Microshocks from induced voltage

I am not aware of any formal guidance, but if electric shocks are being felt, then earthing the fence seems sensible.

An all metal fence such as chainlink secured to metallic uprights will almost certainly be fortuitously earthed sufficiently to avoid shocks.

A greater risk is dry wooden uprights with plain or barbed wire secured to them. The currents involved are minute and no elaborate precautions are required. An offcut of fencing wire twisted around the top two strands of the fence and with the loose end buried will normally suffice. The lower strands are generally a bit more earthy due to long grass and the supports being damp nearer the bottom.


It might be prudent to investigate and ensure that the shocks are indeed due to capacitive* coupling from a grid line. Not for example diverted neutral currents or similar faults in a nearby LV consumers installation.


*Often called "induction", but strictly speaking it is capacitive coupling.

Thanks Broadgage, the earthing will be for a proposed design. I was looking into understanding what the earth target value would be to safely discharge any charge accumulation.

For a proposed design near a high voltage grid line, it might be easier to select EITHER an all metal fence that will be fortuitously earthed by its basic design, OR a wooden fence that cant become live.

If metal wire on insulating posts is required for other reasons, then earthing may be achieved by specifying that say 2% of the posts  should be metallic, with a minimum of two, even if most are wooden.


In view of the minute currents involved, a high resistance connection to earth will be fine, much higher than for earthing any likely electrical installation.

Consider the following worst likely case.

The capacitive charging current is likely to be a lot less than 1ma. Assume 1ma to be generous and assume a very high resistance earth connection of 1000 ohms. That will result in the fence becoming live at one volt.


In practice even wooden posts will usually have a low enough resistance to shunt the voltage to earth, unless newly installed and very dry.

I megaohm might be a reasonable guestimate, or for a medium length fence with 100 posts a total resistance of 10,000 ohms. That will shunt to earth 0.1ma with a voltage rise of one volt.

A longer fence will collect more current, but will have more posts to shunt this current to earth.


I have only once received a shock from a fence under a grid line, and that was while helping to build it. No shock was perceptible by the next week, presumably the insulation resistance had dropped.

For metallic fences I would consider referring to ENA EREC TS 41-24 (available via the Distribution Code site) which (in §6.6) specifies 2.4m rods 1m either side of the overhead line and ensuring electrical continuity between panels. This assumes the fence is otherwise electrically continuous.


The specification is written for HV substations so do check first as it may be inappropriate in some cases.


I have also seen GRP sections used (with buried continuity conductors across the gap where necessary)

HI Mike (MG)


At some point late last year, I posted a question along much the same lines, except I was earthing fences on a Railway line platform - as these have overhead H/V near by for the trains.......


There was some really good information on that topic about the why we need to do this and why I needed to use 25mm and what we were trying to achieve. 


If you can use this stupid new search function better than I - search my name and look for the topic along the lines of earthing a fence or temporary fences.


Something along those lines.


On the old IET forums platform I'd have found that in seconds - I don't know where that thread has gone now. 


Kind Regards

HI Mike (MG) 


I think - if I remember correctly what we were trying to achieve was to allow an equi-potential to from and between everything so that in the event of an HV fault, everything rises and falls in potential as evenly as possible. (Between HV/train/metal fencing/building structure etc etc.) 


Induced voltage may have been a small consideration but I don't really remember if it was the main point of Bonding?


Sorry I think I should have used the term bonding on the above post and I cant edit the post.


A very long time later ------ I've found it - here: 

https://communities.theiet.org/discussions/viewtopic/1037/24683?post_id=123994#p123994


Kind Regards


Tatty

All thank you for your responses, the guidance in TS 41-24 is a bit similar. I managed to get someone specializing in EMC to model the induced voltage on the fence from the Overhead transmission tower and recommend the maximum continuous metallic section lengths where the standing voltage does not exceed 60V and also earth rods to safely carry that induced current to ground. The continuous section lengths will be separated by 2.5m of non conducting fence.

Tatty, this is indeed a railway, there is not need to traction bond the fence in this scenario as it is situated far away from the drop zone (OCLZ as specified in 50122-1) therefore the only concern is any induced voltage on the fence parallel to power lines that would likely exceed the 60V limit. Minimum cable size for a 200ms disconnecting system and 12kA Network Rail job is 35mm sq for a XLPE sheathed cable. For a 6kA system a 25mm sq is acceptable although NR prefer bonding rated for a 12kA system. A document No. PAN/E&P-E/EE/ESD/0102 issue 4 is very helpful if you don't have it already. The cable minimum cross sectional area can be calculated with the adiabatic equation in BS 7671 and relevant k from the provided tables. Cable lengths are subject to voltage drop across the specific cable not exceeding 645V in line with the voltage limits specified in 50122-1 for 200ms. Once the voltage limit is exceeded on a cable then use a size up until the criteria are satisfied

if your only problem is the capacitive coupling then you will be looking at a few k ohms per kilometer is all that will be needed to pull it  down to 60V RMS.

For that a meat skewer in the flower bed with bell wire would do.

However, for good mechanical reasons, and to survice a bit of weather, a proper electrode and a sensible wire diameter will be needed, or a metal fence  post in every 10th position or something.

I am very well aware that neither BS7671, nor the railway rules, correctly handle the case of things that only need to be loosely earthed, and tend to specify every thing as if hundreds of amps may flow, when the series impedance means it simply cannot.

We could argue that there may be an element of induction here, if a nearby conductor is carrying a heavy current. However I agree that capacitive coupling is probably the main factor.


This reminds me of the days when I worked for the CEGB. Some one told me that ordinary neon mains tester screwdrivers were useless when one worked on a grid site; they glowed on anything metal that they touched. I tried it and sure enough this was the case - even when touched onto metal earthing strip going straight into the ground. I presume this is because a human body can capacitively couple to a high tension source just like anything metal, and the mains tester provides a path to earth in this case.


Fluorescent tubes in buildings associated with high tension supplies often glow dimly in the dark when they are not switched on.