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.

 

[telectrix]

the cable to the garage doesn't need RCD front end if surface mount. you would need to use SWA on the exterior section though

 

[malcolmsanford]

For me it is about convenience. If your installation is 35 metres from the main house, then I would certainly go for RCD protection local, if the installation is just a metre away I would not be duly concerned.

As for the cable, provided the T + E was mechanically protected and erected safely across that 3ft gap, then it not being RCD protected again would not be an issue for me, provided you met the disconnection times for the protective device, in this 5/1 sec

 

[telectrix]

agreed malcolm. i was thinking about T/E just being strung across 10ft without mech. protection.

 

[Guitarist]

That's what I thought. The T&E is existing to the garage and inside a length of flexi plastic conduit where it passes outside, fixed to a catenary wire for additional support. Don't really want to change to swa for the sake of a 3ft span over a path between the bungalow and garage.
I value your opinion here tel.

 

[telectrix]

as malcolm said. if mech protected, then no problem. just feed from non-RCD in CU .

 

[Richard Burns]

I like that, theoretically it may be marginally safer with an RCD and to take that away from the cable would be reducing safety which is not permitted and to leave it in would be to fail to discriminate, which is not permitted!




Is the 3ft run across the gap at the recommended height of 3.5M? If it is then there is no real risk of damage (apart from UV) and you have not significantly reduced the safety by removing the not required RCD protection from the cable.


If the existing cable is run in conduit from the CU then it is presumably already surface run, why does it need to be changed to mini trunking? Ah just re read the garage cabling is going to be in mini trunking.

 

[Richard Burns]

Oh Tel just noticed your status is this after 15,000 posts you start to be appreciated!!

 

[Guitarist]

The problem Richard is that the cable height is only about 2.5M between the buildings as it comes out of the eaves of the bungalow and there is no practical way of getting it higher without spacing and poleing. By the time I do that it will be nearly at the garage anyway. I realise that it doesn't conform with height regs but practically it is just a very narrow walkway.

 

[malcolmsanford]

As this is T+E and I assume not in a special location, which regulation concerning height was you referring to, not 417.3.1 I hope

 

[Guitarist]

Of course not Malc. Table 4B in OSG

Just to clarify, it is a 3ft span, not 3m.

 

[Richard Burns]

Really from a practical point of view I would do as you suggest: RCD in garage, surface mount non RCD protected cable from CU to garage, I don't think I have seen any cables at 3.5m in a domestic situation. Note on the certificate that the existing cable does not meet the height recommendations of OSG but is (somewhat) mechanically protected by the flexible conduit.

 

[malcolmsanford]

OSG is a great guide, but a guide only and you notice in table D2 that even reg 417.3.1 as been replaced with reference to the ESQCR-2002 statutory documents.

The 3 metre span guide for your scenario is a guide and is not a regulation, and the height part of the guide refers to certain criteria for certain applications, but none are for ambulatory paths.

The only regulations concerning height restrictions are in special locations such as Marinas or Caravan sites. So basically you can have it at any height, or regulation 417.3.1 and 2 regarding basic protection of placing out of reach for bare conductors

 

[plugsandsparks]

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

 

[Guitarist]

Appreciate all your comments guys. I had it clear in my head what I wanted to do, but always nice to get other viewpoints.

 

[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....