Earthing Armoured Cable

[YoungManGW]

I trust that I've placed this in the correct category.

A bit of advice, please, just to check I have something right. I need to connect up some armoured cable, between the back of a waterproof exterior double socket and a series of three garden lights. The cable is three core, 2.5mm2. The lights are high pressure sodium lamps, 70w each, rewired with 2.5mm2 twin and earth.

I intend to use waterproof plastic junction boxes to effect a series of three junctions between the armoured cable and the T&E. One core of the armoured cable will be used as an earth. This will, obviously, connect to the earth of the T&E.

The needed clarification comes with respect to the armour. My understanding is that, even with one of the cores used as an earth, the armour should be earthed too. So, the armour needs to be connected, through the junction boxes, to ensure continuity, using banjos or earthing nuts. So far, so good. But here's the first question:

Does the core earth need to be kept separate from the armour earth? Or, for ease of effecting the connections, can both the core earth and the armour earth make use of the same connecting block, i.e. be all linked together? Or do they need to be kept separate?

A second question follows if they need to be kept separate: in that case, to where should the armour earth be connected at the supply end?

Assistance appreciated. BTW, I'm not seeking information on trenches, Part P, rules and regulations, just practical advice. Many thanks.

Regards,
Richard

[Scimitar]

You ought to be using metal boxes with knockouts. That is the only way to reliably preserve the earth integrity.

[Craig Sawyers]

My son, who I ask in this sort of problem - who is qualified to Part P - says that it is not necessary to ground the armouring.

If you are using one of the cores as E, there is absolutely no need to also earth the armouring. If it is three phase, where each of the three cores is a phase, then yes - the ground is managed via the armouring. But since your installation is single phase, don't earth the armouring.

So cut it flush, and use a gland to make a waterproof joint to the outer insulation.

[YoungManGW]

Thanks Scimitar. My understanding is that, provided there is a good firm connection between the banjos/earthing nuts, there is no problem using plastic boxes. There is a good reason for using such, in that plastic boxes are better suited to external conditions.

Thanks Craig. Yes, is single phase. There does appear, on various electrical fora, to be much disagreement on the point, when one of the cores is used as earth. Some say the armour should be earthed too, some not. Of the first group, some say to earth the armour at just the supply end, some at both.

Regards,
Richard

[OscarFoxtrot]

#3's not right. And there's no such thing as a Part P qualification anyway.

The armour must always be earthed, and usually at the supply end. The point of the armour is that if the cable is damaged a fault current will flow between the core and the armour and open the circuit protective device.

The question is whether the armour earth is connected at the load end of the cable.

If the supply earth is exported to the load end (TT or TN-S supply) then the armour and the cpc will be linked at the load end and use the same nuts/connections.

There are cases where the supply earth must not be exported to the load end because of the cable length, supply characteristics or other rules. In such case the armour is earthed at the supply end to protect the cable. The armour is insulated at the load end, and the load end installation has its own separate earth rod and earthing system completely separate from the supply.

Use Piranha nuts not banjos. https://www.tlc-direct.co.uk/Products/PRPEN20.html

I don't understand why the lights are 'rewired with 2.5mm twin and earth'? An exterior rated cable should be used between the junction boxes and the lights if necessary.

[Argus25]

Originally Posted by YoungManGW

I need to connect up some armored cable, between the back of a waterproof exterior double socket and a series of three garden lights.
Richard

Richard, I could give some advice here, from bitter practical experience.

If you are setting up garden lights (unless it is an indoor covered weatherproof garden inside the dwelling) I would strongly advise not to use mains wiring to do it, no matter how robust the earthing & armor and no matter how perfect the setup seems and no matter if it meets electrical regulations or not. Because there is a better way to do it, that won't give any safety risks or any trouble. I will explain:

Even in a perfect setup for mains power outdoor wiring, there is always condensation that causes earth leakage. So after a year or two you might find that your breaker box RCD starts to trip. This is often great it you are away from home and your refrigerator goes off and your set top box stops recording your favorite programs, the backup battery in your security system goes flat etc etc.

