Electrical

The specific physical geographical location of the fence?.

the fence is parallel to the 250kv o/h line for a total distance off 10 km

specific physical geographical locations, continent, country, state, province, county, city, locale etc..

the fence we are talking about and the over head line 230 kv are in saudi arabia 300km north of Daharan near the arabian gulf 10km away from sea sandy soil in general

Three phase transmission lines are commonly rotated every so often. Is the bottom wire the same phase for the entire 10km?

If the fence is steel wires mounted on metal posts with wire ties, the ties usually scratch the post paint enough to connect them, so the wires may already be grounded by the posts.

Hi Guest,

I echo bwire on a description or pic' of the area the fence is in.

Out of interest,................... What is the fence keeping off the highway? After all, the first wire is about 80' high (a guess), unless there is a wire mesh as well?

Take care.

Is this mesh fence running between two posts? It should be grounded by the posts themselves for trip current relay to function if any wire drops on the fence accidentally. Grounding of the fence may be essential only for tripping if required. However for isolated supply large ground current may be unwanted.

yes this is a mesh fence pvc coated between the posts the fence is protecting the high way from animals the fence is 10km long and paralel to the 230kv o/h line which is a double circuit the consultant ask to ground the fence since voltage will be induced in the fence

Dear hussam,

If it is a fence for warding off animals then it must be insulated from earth and a pulsed high voltage feeder source of low current (that will not kill animals) is connected to this fence somewhere. Impedance of the fence to ground must be very high else current will leak through ground path. Fence feeder voltage may be few hundred Volts and perhaps for less than 1ms duration, low duty cycle and its current limited to about 100mA. It sure will give a heavy jerk but will not kill in few impulses. It will give enough time for animal to recover and understand the danger and will force it to move away.

Grounding this fence will render it useless and ineffective electrically. This may also activate an alarm in the control room for fence fault.

If fence is wired to an intelligent system for border security then tempering the fence sure will generate an alarm. For animals, intelligent fence is not required and only fault prevention is required else it will become useless.

As I read it, this is an ordinary PVC coated wire mesh fence on steel posts. Not an electrified fence. The concern is electrification by induction or capacitance from the nearby high tension lines.

If this is like what we call cyclone fencing, each wire of the mesh is essentially a vertical coil, which may or may not be electrically insulated from its neighboring wires. Since each wire is essentially perpendicular to the conductors of the transmission lines, there will be very little inductive or capacitive coupling. If the PVC does insulate each wire from its neighbors, there will be no additive accumulation of voltage along the length of the fence. And, if the PVC does insulate each wire from its neighbors, any attempt to ground the fence will be fruitless unless the grounding method connects separately to each wire.

I have the impression that the fence is already installed. If so, some measurements are in order:

1. With a very high impedance voltmeter and a small metal stake driven a short distance into the ground, determine if there is indeed any voltage between the fence and ground currently. Make this measurement at several locations and in dry weather. Unless more than 50 volts can be measured, there is no problem, and any grounding efforts are unnecessary. Moisture will presumably increase the connection between fence and ground, reducing any voltage.

2. Measure the resistance between neighboring wires (coils) of the fence. If this resistance is over a few thousand ohms, then the grounding attempt will be futile, as above.

Hi mr Dkwarner

Yes it is not an electrified fence, it is just to protect the high way from animals like camels, people and cars from interring to the high way, and it is already constructed. The problem as you said is electrification by induction from the o/h line 230 kV; we don't want people who are touching the fence to get electrical shock,so Grounding is required the question how and what will be the voltage induced on the fence how we can calculate it your answers are very good gave me a good idea what i have to do measuring the voltage and resistance any new idea or more explanations?thank you

If fence is floating then even induced voltage can not give shock. However, if you ground one point of the fence at just one point and there is enough induced voltage then it sure becomes of danger as it has a reference point to earth to make a short circuit through human body. You have to ground this fence at many places and induced current will only heat the fence a bit and there will be some power loss if there is any real induced voltage in the fence enough to force current. It like having many small voltage secondary coils of the transformer shorted out and each having only small voltage. I am not sure how much induced voltage or induced current you will have. Do some computation to get these figures.

If you do it then do it properly else do nothing is a much better idea. Accidentally if fence is grounded at one place then sure it is dangerous one at far points where it is not grounded.

thank you Mr shyam

Welcome.