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

The simple answer is put your lighting column and it’s cable outside the 9m zone. What I suspect you have been given is the DNO’s standard “hot zone” of 450V round the transformer. It is difficult to comment without knowing the DNO’s design guides or the system voltage. I will assume 11kV, which gives a phase to earth voltage of 6,350V. The worst case would therefore raise the transformer steelwork to 6,350V in my example. If we then assume the 450V contour is at 9m, this gives a rise of earth potential of 655V/m measured away from the transformer. If you want the maths, this is (6,350 - 450) / 9. 


if you place your lighting column within the 9m zone, it is conceivable that a person standing 0.5m away from the column and holding onto it during an HV fault, may receive an electric shock of 330V or so, due to the difference in potential between the ground they are standing on, compared to the lighting column. 


if you want a more accurate figure, you will need to ask the appropriate DNO to do the actual calculation for you. Depending on the location of the actual HV earth rods, my figures may be either too low or two high. 


Regards,


Alan.

Thanks Andy, 

I was considering the each one a TT in it`s own right against the muliple electrode effect of reducing impedance to earth and the effects of a bigger area earth reference as opposed to possible differences accross this field itself. both advantages and disaadvantages under different circumstances. In practise most of our TNS and TNC-S supplies will have many varied contacts with earth over a very large area I would think

In practise most of our TNS and TNC-S supplies will have many varied contacts with earth over a very large area I would think

Indeed - but hopefully none of them within the HV area of a 'hot' transformer site - the DNOs are usually very careful about that.


Actually I've been thinking about the little TT islands again and I'm not sure now it's a good idea. During a HV fault the ground (and hence the TT island's LV earthing system) goes up to some considerable voltage (perhaps several thousands of volts) - yet the L & N lines remained referenced to normal LV earth which hopefully is well outside the HV areas and so remains close to true earth potential. The insulation between L&N and c.p.c/exposed-conductive-parts is then stressed by the HV fault voltage which is probably well outside what the LV insulation can withstand - so we should perhaps expect insulation failure, so not good.


   - Andy.

The issue as I was earlier thinking, is to limit any touch type voltage if unfortunately in contact with the metal work of the lamp post on its rods and pad (and anything else connected to it) during a HV fault and the ground or any other earthed position (but that would be unlikely with the distances between posts etc).  The closer the lamp post is to the HV electrode the worse it all gets.  Local LV circuit earth rods just add to the connection to the ground of course (like the concrete pad and threaded rods) of the lamp post setup so are not really solving anything in the HV fault case ?


Not connecting each lampost together (using the insulating gland idea on the SWA earlier mentioned) would seem to be an option to limit cable based propogation of the voltage to other posts on the circuit.  It would be unlikely that any more than one post would be close to HV electrode area too.


As I mentioned (other than all fully insulated lamp post setup perhaps if that were feasible) avoiding the vicinity (~9m in this case as suggested) of the HV electrode for a lamp post is really the better the choice (perhaps along with some other bolstering options as mentioned if really needed).


AlanCapon has captured what I was trying to get my head around.


Does everyone agree ?

A certain Nuclear Power Station near Edinburgh has a similar problem as the instrument earth cables (and there were the best part of 50,000 of them) all had their screens and armour separately earthed only to a central Earth Bar. This was achieved using insulated glands at every junction box and carefully designed earth bonds between the glands of the multicore cables back the the control room and the smaller cables to the instruments.

The risk of this arrangement arose if someone was touching one of these glands when a fault occurred on the 400kV substation nearby but this was calculated and deemed small.

As an added safety feature in the situation being discussed, if the insulated gland is made off first then the risk of anyone getting a shock while touching the gland during an HV earth fault is even lower.

Looks like it being a solar rechargeable light on a fibreglass pole.

Do you need  a lighting stand within 9m of the TX electrodes, or just a cable route? The cable could be in a plastic pipe, then we would not care if the ground around it is ever live or not.  - the problem only arises is if a touchable lamp post is 'planted' within the  near field of the electrode.

Even then so long as  the  exposed lamp post voltage can rise and fall with the local surface voltage, this is  the best case. The only hiccup is as Alan alludes, as there will be a definte slope in the ground potential ,  the lamp post poses  a comparable hazard to the bare foot step voltage.  

Actually a concrete column or a one with a fibreglass clad base might be quite sensible, both exist.

Are wooden poles and an overhead cable acceptable from an engineering point of view?


Regards aesthetics its just some more poles and overheads by some existing overheads and poles, of which I presume two of the poles form a H with the TX balanced between them, so a couple or three new poles won't be particularly spoil the view.


Andy Betteridge

Thanks all for the input. Much appreciated.


I think you might all see that I'd got to the point that putting a lamp post as described within the 'hot zone' (using words above) was, all things considered, something to be avoided if possible.


Burried cable already mentioned as going in plastic duct or as suitable.


I will have a look at the fibreglass cladding is an option to bear in mind (thanks mapj).


The property owner, will no surprise be set on any aesthetics of lamp posts they want so will try to work with that of course.


Will have a final check with distributor contact and just get a final review.


I think best be safe than be risky is how I felt when I first posted and all the comments have been useful.


Thanks again :)

Or  for light duty situations  the all-insulated lamp post  You could do worse than call them to see what they offer for your situation Tel +44 (0)1623 511 111  I have no connection with the company other than our maintenance folk ordered a replacement post for a funny situation from them once.