. so any earth faults in the garage will go via the garage rod so the rcd in the house will not trip, so i need an rcd at the garage cu.

Sorry if a bit confusing, been a long day and just can't get my head on this and don't know whta to do for the best.

Just looking to bounce some ideas off the more experienced guys on here

Thanks

 

[tony mc]

I think your understanding of how a RCD works is a little confused!

The earth or seperate earth rods will make no difference as far as the RCD's are concerned they will only see a imbalance in the coil and trip.
Their would be no discrimination between the 2 RCD's and its anyones guess which one would trip first. (If both 30mA)

If you have the supply cable RCD protected with a 30 mA device then it would be pointless having a second RCD in the garage.

As far as the Rods are concerned link them all together and you will hopefully bring down the overall RA!

 

[Phillcbr400]

If running swa to the garage I would gland the sheath at the house end and gap at the garage end. You need to take into account of mechanical protection for the gapped section.