nyy-j cable - direct burial?

[dom]

I'm installing exterior lighting in the form of four, ground-level (buried), drive-over type units, within 300mm of the front wall of the building.

All the exterior space (soon will be) shingle. No grass, no flower beds, no earth.

The front of the building is also the car parking space, but you can't realistically drive over the (proposed) position for the lights.

I've already installed an IP66 junction box (MK Masterseal) on the outside wall, close to ground level.

Plan is:

2 of nyy-j cables from junction box to 2 lights (which each have 2 of compression glands on the base), either side of front door.

2 of nyy-j cables from each of these lights, to a 2nd light also either side of front door.

Cables direct buried, bedded in sand, but no additional protection. Warning tape buried above.

Whole area covered in ground-grid type support for pea shingle.

Some sort of capping where the cables come above ground, up the wall to the junction box (maybe 250mm exposed).

Realistically, I don't think I'm going to get the cables 450mm deep - maybe 300mm.

I don't want the hassle of SWA cable.

Adequate?

I guess I could slip the nyy-j inside loose-fitted flexible conduit for additional mechanical protection. Using the existing glands on the lights to clamp up on the nyy-j looks best for waterproofing (with maybe a smear of ls-x).

[ARW]

Any chance of a link to the lights you intend to use?

[harryagain]

Why is SWA cable a hassle? No special tools are required.

[Andy Wade]

[NYY-J] Application range Power and control cable for fixed installation in the following applications: For indoor and outdoor use Burial without additional, suitable underground protection according to VDE standard HD 603/VDE 0276-603 - Part 3-G (point 4) governing PVC cables for direct burial: normal minimum installation depth 0.6 m, but at least 0.8 m under roads In concrete with a temperature below the maximum cable operating temperature of +70 °C according to the VDE standard HD 603/VDE 0276-603 - Part 3-G (point 4) governing PVC cables for direct burial In water: no longer than 2 weeks at a time, maximum submersion depth 10 metres, only in static water/bodies of water without shipping traffic

Since you won't meet the above depths I'd suggest some hard protection: concrete cable covers or similar.

[2] Might also be of interest - seems you're not the first...

*****

[1] [2]

[dom]

[Tim Watts]

On Friday 09 August 2013 22:06 Andy Wade wrote in uk.d-i-y:

Fundamentally this stuff does not have an earthed screen (which the armour in SWA acts as). An earthed screen will guarantee that a metal tool that penetrates the cable will be earthed *before* it hits a live conductor, tripping the protective device (fuse/MCB/RCD).

My conclusion, irrespective of any claims or regulations, is *I would not use this cable underground* unless it was inside earthed metal conduit or similar.

Besides, small SWA is hardly difficult to work with - I don;t see the OP's problem...

Cheers,

Tim

[Andy Wade]

BS 7671 does not impose an absolute requirement for an earthed covering.

Reg. 522.8.10: "Except where installed in a conduit or duct which provides equivalent protection against mechanical damage, a cable buried in the ground shall incorporate an earthed armour or metal sheath or both, suitable for use as a protective conductor. The location of buried cables shall be marked by cable covers or a suitable marking tape. Buried conduits and ducts shall be suitably identified. Buried cables, conduits and ducts shall be at a sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground."

A concrete channel and cover could be used to form a duct providing mechanical protection and also marking the location.

That depends a bit on the cable entries provided on the luminaires.

I'm taking it as read that 30 mA RCD protection will be provided for the feeding circuit.

[ARW]

I am wondering if ducting and flex might be a better option.

[dom]

Yes the fittings are well suited to NYY-J cable, although no doubt they could be adapted to SWA.

My own opinion is that the relatively short cable runs, max 2.5m from external junction box, and 1.5m light to light - make it pretty obvious there's a buried cable run.

On it's own RCBO.

[Tim Watts]

On Saturday 10 August 2013 14:12 Andy Wade wrote in uk.d-i-y:

Yep - that's why I very carefully made my statement as I did :)

Essentially, if I stick a fork in that cable, I want the tool to hit mains- earth before it hits a live conductor.

At times like these, I will happily exceed the regs if I'm unhappy with their strict application in a particular scenario. You may disagree in this case.

But I think a residential garden having un-armoured cables buried direct in the ground is a very very bad idea.

...

Or that - but we're back to "is SWA really so hard that concrete troughts would be easier!"

Well - this is true. if they are anything like internal lumineres! I'd take the SWA into a Pratley box which is ideal for waterproof maintainable SWA terminations. Have that as close to the light as possible and make the final very short connection as suits the fitting. The Pratley box makes a good junction/loop through JB too if that suits how the cabling is to be laid.

[js.b1]

NYY-J is more for surface cable runs (unless very deep as indicated by A.Wa de), is expensive £1.20-1.50/metre? for something offering no protection compared to SWA.

You could use a BS8436 cable, the conductor bundle is wrapped in foil which is specifically designed to withstand short circuit fault currents ONLY wi th suitable Type-B MCB protection (eg, 20A for 2.5mm). It is not a trivial foil screening, it is continuous with an uninsulated tinned copper earth co nductor. Glands are plastic as opposed to the unwieldy brass/SWA combinatio n.

I think Discount Electrical sell it by the metre (£1.25-1.75?), Ebay list it by the metre (£1.50-2.50 from memory). It will still need running in flexible conduit if only for general routing and possible replacement. It w ill still need to be buried at ideally 450mm or as required for the usage o f the land - and in river washed sand with "electric cable below" warning t ape above (TLC do that by the metre).

