Hi all
At the game reserve where I live we have around 200 km of electric fence around the whole place.
We have blocks of properties with gates in the main fence which the residence maintain.
Our gate is off grid with solar, but we have lights at the gate which is on AC, running from the closest homestead around 150m away.
Problem we have, if lightning hits pretty much anywhere in the reserve, even kilometers away, it will fry the AC cable run somewhere close to the gate and fence. Apart from tripping the EL, it does no damage at the DB 150m away where the cable run is connected.
The theory seams to be that any big spike on the electric fence jumps over to the AC cable where it gets close to the fence at the gate, then it fries the cable.
One of the solutions is to entirely skip AC and take the lights on to battery power as well, but this is the most expensive option because weâll need bigger batteries, around 2 kWh of lithium plus a inverter, so we are looking at upwards of R25k, quick and dirty cigarette packet calc.
Myself and some others think first prize will be if we can keep the AC and make a plan to minimize the chances of this happening, protection devices, separate earth etc. that this will be a lot cheaper than going the battery route.
Any comments, any ideas, earth / lightning experts?
This does not make a lot of sense how lightning can damage the cable there. Do you have photos of the point in the installation where the cable gets damaged? Are there any other metal parts crossing over at that point?
I donât think a separate earth would help. Protection devices would just prevent damage from propagating to other devices, which it does not seem to do anywayâŚ
You say at a gate.
Then there is the possibility that one side of that fence has a poor electrical connection with the other side.
I suggest you dig a trench from one side of the gate to the other side of the gate and bury a few strands of bloudraad that connect both sides of the fence to each other.
So electrically there is no gate gap in the fence.
My guess would be that lightning induces a magnetic impulse, and any wires in the vicinity of that magnetic field will have a voltage induced in them. If the wires run in a big loop, that makes it worse. If the wires run side by side, then in theory the same offset is generated in both wires and at least there is no differential difference (in theory anyway).
When you say it âfriesâ the wiring close to the gate, do you mean it burns the insulation and damages the wiring in that manner? Because that would make sense: Suppose there is 2000V or thereabouts on the cable at the time, it is probably going to jump across wiring thatâs rated for much less.
One solution may by to put a surge arrestor on it, that is, to make a controlled point where the energy can jump across rather than forcing it to jump through the insulation.
Thatâs what those spark gaps were installed for.
The reason I asked about earthing is if they have connected to a local earth then this will spike to a really high voltage with lightening. Since the earth is in close proximity to the live & neutral this will produce a high voltage gradient that in all likelihood will arc across wherever is the easiest.
My suggestion would be not to have an earth at all and keep the mains power cable insulated from everything as far as possibleâŚ
Around 3 or 4 meters of the cable basically melt so that the copper and insulation is one fused mess and the black water pipe acting as conduit also clearly takes some heat.
It is earthed yes, 3 cores running from the DB 150m away.
Must we maybe try not earth it at all, only run a live and neutral?
The fence represents a lot of eath spikes connected together. It is already a better earth than any big spike you are contemplating.
Lightning has just jumped 1000m+ through the air, which is also a massive amount of insulation to overcome. You are also not going to replicate that sort of insulation. Lightning will defeat it.
I am saying you wonât be able to stop it from going where it wants to go. You have to help it go where it wants to go.
The charge wants to go somewhere, and your equipment is on the shortest path.
So you must provide a preferable path that is effectively parallel to your equipment.
I am guessing that the other side of the gate is where it wants to go. But it may not be. For instance, it could be your houseâs borehole casing en route to an aquifer.
Whatever it is, that cable represents the electrically shortest path.
So the low-hanging fruit is to bond both sides of the gate together. This may solve your problem, or it may not.
If it doesnât, then you have to bond this perimeter fence to your house earth and any local fence and borehole casing, local earth spike (and things that are natural death spikes like that together). Run bloudraad from the gate parallel to the cable that is obviously on the route that the lightning wants to go.
Thick bare bloudraad underground is sufficient. You have now removed the reason for lightning to break down the cable insulation by providing an alternative path.
I would also say that 200km is just asking to import lightning. Others electrically isolate the fence into smaller sections ( even with regular barbed wire fencing). So an electrical break (equivalent in size to your gate) in the fence, say 500m away on either side gate, may also help.
I understand that with buried cables this âbloudraadâ is often laid above the cable you are wanting to protectâŚ
So itâs at a shallower depth making it a better route for the discharge than the cable.
Citation needed!
Maybe, but it depends
It is good practice to lay bloudraad in any trench you dig for whatever reason, even if it never surfaces at either end.
The red a white tape to mark your cable routes should be shallow.
Over the years, youâll add an earth mat to your property, which is good. The more conductive your soil, the less likely the charge will seek a path through your equipment.
And you can find your plastic waterpipe with a metal detector.
I presume that thereâs always the possibility of the bloudraad also being fried?
All the more reason to expose the ends so you can do a continuity check?
The AC cable is now useless as an AC cable, but (probably) still fine as a lightning conduit. The bloudraad doesnât have another purpose, so should be fine for a lot longer / couple of strikes I think.
Hang on⌠200km! I missed that the first time round! That is one massively long inductive loop.
I remember as a child, opening a gate on the farm, and receiving a mildly uncomfortable (but not fatal) electric shock. There was some distant thunder at the timeâŚ
In radio terms that would be a Long wire antenna⌠Super duper efficient in picking up on the induced currents from any electric storm, lightning. All the electric fence active wire are insulated from earth, but they run close to the other wires that would be at earth, so a short distance along all that length.
Are the lights metal framed housings? Are the cables inside metal conduit, at least the exposed above ground parts. It is important to ensure continuity between the light housing and ensure that is metal, and the conduit. Also ensure the metal conduit is earthed to the same point as the electric fence earth wires on the fence side.
NieSparkienieGroetnis
PS: Felt with this problem a lot across Africa in comms towers, including IT.
Yes, the game reserve electric fence is around 200 km in total. Iâm not entirely sure how or where itâs broken up, but know there are 2 energizers, but those energizers are quite far from us and where we are having this problem.
The lights have metal housings yes, no the AC cable run isnât in metal conduit, itâs in black water pipe.
Must we try putting it in metal conduit and earth it to one of the fence earth spikes? What about earthing the metal conduit on the other side 150m away, must we earth it there as well?
Hi, nope black pipe is even better. Just make sure the black pipe is connected to the light housing as well. Then us the thickest cable, or multiple thinner ones, and ground the pipe closest to the fence to the fence earth. The other side should not be earthed. Being underground is somewhat of an earth already.
