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

Hello - good day to all !

Any experience/experiences that might provide a few tips/watch out fors etc, most gratefully received.

Contemplating the run of buried SWA cabled over 100m for outdoor lighting (and electric gates) passing by a pole mounted transformer, where it's been confirmed by the distributor that there is a Tank/HV earth electrode setup at the pole. Whilst explaining this was not a rigid rule (more a guide and just as well as it shuts off the only feasible route), it was suggested best [in this case] to stay out of a 9m radius if burying cables - but the situation would be helped if this had to be compromised, by running the cabling in ducting/conduit when in the ground.

What are opinions on the siting of concrete pads with threaded rod to secure [conductive] column lighting in this context; this would seem a more serious concern within the 9m advice, is it a no-no, is there anything that can be done to mitigate (aside to using all non-conducting materials), or is it a non-issue (I feel not) ?

I have no experience of such a long distance, but any tips and advice on pulling SWA along duct for over 100m would be uiseful too :-) \\m/

Thank you all

Habs

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0 Alan Capon over 5 years ago

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.
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0 Alan Capon over 5 years ago

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

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