tank/hv earth electrode and nearby concrete pad with mounted lighting column

advice on pulling SWA along duct for over 100m would be uiseful too :-)   \\m/


I'll see what you receive regarding proximity to HV and LV earthing but regarding cable pulling in duct, I have a very simple work rule approach:

"whoever digs and buries the duct, pulls the cable in"


I'll even lend them a cable sock to suit the cable size and put a heatshrink cap on the end of the cable!


I've done many pulls over 250 m with up to 120 mm2 4c SWA but I've laid the duct.


Last month's one some 50 miles away of 240 m with 16 mm2 4c SWA was the same policy and so glad.The builder put in 50 m coils of 104/110 duct and the blue 6 or 7 mm blue rope and 3/4 of way down the duct, the rope broke. I wasn't there to watch but he had to dig several holes and open up the duct to find the end and then cobra rod to put the rope back in. He blamed the rope but I think it could have been due the the drum jamming on unwinding from a lash up on a scaffold pole, or backfilling material not being graded crushing the duct and neither helped by using a tractor to pull the cable in!


I use a 110 V capstan winch that fits onto the tow bar and am making up another, as I found a 0.37 kW 240 V one to adapt in exchange for half hour fault finding on a farm last week. I love bartering!!


Regards


BOD

Interesting question about the Earthing arrangement.


I guess another possible approach would be to make each lighting column etc its own little TT island with its own independent local electrode - that way it's remains at the same(*) potential as the ground it is stood on - whatever that might be.


(*) well same-ish - TT earths can be a little different from the actual potential of the soil around the rod due to leakage currents and the electrode resistance - but usually that's not too significant.


   - Andy,

So if we have a number of lamposts in a field we could have :-

1/ Each lampost is it`s own TT electrode    or

2/ As in 1/ above but  all linked (from 1st to last) by a common cpc.


I`d instantly envisage the benefits of 2/ being a lower ohmic connection to "true earth" but what are the de-merits?

I assumed the concern is the SWA armour being rather lively.

@Sparkingchip it is...


It is the concern about the HV voltage being impressed (to use the distributor word) onto *any* grounded metal work (inc. the cable armour) in the vicinity, particularly the closer any of it is to the HV electrode.


My thoughts have been:


To understand where an uncleared harmful potential diff. might be that someone might unfortunately experience on this circuit  (presume the whole thing is TT'd, as intended)  ?


Any LV fault on this circuit should be cleared as required, so it is really about this 'impressed' HV fault voltage on the circuit's earthed equipment until that is cleared in whatever time ?   It's more like a bonding exercise I feel ?


Is there going to be any harmful potential difference from something touching the lamp post or other connected metal work along the circuit and in contact with the ground (as not likely to be anythng else I imagine) ?    It would seem to be worst case closer to where the impressed voltage is picked up ie. if < 9m from the HV pole electrode ?


I don't see what can be done about reducing the effects at the lamp posts, so avoiding the 9m is the best bet unless limiting the impressed voltage is possible (as with the comments of putting insulating barriers in place for the SWA in the ground eg. plastic ducting etc)...which does not seem possible with  concrete pads threaded rods and posts into the ground. 


Perhaps the 9m is a good starting point where by the voltage impressing effects, in this case of a fault passing down the the HV electrode, on metal work are considered not to be an issue.... I wonder on the maths (gulp) !


What does anyone else think further, if anything .  Thanks all.

What about using insulated glands on one end of each SWA so that the lamp post, which is earthed by its bolts into the ground, serves as the earth connection for the armour of one of the SWA cables connected to it but there is no earth connection from any of the lampposts back to the source of the LV.

The maximum voltage difference which anyone could touch would be across one of the insulated glands but the chances of anyone removing the shroud and touching the gland when there was an HV fault will be minute and the voltage not that great either. If the holding down bolts do not give a sufficiently low resistance to clear a fault then additional earth rods might be required at each lamppost.

The SWA in the duct is the only thing in the installation that crosses the zones.


Is there an alternative cable that can be run underground in a duct?


Andy B.

It looks like Harry’s TT arrangement wins.