Old MICC cable underground to garage, extraneous conductive parts

[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!

 

[freddo]

There's loads of micc chucked in the ground feeding outside lights in local pubs, all working fine. A lot is bare imperial so must have been installed a while ago.