DC Cable buried <50mm in wall

[BruceB]

I agree the nail will win. but provided the armour is earthed there should not be a drama

 

[SolarCity]

You wouldn't want to put them in separate pieces of metal trunking or you will be giving yourself a couple of nice size electro magnets!!

Surely this wouldn't be an issue with DC?

 

[VMan]

Surely this wouldn't be an issue with DC?

I think with DC would produce electromagnetic effect of some sort, with AC you would get eddy currents, correct me if i'm wrong.

 

[SolarCity]

I think with DC would produce electromagnetic effect of some sort, with AC you would get eddy currents, correct me if i'm wrong.

You're probably right but I'd be interested to know.

I'm thinking that the electromagnetic effect wouldn't be an issue as the magnetic field would be constant.

 

[Deleted member 26818]

you can run DC in SWA.
The regs require cable buried at less than 50mm to be mechanically protected against nails screws and the like. It also says SWA may not satisfy this requirement. Also how will you keep within your zones?
I am very reluctant (as in, never have) to run DC anywhere inside a house except clipped onto the rafters in a loft and clearly labelled.
Is the client also going to be running his drainpipes inside the house, because in all honesty a bit of conduit looks no different!!

Providing mechanical protection sufficient to prevent penetration be nails or screws is just on of the acceptable methods of providing additional protection.
As such SWA doesn't have to satisfy that requirement, as it satisfies another, insofar as the armour is an earthed metallic sheath.

 

[Gavin A]

I agree the nail will win. but provided the armour is earthed there should not be a drama

As such SWA doesn't have to satisfy that requirement, as it satisfies another, insofar as the armour is an earthed metallic sheath.

I'm struggling to see how earthing helps in this situation as the DC circuit from the panels is separated from earth / there is no return path via earth.

If we were to be adding this sort of protection on the DC circuit would we not need to actually bond the sheath to the negative pole of the panels?

Not that this is something we've been doing, we've been using it purely for mechanical protection when running the cable externally, but surely that would be the approach that would be needed if you were to replicate the impact of earthing the sheat on an AC system - in a similar way to the metalwork on cars being bonded to the negative pole on the battery.

or have I missed something here?

 

[spark 68]

Exactly Gavin,

As the DC supply is floating WRT Earth, earthing the armour of the SWA would achieve precisely nothing, unless you either tied one of the DC poles to earth, or tied the armour to one of the DC poles.

Although I have nothing to do with the solar industry, I have worked with DC.

 

[Gavin A]

yep. I think this is the aspect of the situation that makes running the DC cabling inherently a lot safer than AC, as there's virtually no way you could possibly get electrocuted from a nail through the cable.

Worst case scenario is you manage to short circuit it with a nail bridging live and neutral, but you'd still be safe to touch the nail. If you just nicked the positive then there would be the potential there, but you'd have to somehow also be touching the negative side of the circuit for that potential to become actual current flow, and the negative would still be fully insulated and it's not like there would be any other earthed metal parts to worry about.

About the only way I can see of this being a major issue would be 2 nails, one through the positive, the other through the negative, then touch both at once. In this situation though the Armor would save you as the vast majority of the chort circuit current would flow though the armor as the path of least resistance, not you.

 

[BruceB]

Most inverters will also detect if one of the dc cables is taken to earth and will then cut out. So the user can notice something is wrong.

 

[Gavin A]

Most inverters will also detect if one of the dc cables is taken to earth and will then cut out. So the user can notice something is wrong.

but they'd also detect a short circuit on the DC side anyway.

If anything I'd think that bonding the sleeve to earth (as in the MET) would significantly increase the potential danger as it would mean all earth bonded extraneous conductive parts in the house would also be carrying any potential DC fault current, which could come as a nasty surprise for any unfortunate engineer called out to find the fault who'd not be expecting the water pipe in the loft to have a potential to the negative side of the installation when testing the system.

I really can't see any justification for doing it, and lots of justification not to.

 

[BruceB]

It should not really matter what potential it was at from a safety perspective if everything was at the same potential, which is the point of bonding.

If you do not earth the armour of the swa cable then it would not meet the conditions in 522.6.100(ii) or 522.6.101(i) and therefore certainly could not be used for carrying dc buried in a wall.

All conductors in a cable have to be attached to something rather than left floating, including the armour.

I really would recommend earthing it in the situation we are discussing.

 

[Deleted member 26818]

Yes you are missing something.
You are missing the fact that the Regulations often make no sense whatsoever.
With the introduction of the 17th edition, we were required to provide RCD protection to all circuits of a location containing a bath or shower.
This requirements included SELV circuits.
We are now following the amendment only required to provide RCD protection to low voltage circuits, not SELV or ELV circuits.
Although we are still required to provide additional protection for the cables of such circuits if those cables are concealled in walls.
If that protection will not do anything, we are still required to provide it.

 

[yellowvanman]

Not a fan of feeding dc through SWA.

On ac the SWA is earthed so if you put a nail through it and it touches the line conductor the fuse/mcb or RCD will trip, if you touch the neutral only then only an RCD would trip - not a big issue in terms of safety though as the neutral should be at the same potential as earth.

You put dc through SWA and the SWA is earthed, nail touching one of the conductors nothing is going to happen! No trip - if you hit the + conductor then that would be put the SWA at the same potential as the + conductor and the earth for the whole house would be at the + voltage.

If you hit both the + and - then you would get Isc following but still nothing would trip!

In both cases the fault is not 'removed' by a protective device.

 

[Gavin A]

It should not really matter what potential it was at from a safety perspective if everything was at the same potential, which is the point of bonding.

But usually bonding is between conductors that all have potential to earth. In this situation there is no potential to earth from the DC side of the circuit, so you could hold the bear live end of the positive DC cable in one hand and touch the water pipes etc all day long with no problem.

So yes in fault conditions you are creating a situation where everything in the house that's bonded to earth would be at the same potential as the positive DC cable, but that potential wouldn't be to earth, it'd be to the negative side of the PV circuit, so all you're doing here is multiplying the danger instead of preventing it.

Anything that doesn't make a situation safer in any way, but does increase the danger should be avoided IMO.

That said, we only use this method for surface mounted SWA eg on external walls, where we're protecting the circuit from someone catching it with a spade or something, rather than someone sticking a nail through it.

I'm not saying that it will meed the requirements for being buried <50mm in a wall, merely pointing out that bonding the SWA cable to earth in any situation where it's being used to carry DC current from the panels is not a sensible thing to be doing IMO.

 

[edexlab]

I've not run the dc in swa buried in a wall as yet but did look at doing this sometime ago , there was a short thread on this and several ideas were suggested , the most obvious thing to do in my opinion would be to limit the risk of damage to the buried dc cables first either by increasing the mechanical protection
ie perhaps by fitting some steel flat bar in front with some decent spacing between this and the cables ,which a nail would not penetrate (unlikely to be shotfired into in a domestic property) and would make someone drilling stop and think

Another idea would be to distance the cables say 200mm apart in separate single core swa or steel conduit (earthed) this would reduce the risk of a single incident damaging +ve and - ve at the same time
(this is a method I've read is sometimes used in Germany and other countries ,the Americans use aluminium conduits or single core metalclad cable ) be interesting to hear from Bruce to see his take about this idea.
although it is still a possibility that this could become a risk if for example the customer decides to put up a few shelves and both cables have been penetrated and the screws can be touched at the same time , less likely but possible

My preference would be to have it boxed in (ie cables clipped or in trunking inside) to the extent that the cables would be deeper than 75 to 100mm in any direction but visible when the boxing is removed or damaged ,this would be the easiest way of satisfying the regs customer permitting!.