Old MICC cable underground to garage, extraneous conductive parts

[catdog1121]

Ok so the scenario is there is an old MICC or pyro 1.5mm 2 core cable supplying the garage run underground on a 16A circuit breaker that is RCD protected.

In the garage there is a water pipe fed from the house, at the house it is bonded using 10mm, at the garage bonded with 6mm. I think the water pipe from the house bonding to the garage has continuity but need to double check this.

In an ideal world I would replace the MICC with 10mm 3 core SWA and then up the bonding to 10mm in the garage just to be safe and put the RCD on the garage side only. However the consumer unit is in the middle of the house and the home owner really does not want to run a new supply.

I dont really want to remove the RCD protection from the MICC cable as its not clear where it is buried and at what depth etc.


My thinking was if I remove the 6mm bonding at the garage and check continuity from the 10mm bond in the house to the water pipe in the garage, if I get a low reading then its effectively bonded with 10mm anyway so not an issue...

If it is not then as it has been in place for a while with no sign of damage it could stay with 6mm or potentially I could keep the micc on rcd protection, then change the earthing arrangement to TT system in the garage and put 30ma RCD in the garage..

What are your guys thoughts?

 

[WallaceP]

Get a plumber in to fit a section of plastic pipe where the water supply comes into the garage and walk away if it were me

 

[Strima]

As it's existing I wouldn't be massively overconcerned about the CSA of the bonding cable in the garage, as long as there's good continuity. As @WallaceP has said, a section of plastic pipe would be an option to get around this if you really need to.

 

[telectrix]

does MICC cable require RCD protection as it is protected by an earthed metal sheath?

 

[westward10]

What is the earthing system. The csa for the sheath of a 2L1.5 MI is approximately 6.3.

 

[catdog1121]

What is the earthing system. The csa for the sheath of a 2L1.5 MI is approximately 6.3.

Earthing is TNS...

 

[catdog1121]

does MICC cable require RCD protection as it is protected by an earthed metal sheath?

Good question, but as I have no idea how deep its been buried and also its only protected by a copper sheath which is not going to be anywhere near as strong as SWA (I would guess).

 

[catdog1121]

Get a plumber in to fit a section of plastic pipe where the water supply comes into the garage and walk away if it were me

Thats actually a really good shout... Although the pipe comes up from the floor, straight up to an outdoor tap in the garage so there is often water on the floor that would be going up to the pipe where it comes into the garage so not sure if that would make a difference

 

[westward10]

Earthing is TNS...

You would get away with the existing 6.0 as the sheath will support it.

 

[pc1966]

According to this the MICC sheath is just over 7mm^2 CSA if 1.5mm:

https://www.miccltd.com/cms-files/MICC_Light_Duty_2_Core.pdf

Also MICC can safely run far hotter than PVC (though surrounding may not like it!) so even for the 10mm bond in TN-C-S I would not worry too much. As TN-S no issue!

 

[pc1966]

Good question, but as I have no idea how deep its been buried and also its only protected by a copper sheath which is not going to be anywhere near as strong as SWA (I would guess).

The issue is not damage protection for needing the RCD as such, but if using a spade or similar to cut in to the cable could you touch the core without being in contact with the CPC and so tripping the protection?

So anything with an adequately rated earthed shield (to clear OCPD) is safe. It might not be suitable for direct burial as not strong enough, but that is a problem that is independent of RCD protection or not!

 

[catdog1121]

The issue is not damage protection for needing the RCD as such, but if using a spade or similar to cut in to the cable could you touch the core without being in contact with the CPC and so tripping the protection?

So anything with an adequately rated earthed shield (to clear OCPD) is safe. It might not be suitable for direct burial as not strong enough, but that is a problem that is independent of RCD protection or not!

Yeah that is true but the RCD is not really my main concern allthough if the outer sheath were to get damaged etc potentially the RCD could trip before the mcb. its more that the bonding is only 6mm2 rather than 10mm2, in reality though I think as you say it will be fine.. Just wanted to check my thinking. Would you code it on an EICR as a C3 or no code?

 

[pc1966]

Yeah that is true but the RCD is not really my main concern although if the outer sheath were to get damaged etc potentially the RCD could trip before the mcb.

If it is shorted both are likely to go.

The MCB is likely to clear first as the magnetic trip is typically opened in under 10ms while RCD often take 10-20ms or more but would be underway due to the size of the fault. In fact as most RCD often can only clear around 1.5kA fault current you really need to make sure the related OCPD is able to stop larger faults in under the typical opening time of the RCD so its contacts are not faced with interrupting too much current.

Realistically any consumer MCB will do that. For example, eve a 50A D-curve (which you often will not have a low enough Zs to use) has 1kA as upper magnetic trip limit, and most likely fuses but they oguht to be checked if sized for 5s sub-main times, etc.

its more that the bonding is only 6mm2 rather than 10mm2, in reality though I think as you say it will be fine.. Just wanted to check my thinking. Would you code it on an EICR as a C3 or no code?

I would say no code.

The water pipe system (which I assume comes in to the house first) is already bonded to 10mm so meets TN-C-S requirements (even though not needed here) and so this is really more like supplementary bonding for a bathroom, etc,and for that it is plenty.

If it were a totally separate set of metalwork, say a steel foundation, and TN-C-S then I might be a little concerned, but you won't see that high a current to trouble the MICC here.

 

[telectrix]

It might not be suitable for direct burial as not strong enough

maybe, maybe not. our water supply comes in in 1/2" copper, buried in 1956. still intact. once copper has got it's oxide coating, it's proof against most things, unless the ground contains highly corrosive compounds.

 

[pc1966]

It might not be suitable for direct burial as not strong enough

maybe, maybe not. our water supply comes in in 1/2" copper, buried in 1956. still intact. once copper has got it's oxide coating, it's proof against most things, unless the ground contains highly corrosive compounds.

Yes, after I wrote that I did wonder as copper pipes are usually OK outside.

I would rather it were PVC covered MICC if in the ground though, but for visuals I like the bare copper polished up!