Outbuilding Power

[JDLDIY]

My garage already has power for sockets and lights, but as part of the overall rewiring I need to make an assessment of it. The consumer unit end of the installation requires attention as I will be moving the circuit to a new unit where it will be covered by a separate 30A RCD protected circuit. The cable runs from the house underground but alongside the driveway and comes up throught the garage floor. It then connects into a switch-fuse unit, from which separate light and power circuits are taken. The cable, however appears to be 2 core but the exterior insulation is metal, so the cable is flexible but fairly rigid. The round conduit which it runs in underground is also metal.

So, how does this work? Presumably the metal cable is acting as the earth? Is this somehow then connected to the consumer unit to connect the earth circuit? Should I remove the whole lot and start again? I would like to be able to remove the cable from the conduit and run a new 3 core through it, but with the conduit being metal presumably that's not acceptable? I would like to avoid having to lay more conduit as my driveway is concrete but if I must..........

 

[Big_Spark]

JD, there is nothing wrong with using the outer sheath of an SWA cable as the Earth core, however without seeing the actual cable and being able to do some checks on it one cannot really say it is or is not acceptable the way it currently stands.

The fact that the cable has been run in metal conduit means that it has been done with some thought as to protection of the cable, however metal corrodes, and with it being buried under your driveway, there is the possibility that the conduit may have damaged the outer sheath of the SWA and compromised it's ability to offer adequate earth protection.

The only way to know for sure is to test the cable properly, and I do not mean with a multi-meter either.

I would suggest that you call a local electrical contractor and ask them to do a full periodic inspection and test of the garage supply.

Specifically you need to know that the Earth path provided by the SWA sheath meets the requirements visve and Earth Loop impedance Test and also for the cable to be fully tested with regards to insulation resistance. It would be a wise precaution to see if there is any contact between the steel of the conduit and the steel armouring of the cable.

A multimeter will be unlikely to do this, but a properly used Insulation Resistance Tester can. Although it should be borne in mind that corroded steel (rust) does not transmit electricity so a negative result will not necessarilly be proof positive.

The other thing to consider is the type of loads you will use in thre garage, what the maximum potential load will be, and then decide if this cable will meet that requirement with respect to load carrying capability and voltage drop.

Personally this type of work should not really be done by the DIYer as the increased potential for shock makes it a priority that everything is done to the highest standard and safety is paramount.

 

[JDLDIY]

Many thanks for the advice FWL, I totally follow all your drift on this. I could as you say call out a local electrical company to do a field test on this installation, but by the time I have paid for it, I could use the funds to go some way towards replacing the supply myself. I agree that the conduit and/or the cable is likely to be affected by corrosion, the thing is just getting the system checked over now isn't really sufficient is it? I mean just because it's working now doesn't mean that it will continue to remain that way....the more I think about this the more I am deciding to replace the lot and therefore be certain of the job.

JD

 

[Big_Spark]

JD..where do you live??

 

[securespark]

Sounds like MICC to me, FWL.

 

[JDLDIY]

The north west. MICC?

 

[Big_Spark]

Sounds like MICC to me, FWL.

Actually, re-reading the post a couple of times that is a distinct possibility which I hadn't considered, I got used to people on here describing SWA in weird and wonderful ways and assumed that was what he was talking about.

The simple way is to ask this...

JD..this cable in the conduit, is the outer sheath, the metal part, solid in structure or a series of metal cores, like a hauser cable?

If you cannot see it because it is covered in an outer layer of PVC, then what colour is this PVC sheath?

 

[JDLDIY]

The outer metal sheath is one long tube as far as I can see. About 1cm diameter, it screws into the switch fuse unit on the garage wall. I haven't taken it apart as yet, I haven't seen the cable from a cross-sectional viewpoint. So I don't know whether the outer shell of metal has inner metal sleeves as well, if that is what you mean. What is SWA, MICC? If you've got any links I'm sure I can identify it. Do I take it that I can't remove the metal cable from it's conduit and thread some 3-core down it? That would save me digging and laying more conduit.

Many thanks again for the advice.

JD

 

[Big_Spark]

OK, well this has clarifed things immensly, and also saved you some money I suspect.

SWA is an abbreviation for Steel Wire Armoured Cable. This is normal PVC insulated cable cores in an inner sheath that is surrounded by a steel wire cable that performs the function of mechanical protection and earth connection. The steel sheath has an outer PVC sheath around it, black in colour and PVC stabilised for external use.

MICC is Pyro, or Mineral Insulated Copper Clad cable. This cable has one to 48 cores which are embedded in a white powder, magnesium sulphate, that acts as in insulator, this mineral powder is surrounded by an outer copper sheath, sometime this copper sheath has a PVC covering, the PVC covering will be Orange, Red, or possibly black or white.

Pyro has the highest current carrying capacity of any cable, size for size as it is made from extremely pure copper and is hard drawn annealed, combining this with the excellent thermal properties of the mineral insulation and radiant abilities of the copper sheath mean that like for like, MICC can carry about 1.5 times the load of similar sized SWA.

The Earth protection is afforded by the copper outer sheath.

Re-terminating Pyro is not a DIY job, the mineral insulation is poisonous and if the cable is not terminated properly first time, you have a serious problem on your hands. However a Pyro installation will last virtually indefinetly.

 

[il78]

SWA (Steel Wired Armoured)

MICC (Mineral Insulated)

 

[JDLDIY]

Thanks il78! Nope, neither of the above look like my cable in question! But now I know what they (MICC and SWA) are and look like.


FWL, thanks very much - I missed your reply just now. So, which do you think I have then? Do you recommend replacement or should I leave well alone?

JD

 

[securespark]

However a Pyro installation will last virtually indefinetly.

Unless a navvy puts his fork through it, digging a trench for a new gas pipe!

Customer had a problem, no power. I tested - everything hunky dory. Next day off again. Hmm. Went outside. Saw signs of digging. Oh yes says client. Gas man dug in new pipe 6 months ago.

Dug up cable, and hey presto, damaged conduit and pyro.

We didn't do it, bleats the digging contractor.....

JDL Your cable may not have any plastic insulation on it may just be naked copper.

 

[Big_Spark]

And if it is naked copper in the rusting conduit..I would be tempted to dig it up as the copper sheath will suffer oxidation and this could damage the integrity of the cable to perform as originally installed.

JD, I think I must slide back to my earlier comment about getting someone to look at it.

Does anyone with some serious knowledge not live near this fella..anyone willing to give him a hand and take a look at this for him?

It's only a five minute job plus a little travel..depending on where you are.

 

[ban-all-sheds]

The SWA illustration you showed him was 4-core.

This is what 2-core looks like:

Is this it?

Failing that, take a look at the illustrations here - does anything look familiar?

 

[securespark]

I'm in Stockport, but don't know which bit of the NW JDL is in??