Conduit itself provides no mechanical protection from a spade, or point con tact with rocks if you have clay - they are prone to sink over years. Steel conduit life is limited by zinc thickness (which is little).

Many lights have small terminal blocks or push fit terminals, so you probab ly want 1.5mm - which will assist routing anyway. Check the earth regularly if using metal light fittings (eg, JCC).

I would use SWA to suitable boxes & Piranha nuts (TLC sell), or BS8436 1.5m m in nylon flexible conduit (can be had cheaply on Ebay too).

[Andy Wade]

Your fork or spade will almost certainly in contact with the soil when it penetrates the cable and the loop impedance in the vast majority of cases will be low enough to trip a 30 mA RCD in milliseconds. I agree that it's a different matter for a non-RCD-protected circuit but even then most digging implements have insulated handles.

Fairynuff.

Well, to be pedantic, almost every house has a service cable under the front garden. These cables aren't armoured, but are of concentric or lead sheathed construction. Time says these aren't a great hazard.

Moreover... how do you connect SWA to equipment underground? Crimped resin-filled joints would be OK, but are expensive and would require the equipment to be compatible. Burying SWA glands is surely a no-no, even 'CW' type. The gland makes the connection in the CPC, so has to be accessible for inspection and testing [543.3.3 and 526.3]. The only get-outs I can see are 526.3(i) - but SWA glands aren't designed to be buried - or 526.3(v) - is there a relevant product standard? 543.3.1 also needs to be considered, re. corrosion - specifically the risk of dezincification of a brass gland in some soils. NYY-J and plastic glands seem more attractive now...

[Tim Watts]

On Monday 12 August 2013 11:20 Andy Wade wrote in uk.d-i-y:

Hi Andy,

Dry earth?

Yes - and the concentric nature means the tool will hit either earth or the neutral in a PME system first. Though I do agree a service cable fused at

300A will go bang quite nicely.

Pratley boxes claim to be suitable for underground SWA jointing without gel or resin filling. Do the manufacturer claims trump the regs?

Cheers,

Tim

[docholliday93]

I'm not in a position to check it now, but I'm pretty sure that the service cable in my first house was like the one in the present one, which is lead sheathed (presumably PILC) with a steel tape armour. (my second house had overhead cables, even though it was in a small town rather than rural location, and there are still quite a number of others with overhead mains connections)

[Tim Watts]

On Monday 12 August 2013 13:48 [e-mail] wrote in uk.d-i-y:

I've seen he end of mine, in the cutout. Outer insulated neutral strands (PME) and inner live core.

[dom]

This pretty much hits the nail on the head. The inset light fittings sit 27

0mm deep into the ground and have a pair of plastic compression glands in t he base. These glands *could* be replaced with SWA glands, but the glands n eed to screw directly into the cast plastic body (so the interior locknut a nd earth tab can't be used).

As opinion here was pretty firmly in favour of SWA, I'd given some thought to how to do it that way - but it's still coming back to the light fittings being most suited to NYY-J.

[Andy Wade]

Here's an interesting document about power cables, suggesting that you're right about the PILC cables being armoured.

[ARW]

One of the reasons I asked for a link to the lights you wanted to fit was because I wanted to know how I could connect them to a cable. And the truth is, I am still not sure how I would wire them up:-). I would not be happy with either SWA or nylon compression glands buried underground.

I have one other suggestion for you. Use LED lights with a built in cable (they will be epoxy resin IP protected) and then you pass this cable through some ducting to a LED driver. It would make replacement easier should it be needed.

Now as most people know, I am not a snob. But I have to be totally honest and I believe that a 20 lightfitting is not really going to cut the mustard in this case. The lights you linked to are PKE Pro Spec lights - and I have never heard of them. They might be suitable for use on decking but I would not fit them on a driveway or indeed in a footpath due to the difficulties of keeping them watertight in what is in real terms a submerged position.

I'll still help you whatever you decide to do/buy.

[dom]

I agree they are a pretty cheap fitting. But I bought one to check the qual ity and was pleasantly surprised. Price jump to next rung in quality seemed to be about £75 (which I'd swallow for 1, but not 4). I'm using PAR30 LE D lamps with them (having checked the brightness/colour/beam dispersion). P urpose is to illuminate the front of an (old) building (in a dark rural are a).

About the fitting - body good, polycarb lens/cover good (incl 2 rubber gask ets), glands good, lampholder not so good (Ikea sort of quality).

I'm thinking a smear of LS-X silicon on the glands and lens to keep the fi tting as watertight as possible.

[dom]

A compromise occurred to me - use SWA, but take it into the light fittings by the existing compression glands (rated IP68), rather than CW SWA glands (IP66).

Use 3-core SWA, and WAGO's inside the light fittings for continuity on the earth core (L & N continuity via the lamp holder).

(the light fittings really need to be daisy-chained)

But without an SWA gland, how can the steel wire screen be clamped to?

Alternatives seem to be:

1. Run 4 individual cables from the above-ground junction box, CW SWA glands at that end, compression gland at the underground light fitting (so screen clamped one end only).
2. Use the smaller BW SWA glands inside each light fittings to clamp the screen (though these wouldn't be fixed to anything).
3. Find some other way of clamping to the screen.
4. Leave the screen floating.