Is it prudent or safe or anything else to add a local earth rod to a buildi= ng some 100m from its mains supply source (farm house)? Power (1ph) is pro= vided via buried SWA to a metal double 13A surface outlet. I'm not sure if= the existing earth is provided by the SWA armour or one of the conductors = but it is from the farm house. There is at present no local earth point wh= ich we are proposing to remedy by the addition of one or two earth rods.
Does the additional local earth compromise the earthing provision at the fa= rm house into the SWA?
some 100m from its mains supply source (farm house)? Power (1ph) is provided via buried SWA to a metal double 13A surface outlet. I'm not sure if the existing earth is provided by the SWA armour or one of the conductors but it is from the farm house. There is at present no local earth point which we are proposing to remedy by the addition of one or two earth rods.
house into the SWA?
I am FAIRLY sUre that not only is that acceptable, its also 'good practice'
Tying earth down to a local stake is never a bad thing. As one does with pipe earth bonding
Which although superficially there to make sure pipework is earthed, is also a way to make sure that earth is in fact earth if the pipework is connected to buried metal pipe.
Whether the earth is provided by the armour or the armour and a core is not that important (as long as the armour *is* earthed) in itself - both are acceptable.
Not generally...
However this is a slightly deeper question than it first appears.
There are pros and cons for exporting earths - especially at that sort of distance. Which is best will depend a bit on the earthing system used in the house. So that is the first thing you need to establish.
If you want to know how to tell what you have, see here:
formatting link
have a look at:
formatting link
also read the following section on exporting an equipotential zone. This would probably apply if your earthing at the origin of the circuit is TN-C-S (it will depend a little on the nature of the remote building and what it is used for)
The local socket in the outbuilding will almost certainly require RCD protection - however on the off chance that its not got it, or for some reason you don't want it, you will need to take great care ensuring that your earth loop impedance is low enough to reliably disconnect the circuit in the event of a fault. For this an other reasons, you would probably be better off with the remote building having a TT earthing system in its own right. This would mean that the supply SWA armour would be earthed at the head end, but would not be connected to anything at the remote end. The earthing at the remote end would be provided by local earth rod(s), and a local RCD would ensure disconnection times.
More information is required for a meaningful answer to your question:
- type of earthing at the farmhouse end - TN-S, TN-C-S (PME), TT, or PNB (see
formatting link
,- rating and type of overcurrent protective device for the circuit supplying the outbuilding,
- rating, type and position of any RCD (or old ELCB) protecting this circuit;
- type (or approx date of installation), conductor size and no. of cores of the 100 m run of SWA cable,
- description and use of the outbuilding: dry floor or damp ground, type of electrical appliances used, are appliances used only within the building, or outdoors too, are there any metallic service pipes (e.g. water) entering the building from the ground.
It has come up before. The Nat. Phil. has it, IIRC.
Protective neutral bonding is a variation on the theme of TN-C-S used where one distribution transformer supplies only one customer. Instead of multiple earth electrodes along the route of the supplier's combined neutral & earth conductor, a single earth electrode [1] is connected at the point of the N-E strap in the supply cut-out.
It's used for some rural supplies, farms (why I mentioned it) and also large industrial sites fed from a dedicated tranny. Chapter & verse is in BS 7430.
[1] Or a single earth system (set of electrodes) if necessary to get a sufficiently low earth resistance.
No knowledge about age or installation date of SWA. I'm at site on Saturda= y so can estimate it's size.
Everything fed from the sockets - lighting and occasional pumps and power t= ools is connected by plug-in RCDs. No idea what the protection is at the f= arm.
The remote building is a WWII Rotor bunker - is the name sufficient to expa= lin what it is? - which requires periodic pumping to drain the lower level.= There are various sewage ejection pipes running to a remote sewage farm b= ut other than that it has been effectively isolated from the outside world.= The power from the farm is (relatively) recent. Photos are a little tric= ky due to the need to discourage unwanted visitors but I'll see what I can = do.
tools is connected by plug-in RCDs. =A0No idea what the protection is at t= he farm.
palin what it is?
There is probablly various cable entries dotted around it, for telecoms and RF connections, probably some whacking installed earths as well, if you can find them.
rious sewage ejection pipes running to a remote sewage farm but other than = that it has been effectively isolated from the outside world. =A0The power = from the farm is (relatively) recent. =A0Photos are a little tricky due to = the need to discourage unwanted visitors but I'll see what I can do.
Wouldn`t want it to end up like Barnton Quarry :-( , pics from 1991 were mine:
The full list was an ideal, any info is better then none.
I just wanted to get an idea of the resistances involved, to assess fault protection. An o/d measurement would be better then nothing, but core CSA and number would be better. Can I take it that the SWA is old, so likely to be BS 6346 PVC insulated type?
[Quoting order changed]
Former radar station? It must have had a more substantial electrical installation at one time. Presumably that fell into disrepair.
If the feed from the farm is recent (how recent?) it should be properly earthed and have RCD protection, otherwise the electrician who connected it was negligent.