The way to solve all the safety and nuisance RCD tripping problems of outdoor wiring is to go over to isolated low voltage (24V AC), which there are many garden lighting transformers for, some in substantial output capacity.

This way all issues are solved with no future headaches or safety issues, and you can impress your friends by unscrewing an outdoor lamp and putting your finger in the socket to prove the point. Or chopping into the wiring with the garden spade. And you will never have any nuisance RCD tripping either.

You may already be stuck with the lamps you have, but there are plenty of alternatives.

I spent some years trying to perfect conduit armored outdoor mains wiring, and in the end learnt my lesson the hard way. Now all my outdoor lighting/wiring is low voltage and the problems have all gone away.

[broadgage]

I would agree that 24 volts or similar is preferable on safety and reliability grounds, only viable for modest wattages though. 24 volt control gear for discharge lamps is not readily available.

If however the decision to use mains has already been made, then I agree that the armouring of SWA cable should be earthed.
The cable ends should be terminated in the proper glands sold for the purpose, and thereby earthed.

If going on holiday I would isolate any external circuits before leaving so as to avoid any problems caused by the tripping of an RCD.

[trh01uk]

Originally Posted by OscarFoxtrot

#3's not right. And there's no such thing as a Part P qualification anyway.

Funny that. There are thousands of professional electricians out there advertising themselves as "part P certified" or similar wording. There are even registers of such people. Perhaps you should take this up with them?


Richard

[Station X]

No more discussion about Part P of the building regulations please.

[ms660]

The armour of the cable must be connected to the main earth, the armour offers little protection to the conductors insofar as forks, spades, digger buckets etc are concerned.

Lawrence.

[emeritus]

When I installed SWA to supply my garden sheds more than 25 years ago, it was (and still is) connected via its own fuse box and ELCB (out of necessity as no spare fuseways were available in the consumer unit). Never had any instances of the circuit breaker tripping, other than when replacing a light fitting, when the Neutral wire accidentally came into contact with the earthed case of the new luminaire.

[Radio Wrangler]

The basic principle is that anything which can cut into the cable and get at the inner conductors will be bubbing against grounded armour wires so that the offending tool shouldn't go live and that a fault current is created which will trip an RCD, or if nothing better is fitted, pop a fuse.

For this to work, the grounding of the armour must be at least beefy enough to pull the breaker or fuse that powers the circuit.

If a junction or connection box is plastic, you still need a serious metal structure bonding the glands of the armoured cables.

David

[Radio Wrangler]

Oh, and a feed to anything classed as another building is required to have an isolation switch at both ends.

Don't think this applies to streetlamps in a garden, but there should be an isolation switch.

Self preservation suggests having one which can be locked off so you aren't up a ladder when someone else flips a switch expecting it to do something else!

David (Chartered electrical engineer, but somewhat out of date)

[eaglework]

When I ran 2 SWA feeds using 3 core to a shed and greenhouse the electrician said the SWA armour should be earthed at the CU end and extended through any intermediate junction boxes.
He was also wary of plastic junction boxes as they tend to deform under the pressure of the SWA gland nut over time and can result in a poor earth connection.
On an electricians forum I occasionally use there are suggestions of using a lock nut as well as the SWA nut to ensure the linking connection remains secure or use 'pirana' nuts.
Hope that's of use.

[Argus25]

One house I rented some years ago had mains powered outdoor lamps. The conduit was a grey plastic that had deteriorated & cracked letting water in and exposing the wiring. There was no RCD on the breaker box back then.

One day the gardener came with a lawn edge trimming tool. About a week after that I saw a red object in the grass near an edge where one of the lamps was mounted on a wooden beam, it was a phase wire with much of the insulation missing and the copper exposed after trauma from the gardening tool. My daughters played on that lawn, sometimes at night when the lights were on. After seeing that I decided mains wiring in the outdoors needed to be in much better quality conduit than was commonly used and preferably buried under concrete, or better still avoid it in the outdoors for lighting & use isolated low voltage for total safety.

[YoungManGW]

Many thanks for all the comments and advice - very helpful.

