RCD to outbuilding

[Guitarist]

Just after some opinions on this scenario...
Existing 6mm T&E leaving an existing CU (with an RCD protecting all circuits) in conduit, through a loft and out across a 3ft gap into a garage. Garage has a 30mA RCD board itself so at the moment no discrimination.
Now fitting a high integrity board so was planning to take the supply off a non-RCD protected MCB, surface mount the cable in mini-trunking (as it would be less than 50mm deep otherwise) and leave garage as-is.
I'd be interested in peoples opinion of "non-RCD protected cable for discrimination purposes" versus "All cables RCD-protected but no discrimination for garage", in other words added safety versus inconvenience if earth fault occurs.
Really due to the fact that it's T&E rather than swa.

 

[GLENNSPARK]

The only thing i would add is that i would not want anything being plugged in from the garage and the external bit of cable on an RCD that could knock off the whole house. So i would go for no RCD MCB to avoid any nuisance tripping of the house. What gets plugged into garage sockets, like garden stuff, welding set, should not be affecting the house RCD wise. From my experience of meeting RCDs in series both at 30mA it nearly always appears to trip the house first. Law of the sod maybe or some science i dont know

its all about accumilated earth leakage when you have 2 30mA RCDs in series mate......the RCD at the house (supply) end will be subjected to accumilated earth leakage on ALL final circuits on that side of the C/U in the house......and so it will let go before the garage RCD...which isn`t being subjected to the same value of accumilated earth leakage......

 

[telectrix]

need to plug a few pc's and a couple of fridges in the garage, then , to equalize the leakage. LOL.

 

[plugsandsparks]

its all about accumilated earth leakage when you have 2 30mA RCDs in series mate......the RCD at the house (supply) end will be subjected to accumilated earth leakage on ALL final circuits on that side of the C/U in the house......and so it will let go before the garage RCD...which isn`t being subjected to the same value of accumilated earth leakage......

Makes sense - so basically a waste of time fitting one in the garage

 

[GLENNSPARK]

Makes sense - so basically a waste of time fitting one in the garage

no..no and no again.....as a garage socket may be used for outdoor equipment....several ways to deal with RCDs in series....depending on the earthing arrangements
If a TN system then no upfront RCD would be required if you use an armoured cable....you would however use an RCD downstream..(garage)...

If a TT system then a high integrity C/U can be used with a 100mA s type RCD on its own.Then use a 30mA RCD at the garage end....this will give you the discrimination you need.....

 

[GLENNSPARK]

but it does of course depend on how far the house C/U is from the garage/outbuilding....as to whether you RCD the garage....in the garage or the house C/U....

 

[plugsandsparks]

no..no and no again.....as a garage socket may be used for outdoor equipment....several ways to deal with RCDs in series....depending on the earthing arrangements
If a TN system then no upfront RCD would be required if you use an armoured cable....you would however use an RCD downstream..(garage)...


If a TT system then a high integrity C/U can be used with a 100mA s type RCD on its own.Then use a 30mA RCD at the garage end....this will give you the discrimination you need.....

Dont panic, was referring to the situation where the house is already covered by an RCD at 30mA, unless the garage cct comes away from rcd protection whats the point if the house always trips first because of other house leakage reducing the threshold of the house RCD.

 

[Richard Burns]

It will not always be the house first, the balance of probability is the RCD with the most circuits on it but the variability in the operating characteristically could invalidate this.
If a garage RCD trips at 18mA and the house at 26mA then the garage could go first. Then you have the response time if the house one trips in 100ms and the garage in 12ms the garage one could still go first.

And obviously vice versa, could be always the house.

There is no discrimination, similarly if you have a normal 100mA at the house and 30mA at the garage and the garage one trips slowly the 100mA could go first.