TPN Armoured Install Customer does not wantto bury the cable

[John-]

Hi
One day i will be able to contribute a bit more than just asking questions - sorry...

A customer of mine has some alterations to an existing installation, where by we will have to install a new 25mm TPN supply from the DNO to a point circa 25m away. The cable run is on private property and not accessible to the general public, is mostly on a steep incline (bank going from top to bottom) and will run adjacent a rough stepped path cut into the ground. The area has a fair amount of trees, bushes and grass.

His preference is to lay the cable on the surface of the ground, not bury nor suspend.

I am pretty sure i will not change his mind.

What precautions can i take to minimise the risk of injury? I am planning to install a delayed 100mA 4 poll RCD at the DNO in case something unexpected happens along the length of the cable. I could also insist that he puts up signage along the path of the cable, warning electrical cable etc etc.

Is there anything else I can / should do?

Thanks

John

 

[Strima]

Why do you need an RCD to protect the SWA? What’s the earthing arrangement?

withe the cable laid alongside some steps no doubt someone may try to used it a steady themselves at some point. Ideally it would be better buried but is the another option for the cable run, along a fence line for example?

 

[Rockingit]

Given that nature will want to bury it anyway, if it's only 25m I'd be tempted to look at running it into some ducting and secure some warning tape to the top of it, sling it into the bushes and walk away. A neater solution might be to knock some lengths of 100mm gravel board into the dirt along the length of the run and cleat it but then only you know exactly the terrain involved.

 

[John-]

Thanks for the comments. @Strima RCD is used for additional protection in case the cable is damaged exposing a live conductor.

 

[Rockingit]

Thanks for the comments. @Strima RCD is used for additional protection in case the cable is damaged exposing a live conductor.

That's what the earthed mechanical protection is for..........

 

[timhoward]

Thanks for the comments. @Strima RCD is used for additional protection in case the cable is damaged exposing a live conductor.

My opinion - while it's always admirable to go the extra mile that is a little over the top.
SWA distribution circuits are normally simply protected by fuses which can easily achieve the 5 second disconnection time required.
It would be more usual to have the RCD at a consumer unit the other end of it.
You'd generally have to be actively trying to damage an SWA cable to the extent that a live conductor is exposed and provided the outer steel is earthed that would operate the OCPD.

 

[Strima]

Thanks for the comments. @Strima RCD is used for additional protection in case the cable is damaged exposing a live conductor.

Additional protection is a maximum of 30mA. Take a look at 411 and 522.6 (522.6.204).

 

[brianmoooore]

Even the horse that once picked up a SWA overnight that I'd laid in a trench, but not backfilled, didn't get electrocuted when they chewed it into scrap. It may or may not get damaged laying on the surface, or after a week or two, just under, but it won't be dangerous.

 

[John-]

Additional protection is a maximum of 30mA. Take a look at 411 and 522.6 (522.6.204).

I used the term 'Additional' loosely, not in relation to 30mA. Just looking at disconnecting quicker than 5s. It is a distribution circuit installed in an area that could in 5 years time say, be overrun with plants and bushes that someone may want to hack back. If it were me I would prefer a few mA rather than a few 100A to flow Circuits fed after this will be 30mA RCBO where required. Just looking at keeping the customer out of trouble in the event of an unfortunate cable strike. The RCD also doubles up as a form of isolation as well.

 

[OldSparks]

Personally I ALWAYS treat SWA as TT. Earth the armour at the supply end ONLY, and provide an additional earth spike at the load end.
SWA steel sheathing is unlikely to provide a good enough earth on a 25m run on its own.

 

[DPG]

Personally I ALWAYS treat SWA as TT. Earth the armour at the supply end ONLY, and provide an additional earth spike at the load end.
SWA steel sheathing is unlikely to provide a good enough earth on a 25m run on its own.

But surely you could use a core as earth. Bit extreme to change to TT every time you use SWA.

 

[pc1966]

Strictly speaking the regulations don't specify any need for burial at a specific depth, etc, only that the cable should be unlikely to be damaged. If the route is one that is not going be be subject to gardening, etc, then laying it on the surface is an option, but not one that anyone (I suspect) would be very happy with. At the very least it ought to be attached to stakes from time to time to maintain its route.

If digging is not an option, then I would look at possible alternatives like running between two rows of bricks, or under some small paving slabs, so at least the cable is somehow hidden and protected against causal damage.

As already covered, the earthed armour should be protection enough (assuming it is able to disconnect as planned, etc). Even with 3-core I would connect the armour at both ends with proper glands as it offers a bit better conduction and allows your R1+R2 to be a bit lower so any internal cable fault will disconnect faster and with less fault energy.