2 RCDs same circuit

[Big Man]

Am wanting to put in a garage CU to divide up garden and garage lighting and power circuits. Currently power comes off kitchen ring that is already protected with a 30mA RCD. For sake of convienience to client, if mower trips RCD they can flick RCD back on at garage RCD rather than having to move furniture to get to main CU and also effecting kitchen appliaces. Has anyone had problems with problem tripping or test results being wrong. Or am I good to go down this route.

Thanks people.

 

[ezzzekiel]

there is no benefit of having two 30ma rcd in circuit.

 

[Big Man]

I appreciate that there is no benefit to this set up. The matter is one of convienience. The question concerns whether there would be a conflict between the two RCDs?

Btw the main CU only has one RCD so tripping that could cause havoc in the rest of property as runs multiple pc for office at home.

Your thoughts are all appreciated.

 

[Spudnik]

You cannot have 2 30mA RCD's in one circuit as there will be no discrimination between the two.

And more often than not, it will always be the main one in the house that would trip anyway.

Your only other solution would be to run in a new SWA from the house CU from the non RCD side and install as you are going to, an RCD CU in the garage.

 

[Big Man]

Thanks I thought there was something like that just couldn't remember what.

Thank you!

 

[claire_r]

I have tested these before, 2 rcds in parallel doing a 5 times test, one tripped, then the other, there is no discrimination between the two, whichever one is more sensitive will trip which is generally the house as Jason said.
I would split the main tails wire its own supply =0)

 

[harrymonkfish]

I have tested these before, 2 rcds in parallel doing a 5 times test, one tripped, then the other, there is no discrimination between the two, whichever one is more sensitive will trip which is generally the house as Jason said.
I would split the main tails wire its own supply =0)

if you chose this option, whats the best way to protect the swa cable going to the outbuilding?

 

[Spudnik]

if you chose this option, whats the best way to protect the swa cable going to the outbuilding?

With a standalone CU with a suitably rated MCB for the size of the SWA.

 

[harrymonkfish]

So does that mean you have would have 2 stand alone CU's? One at the house intake and one in the outbuilding?

Thanks
Harry

 

[Spudnik]

So does that mean you have would have 2 stand alone CU's? One at the house intake and one in the outbuilding?

Thanks
Harry

Yes you would.

Although the one for the garage would be considerably smaller.

 

[monkeyelectric]

How about the mcb for the swa protected by a 100mA rcd at the house and the 30mA in the garage? that way you protect the swa against fault and overload, or a time delayed rcd at the house end?you'll have your discrimination sorted

 

[harrymonkfish]

Yes you would.

Although the one for the garage would be considerably smaller.

The job I am planning for will be an extension separate from the main house. Ring, lights, shower and workshop.

So my plan is:

Split the tails at the intake into a Henley block.
63a MCB in stand alone CU.
10 mm2 SWA 2 core through house, buried for 2 meters max, into extension. 15 meters approx.
Small CU with 30mA RCD
Split into 1 ring, 1 lights, 1 shower 40A
Separate earthing via TT electrode 16mm2
Bonding to water intake 10mm2

Acceptable?

Harry

 

[Spudnik]

The job I am planning for will be an extension separate from the main house. Ring, lights, shower and workshop.

So my plan is:

Split the tails at the intake into a Henley block.
63a MCB in stand alone CU.
10 mm2 SWA 2 core through house, buried for 2 meters max, into extension. 15 meters approx.
Small CU with 30mA RCD
Split into 1 ring, 1 lights, 1 shower 40A
Separate earthing via TT electrode 16mm2
Bonding to water intake 10mm2

Acceptable?

Harry

Sounds good to me although...

Main earth could be dropped to 10mm (depending on tests) and main bonding to 6mm.

Dont forget to earth the SWA, preferably at both ends.

 

[harrymonkfish]

Thanks for that Jason.

Just one thing. I thought that as the earthing in the extension will be separate from that of the house, then the SWA should only be earthed at one end ie the house? I was under the impression that you cannot mix between different earthing systems.

Harry

 

[ezzzekiel]

is it totally seperate or attached? this is still a greyish area about exporting earths. prob best idea is to tt the extension

 

[harrymonkfish]

is it totally seperate or attached? this is still a greyish area about exporting earths. prob best idea is to tt the extension

It will be totally separate. the outbuilding will be TT, the house PME.

If I do it this way am I right that the SWA should only be earthed at the intake to house and not in the outbuilding?

thanks
harry

 

[Guest123]

Hi.

I be looking at a 50A MCB max for the 10mm SWA.

In free air the max CCC for a PVC SWA without applying any correction factors is 60A.

XLPE SWA does have a higher rating but not by much as it's rated to 90 degrees whereas PVC SWA is rated at 70 degrees.

 

[ezzzekiel]

yes dont connect the earth at new end, drop in a rod

 

[harrymonkfish]

The customer has now informed me that the workshop is actually going to be a separate building!

The workshop will just have a ring and lights so quite straight forward.

Can I take the supply from the house to the extension containing the shower, lights and ring, and then take another sub main from there to the workshop?

Or, take 2 SWA cables out to the two locations from small CU at intake with separate MCBs and earth both separately?

Thanks
Harry

 

[electricAl]

You could do it eiher way, Harry. You don`t mention distance from 1st outbuilding to workshop - if close then may be better to submain workshop from 1st D/B.

If using seperate submains for each then obviously TT each seperately as discussed.
10mm or > earthing conductor, & 6mm or > for MEB when TT`ing

As you thought, earth SWA at supply end only - insulate at individual D/B`s (as per 542.1.8)

If you do feed a sub from the 1st D/B then you really ought to consider upping to 16mm. As Lenny intimated, even XLPE would only give you 58a if buried, so you`d have to drop down to a 50a breaker. That`s too low IMHO to class as a well designed install with all the loading possible. A 16mm XLPE submain would offer more than enough to allow that 63a as you`d first planned.

Hope it`s useful