Time delay RCD or Mechanical Protection

[daavvee]

Okay!

So, I’m looking to plug a knowledge gap.

The set up is a DB in a main dwelling and a second DB in an outbuilding.

The supply from the main dwelling to the outbuilding is 16mm2 split concentric. It is buried in sealed ducting at 1500mm at the deepest point (under the lawn). It comes above ground inside the outbuilding. The other end comes above ground outside the main dwelling between the flags and the gable end. It is protected by a PVC cable guard. The cable is installed in ‘safe zones’ in the outbuilding but less than 50mm deep into the wall (behind plasterboard).

The outbuilding has all circuits protected by 30mA RCBOs.

So, my question. As the cable is not armoured does this cable need RCD protection in the main dwelling from where it originates, and if so, does it need to be 30mA or can it be 100mA and/time delayed, or can it just be put on an MCB?

Thanks in advance.

 

[loz2754]

Split concentric would need a 30mA RCD for additional protection, if buried in a wall at <50mm.
Or be otherwise mechanically protected - for example in earthed metallic conduit.

 

[daavvee]

Split concentric would need a 30mA RCD for additional protection, if buried in a wall at <50mm.
Or be otherwise mechanically protected - for example in earthed metallic conduit.

And I assume the RCD would need to be in the main dwelling from where the split concentric cable originates.

 

[GBDamo]

Never used split concentric but I thought the design was such that anything that penetrates as far as the Live centre core would have to pass through either the Neutral or Earth creating a high current fault path and near instantaneous disconnection.

This is unlike T&E where you can screw directly into the Live without creating a fault path, until someone poor sod touches the screw/mounted equipment.

I know the regulatory requirement but what does an RCD offer with split con?

What other fault scenarios would make the RCD more relevant?

I guess partial penetration, shorting Neutral and Earth?

 

[daavvee]

Never used split concentric but I thought the design was such that anything that penetrates as far as the Live centre core would have to pass through either the Neutral or Earth creating a high current fault path and near instantaneous disconnection.

This is unlike T&E where you can screw directly into the Live without creating a fault path, until someone poor sod touches the screw/mounted equipment.

I know the regulatory requirement but what does an RCD offer with split con?

What other fault scenarios would make the RCD more relevant?

I guess partial penetration, shorting Neutral and Earth?

I’d read something like this before I posted.

It would be interesting to see if the split con would negate the need for an RCD. It seems as though doubling up, and with not being able to use a time delay RCD the possibility of the RCD tripping in the main dwelling before the RCBO is likely and that’s a 100m round trip (pun intended) to reset the RCD.

 

[westward10]

Previous editions of the Regulations permitted the use of split concentric without additional rcd protection but it was removed in later editions hence it now requires additional protection.

 

[GBDamo]

Previous editions of the Regulations permitted the use of split concentric without additional rcd protection but it was removed in later editions hence it now requires additional protection.

Would be interesting to know the reasoning behind that change, are you aware of any articles?

 

[HappyHippyDad]

What is the earthing arrangement? TT or TN?
If it's TT it would need some form of fault protection even if mechanically protected.

 

[westward10]

Would be interesting to know the reasoning behind that change, are you aware of any articles?

No I will try and find it later in some old Regs. I believe the reason is the live conductors is not totally surrounded by an earthed conductor, some of it is neutral as this pic shows.

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[pc1966]

If, as it now seems to be the case, it would need 30mA additional protection due to being < 50mm behind walls then adding physical protection (either thick steel to prevent penetration, or earthed metal trunking to guarantee disconnection like SWA armour) would be the best option to avoid the lack of RCD selectivity.

More so if the outbuilding has a few circuits and lots of electronics so total leakage could well be creeping up to 15mA+ and a risk of nuisance tripping without any fault.

 

[brianmoooore]

Are we saying that the only bit of the split con. causing the problem is the bit less than 50mm in a wall, or is the 1500mm buried section in ducting non compliant as well?

 

[loz2754]

Just the bit on the wall at <50mm

 

[brianmoooore]

Just the bit on the wall at <50mm

I that case, I'd be adding a bit of trim to the wall in the appropriate place to increase the depth to 50mm+

 

[GBDamo]

I that case, I'd be adding a bit of trim to the wall in the appropriate place to increase the depth to 50mm+

Chase it out, cap it with some perspex and add LED running lights...

....make a feature of it?