To address the various (in some case implied) questions:
- the rewiring of the lamps is internal to the heads and supporting poles, at the base of which will be the junction boxes with the armoured cable
- am committed to mains, as I pride myself on making use of what I already have to hand, rather than buying new, which in this case are some rather nice Philips street lamp heads, sourced indirectly via this forum
- there will be an RCD
- there will be an isolator switch at the supply end
- the metal glands connected to the armour either side of the junction boxes will be substantially connected to each other

And to check my understanding of the advice so far:
- earth the armour at the supply end
- connect the armour earth and the core earth at the junctions
- use piranha nuts (I was racking my brain for this name when thinking of earthing nuts, but couldn't get 'penguin nuts' out of my head).

Regards,
Richard

[Radio Wrangler]

That sounds sensible.

Many years I rebuild an electrical system at a stables.
Pole-top earth from the supplier (pole about 300m away)
Armoured feeds to sundry outbuildings including building a new pump-house for a private spring water system.
Power and sensing up the hill from about 10,000 liter steel tank with float switches, power for remote TV antenna amps etc and a socket for the vacuum cleaner to clean around the seal before opening the top hatch on the tank.
6kVA diesel generator with earth rods etc for when (not if) power fails. Loss of lines can include the earth wire. hoseable switches everywhere (even indoors)

Manual changeover for the generator, it has to be hand-cranked anyway.. 1954 Royal Enfield, and needs a burst of revs offline for self-excitation to come up. The generator is needed to run the water pump. At least with several days water available by gravity feed, there is a window to fix things.

Worst bit was digging (pickaxe work) a trench 100m up a rocky hillside too steep to get a machine in. Trench carries water up and down pipes. power up, pump trigger signal down and buryable TV coax down. There's a pair for a contents gauge some time in the future..

A bit of an epic!

David

[OscarFoxtrot]

Originally Posted by YoungManGW

And to check my understanding of the advice so far:
- earth the armour at the supply end
- connect the armour earth and the core earth at the junctions
- use piranha nuts (I was racking my brain for this name when thinking of earthing nuts, but couldn't get 'penguin nuts' out of my head).

I hadn't realised you were using streetlights on poles.

All good.

To add:

All cables buried 2 spits deep with "buried cable below" tape at 1 spit deep.

There are specific fused cutouts for streetlights to combat condensation and tracking, and to be easier to fit in a confined space:

https://www.electricaldirect.co.uk/p...cut-out-488775

https://www.youtube.com/watch?v=BioanX0RJgI

https://www.youtube.com/watch?v=s0EBpt1tMQ4

[eaglework]

'-connect the armour earth and the core earth at the junctions'

Not sure you really need to this but I cannot see it would cause a problem if you did.
The important point is that your 'functional' earth in the cores of the SWA is continuous and adequately terminated as is the armour of the SWA.
I suppose what you propose would give some additional protection if the SWA armour were to fail at one of your intermediate connections.

[ColinB]

Originally Posted by Craig Sawyers

My son, who I ask in this sort of problem - who is qualified to Part P - says that it is not necessary to ground the armouring.

If you are using one of the cores as E, there is absolutely no need to also earth the armouring. If it is three phase, where each of the three cores is a phase, then yes - the ground is managed via the armouring. But since your installation is single phase, don't earth the armouring.

So cut it flush, and use a gland to make a waterproof joint to the outer insulation.

Sorry to disagree, but this is wrong I believe.

The point of SWA is that it can be buried in the ground.

If sliced through by a shovel (or trowel, JCB....), the metal blade will short the phase conductor to the earthed armouring and trip the RCD or blow the fuse.

If the armour is not connected to earth at the supply end, this will not happen, making the cable no safer than T&E.

Using a core of the cable to deliver the earth to the socket and lights is fine, but all armouring MUST be earthed as well. This also offers the advantage that the core and armour are in parallel, and the impedance will be lower.

Look for 'banjos' and proper glands to make the connections with. Good luck with the job, it's tougher to work with than it looks! Don't forget your Weetabix