Grounds for Electric Fences

Power line and building grounds are supposed to be strictly for safety. The ideal is to have minimal (preferably zero) current flow through the ground rods serving these locations unless there is a fault condition.

Ground rods can deteriorate, drought can dry out the soil, and connections can oxidize and develop high resistance.

If your fence charger is part of the circuit with a ground under high resistance conditions, everything connected to that electrical ground, conduit, plumbing, ductwork, etc. can become elevated in potential and cause shocks to people and livestock.

That's pretty much correct. Premises grounding systems are not intended to carry current under normal operating conditions.

Building and utility grounds don't normally carry current. Your electric fence 'charger' does.

Possibly the small current would increase electrolitic corrosion of the buildings grounding system?

daestrom

Why should the charger produce anything but miniscule leakage current (allowing for the fact that the wires are usually on porcelain or plastic knobs which can get dirty and damp) except when something is in contact with it? Not that I advocate tying it to the safety grounds. Am I missing something?

IIUC, if the mains power ground does go high resistance *and* there is a maims equipment fault to that mains power ground *then* the mains ground point could rise to mains potential.

However, if that ground point is tied to the electric fence ground, then the electric fence unit can become live. So someone touching the unit could get electrocuted - with mains current. Assuming, of course, that they are connected to a low resistance ground (wet wellies and standing in a cow pat should do nicely...)

However, as the unit it live with mains voltage, the electric fence can become live - not only with the (limited current) fence voltages but with the (lethal current) mains voltage.

Result - well-done steak instead of rare.. I much prefer the latter..

This presumes that ground at the fence is quite different from the ground that the charger is tied to. Aren't all these chargers tied to a ground rod? in the vicinity of the charger and fence?- I can see the reasoning behind having this rod independent of the system-as long as the secondary is completely isolated from the supply and the external parts of the charger tied to the independent local ground rather than system ground. However the point you have made would apply to any equipment in use around the farm -milking machines, pumps, etc. Cows stand around in wet patties in the milking barn and at the watering trough without the benefit of wellies.

Erm, the point *was* that, in this case, the charger wouldn't be tied to a ground rod (as it should be) but to the (faulty) mains supply earthing point (which it shouldn't).

If the mains ground at the farm went high resistance, it could easily be because of the ground conditions in the vicinity of the farm buildings. So all the kit in use in the farm buildings could be "live" - the lack of a good earth connection in the vicinity preventing shock.

However, the electric fence could stretch for hundreds of metres - taking it into an area of different soil conditions where a good earth connection is readily available.

Maybe we're thinking of different things. My limited experience with electric-fences (horses), was that the wire wasn't always on porcelain insulators. The fences were maintained by 'ranch hands' that were non-to-bright when it came to electricity and often the wire was simply nailed to the wooden fence post. Counting on the insulation properties of the wood. And needless to say in wet weather that wasn't very good at all. The whole idea is that once the animals learned not to touch the wire, temporary failures of the electric 'charger' wasn't much of a problem because the animals never touched it any more.

I was just speculating that having a steady current through the safety grounds (even just a few mA) could speed up the corrosion of said grounding rods/ system.

daestrom

I'm trying to answer 3 people with this one. Your points are good. and I'll go along with them My early experience as a self volunteered guinea pig was that the hot wires were on insulators.

For Sue-I agree- the charger output ground should be independent of system ground. What if the it wasn't and there was no local ground rod? One simply would have to consider the range of conditions that could occur from low to high system ground impedance in series with the body. Result would be somewhat lower body current with high system ground resistance.

However her point re the grounding of equipment in a barn is not one I would trust. While the mains ground can be faulty- the grounding conditions for the person handling equipment enclosures may actually be relatively good and one must make that assumption-i.e. possible serious shock exposure -body current path in series with cow pat and (poor but not open ) system ground Probably a low risk situation and low rather than harmful currents-- but??

For Archimedes Lever- The current that I called negligable was the insulator leakage (at least in the units I tested directly as a youngster which were all on standoff insulators) and the capacitive current on the secondary when there was a pulse. As to any primary ground current- it depends on the unit- and any primary ground paths (not a problem with the old battery units).

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Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

For Sue-I agree- the charger output ground should be independent of system ground. What if the it wasn't and there was no local ground rod? On simply would have to consider the range ofg conditions that could occur from low to high system ground impedance in series with the body. Result would be somewhat lower body current with high system ground resistance.

However her point re the grounding of equipment in a barn is not one I would trust. While the mains ground can be faulty- the grounding conditions for the person handling equipment enclosures may actually be relatively good and one must make that assumption-i.e. possible serious shock exposure -body current path in series with cow pat and (poor but not open ) system ground. Probably a low risk situation and low rather than harmful currents-- but??

For Archimedes Lever- The current that I called negligable was the insulator leakage and the capacitive current on the secondary when there was a pulse. As to any primary ground current- it depends on the unit- and any primary ground paths (not a problem with the old battery units).

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Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

They are not steady state.

They "tick". A single pulse, every second or so. That keeps consumption low, particularly in situations where leakage paths have been established with Carbon tracks, etc.

Even on high tension lines, as the insulators get old, and dirty, one can hear the level of leakage increase tremendously over the years between servicings.

Yes, I was aware of this. But the periodic 'ticking' is still going to pulse a current through the grounding rod. AFAIK, it's always the same polarity (i.e. DC pulses), so is it going to cause corrosion problems in moist soil?

Been there, heard that. And it used to be a practice in some jurisdictions to turn off the circuit and 'wash' the insulators with a fire hose. This was often done in areas along the sea-shore where salt would build up on the insulators. The salt, being hygroscopic, would create a lot of leakage.

daestrom

the ground system is suppossd to hane minmal or non current over it. if you bond them you are gonna create a potential between the rods, that can create corrosion and oxidation damaging your fence and the ground system, you possibly can elevate the voltage of the fence too overpassing whit it the limited fence voltage.

I have already agreed that bonding is not desirable. I also agreed that corrosion problems could occur. As for elevation of the voltage on the fence beyond what the charger nominally gives it- I disagree. The fence --charger ground potential is that produced by the charger only. I am assuming that the charger output is isolated from the input side with its own ground rod and bonding this to the system won't change the fence-ground potential. (e.g.common practice in distribution- transformer 6900KV with one side grounded. Output 120/240V with grounded center tap. Common ground point but no common current loops and you don't get 6900V on the LV side).

Most of the fence systems around here are portable and run off one or more "car batteries" (OK, they shouldn't actually use car batteries). So input isolation is less critical. The units tend to be water-tight plastic boxes, for obvious reasons (especially in glorious Devon - where it rains a lot). The output transformer is allowed to "ring" - ie the output "pulse" consists of a rapidly decaying sinusoid with insignificant dc component. The designer doesn't want the unit's ground rod corroding, either.

However, they still have a warning label instructing that the unit must not be connected to a mains supply ground..

I suspect that the reason is that, under some circumstances, such as a bad charger ground, some of the ringing potential could affect the mains circuit-particularly when there is a "victim" in contact with the fence. Check out the possible ground loops.