OK, it sounds like it could be potentially quite dangerous if there is significant 'earth lift' under fault conditions. Damp conditions, other pipework entering, etc. The danger is greatest when using Class 1 equipment (appliances with earthed metalwork & 3-core flex) and an RCD doesn't help for the particular situation of the earth conductors being at a dangerous voltage relative to the local ground and metalwork.
If the farm earthing is TN (and in the absence of info we should assume it is) then a local earth just connected in parallel to the SWA armour - as you appear to have been proposing - won't help much. The safest option by far will be to isolate the armour and any earthed cores in the SWA and then earth the local socketry to a decent local electrode - say an 8 ft. rod, for average soil. This creates a local TT system, independent of the farm, working in conjunction with the 30 mA RCD that you absolutely must also install.
So terminate SWA gland into a stout plastic box, insulate the armour and any unused cores /as if they were live/, then the L & N cores go directly to the RCD, then safely out from the RCD to your socket(s). Local earth (16 mm^2 earth cable is required from the rods, for reasons of mechanical robustness) goes to your sockets, and can be bonded to any incoming metal service pipes. This earth should not go anywhere near the incoming SWA cable nor the input side of the RCD.
Photos are a little tricky due to the need to discourage unwanted visitors but I'll see what I can do.
The supply location at the farmhouse would have been of most interest, but perhaps you don't have access there?
If its a recent SWA then there will probably be marking on the outer sheath that will tell you what it is.
Sounds like you really don't want to try and export the farm's equipotential zone there then ;-)
Even if the farm earth is particularly good, with that length of cable it is going to be very difficult controlling touch voltages on the casework of any class 1 equipment under fault conditions (unless the SWA is pretty substantial).
So I would go along with Andy's recommendation that you make the building a self contained TT installation with its own RCD and earth connection, and isolate it from the supplies earth altogether.
(what are the local soil conditions like?)
There is a separate consideration as to if there is adequate earth fault protection for the SWA itself - but you would need to know how its setup and fed from the farm to work that out. (Assuming you don't want folk trekking to the farm to reset a RCD in the event of a trip, it can get a bit complicated if you also need a RCD at the head end just to protect the SWA - you then end up with a cascaded pair of them, and so need a time delayed type at the head end, and a normal one in the outbuilding)
Thinking back to previous discussions on this, did we ever come to a conclusion as to what the best practice solution was for guarding against access to the supply's earth in these cases?
(given the relative ease of "tool less" access to the SWA armour simply by sliding the shroud along the cable and away from the gland)
Not heard of PNB before. I'm still trying to figure out whether I have TN-S or not in my supply.
Coming into my house it looks like TN-S - I have separate L, N and E. However, I'm fed from a dedicated pole mounted transformer on my land, which in turn is fed from 2 overhead lines (presumably 11kV). There are no other properties supplied from this transformer. E is connected to N at the tranformer, and the whole arrangement appears to be earthed by a spike at the bottom of the pole.
Remember that if its two wire and shath, or even coaxial, there is enough leakage between the sheath and the ground to more or less provide a decent path. Not as good as a nice steel stake or a ground mat of course, but pretty good.
I would do that except Id still use supply earth as well. Belt and braces.
That sounds sane at least
However a leak in an underground cable is no big deal.
MINE blew the 60A fuse in the transformer. Then when they popped in a
100A one, it blew the cable! The enabled them to ID the fault to within
300mm. Because in my case with a coaxial the nearest 'earth' to leak to was the outer neutral.
Sodding digger driver. I TOLD him there was a cable there.
What I would do is actually install a cable direct to te house side of the meter, and put a completely independent RCD protected consumer unit at the remote site and earth it to a stake AND carry the neutral bonded to the sheath across as well.
Not sure I follow that, are we talking at crossed purposes?
I was considering the effects on the touch voltage on the casework of class 1 kit in the event of a L to E short at the outbuilding if the SWA head end does not include RCD protection.
You would just be introducing another source of potential into the system though for no real gain.
The actual implications would vary with the nature of the earthing system at the head end. If the head end is TN-C-S, then you also need to extend the farm's equipotential zone out into the outbuilding, and given the nature of the outbuilding's construction and ready access to independent paths to earth, that would be a major cause of difficulties. If the farm is TN-S, then you are connecting two potentially quite remote earthing localities. TT at both ends would be less of a problem.
A small leakage to earth you might tolerate so long as its not sensitising a RCD shared with other circuited at the head end. A large leakage caused by someone looking rather startled and emitting a small plume of smoke, with his pickaxe firmly embedded in the cable however is another matter. Moderate leakage due to just a minor cable fault might be ok, but for the fact its probably downstream of the farm's electricity meter, and someone will be paying for warming up that spot of ground.
Somewhat akin to finding the hairline trace bridge on a PCB with a car battery ;-)
What about feeding the whole outer sheath through a plastic gland? Shroud no longer needed although you could slide it over the plastic gland to look "normal". Need to make sure nothing can contact the armour inside the enclosure. Could heatshrink over the point where the outer sheath and armour are cut off.
My other way of removing access to a conductor at the end of a cable is to push back the insulation, cut the conductor, and allow the insulation to slide back into position so the end of the conductor vanishes a long way inside the insulation. I just tried sliding the outer sheath on SWA, but on the off-cut I have to hand, it won't, so that won't work in this case.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.