Electric fences

[DarloRich]

Not fo boundaries, but there is at least one protecting an overhead line feeder station.

fair point

 

[Elecman]

Not fo boundaries, but there is at least one protecting an overhead line feeder station.

You sure its the NR part off the feeder station and not. The DNO/National Grid part? I know a couple of Grid Feeders so protected but no NR feeder buildings, even those co located on the same site

 

[najaB]

I expect it's quite common for owners of electric fences to put up signs with exaggerations such as "Danger: 100,000 Volts" to act as a deterrent

While 100,000V might be an exaggeration the voltages used in electric fences can be quite high - well into the thousands of volts.

 

[Deafdoggie]

While 100,000V might be an exaggeration the voltages used in electric fences can be quite high - well into the thousands of volts.

Around 8,000 volts. But a very low amperage or current. Not enough to even kill a mouse.

 

[najaB]

Around 8,000 volts. But a very low amperage or current. Not enough to even kill a mouse.

Oh of course. Lots of volts but no real power.

 

[Spaceflower]

Voltage is the difference across a circuit - amps is the the power.
So a high voltage/low voltage would be a waterfall/gently flowing stream?
So a high amperage/low amperage would be the mighty Amazon/kitchen tap?

Or have I got them mixed up

I'm so bored I'm now trying to get my head around this

 

[najaB]

In the water flowing through a hose analogy:

• Volts = pressure
• Current = width of the hose
• Power = total amount of water flowing

So for a given amount of water you can either have a narrower stream with more pressure, or a wider stream with lower pressure.

 

[Tester]

Voltage is the difference across a circuit - amps is the the power.
So a high voltage/low voltage would be a waterfall/gently flowing stream?
So a high amperage/low amperage would be the mighty Amazon/kitchen tap?

Or have I got them mixed up

I'm so bored I'm now trying to get my head around this

In very simple terms sticking with the waterfall analogy.....

Voltage (V) is the height of the waterfall - current (A) is the amount of water flowing.

So think of the four possible combinations of high/low and heavy/light, and which ones you would be comfortable to stand under!

An electric fence is like a high waterfall with a light (and indeed intermittent) flow.

 

[Spaceflower]

Brilliant, thanks.

Which is most likely to 'jump' through air then?

 

[najaB]

Voltage (V) is the height of the waterfall - current (A) is the amount of water flowing.

This is one that I've always found a little counterintuitive - while it's true that the Amp measures charge flow (1 Coulomb per second), we can't actually do anything with charge, we do things with power. So the Watt (volts x Amps) is a better analogy to the amount of water for getting your head around the idea.

 

[Spaceflower]

So does the amp depend upon conductivity/resistance then?

 

[najaB]

Brilliant, thanks.

Which is most likely to 'jump' through air then?

High voltage. Every insulator has a breakdown voltage - for air it is (going by memory) about 1,000V per centimetre - or that might be per inch. So a sufficiently high voltage will be able to spark through air, even if it doesn't carry much current (and hence power). That's why people can pull big sparks off Van der Graff generators without frying themselves, it's a high-frequency, high-voltage with very little power behind it.

== Doublepost prevention - post automatically merged: 28 Dec 2021 ==

So does the amp depend upon conductivity/resistance then?

That's Ohms law: V (voltage) = I (current) * R (resistance). So if you rearrange the formula you get: I = V / R which means that if the voltage is held constant, the higher the resistance the lower the current. (That's how old-school light dimmers used to work).

Going back to the water analogy, increasing the resistance is like pinching the hose.

 

[Spaceflower]

High voltage. Every insulator has a breakdown voltage - for air it is (going by memory) about 1,000V per centimetre - or that might be per inch. So a sufficiently high voltage will be able to spark through air, even if it doesn't carry much current (and hence power). That's why people can pull big sparks off Van der Graff generators without frying themselves, it's a high-frequency, high-voltage with very little power behind it.

== Doublepost prevention - post automatically merged: 28 Dec 2021 ==


That's Ohms law: V (voltage) = I (current) * R (resistance). So if you rearrange the formula you get: I = V / R which means that if the voltage is held constant, the higher the resistance the lower the current. (That's how old-school light dimmers used to work).

Going back to the water analogy, increasing the resistance is like pinching the hose.

And traffic lights

Cheers, very nicely put.

 

[Tester]

Brilliant, thanks.

Which is most likely to 'jump' through air then?

Voltage is the consideration for whether/how far it may 'jump' - current then determines the effect.

Again in simple terms (and this is NOT formal safety advice!).....

Electric fence - might 'jump' a few mm depending on dampness, but unlikely to do you harm.
750V third rail electrification - don't touch it!
25kV overhead electrification - don't get within 600mm of it!

 

[Elecman]

High voltage. Every insulator has a breakdown voltage - for air it is (going by memory) about 1,000V per centimetre - or that might be per inch. So a sufficiently high voltage will be able to spark through air, even if it doesn't carry much current (and hence power). That's why people can pull big sparks off Van der Graff generators without frying themselves, it's a high-frequency, high-voltage with very little power behind it.

== Doublepost prevention - post automatically merged: 28 Dec 2021 ==


That's Ohms law: V (voltage) = I (current) * R (resistance). So if you rearrange the formula you get: I = V / R which means that if the voltage is held constant, the higher the resistance the lower the current. (That's how old-school light dimmers used to work).

Going back to the water analogy, increasing the resistance is like pinching the hose.

It’s circa 1mm / 1000 volts

 

[najaB]

It’s circa 1mm / 1000 volts

Thanks!

 

[Annetts key]

Voltage is the difference across a circuit - amps is the the power.
So a high voltage/low voltage would be a waterfall/gently flowing stream?
So a high amperage/low amperage would be the mighty Amazon/kitchen tap?

Or have I got them mixed up

I'm so bored I'm now trying to get my head around this

It’s the current flow that does the work (and damage if talking about being electrocuted).
With static electricity, you can have a very high voltage. But if there is no conducive path for the current to flow, it’s harmless.

But for a current to flow, there has to be a voltage. Also power cannot be ‘developed’ if there is no resistance to the flow of the current. You can’t make a very good electric fire using thick copper wire, as it does not have enough resistance.

Hence in simple terms, you need both a voltage to drive the current, and the current flow. Together this results in electrical power.

The formula for power (P) (in Watts, W) is P = V x I where V is voltage in volts (elsewhere U is used to mean voltage, with V still used as the symbol for Volts) and current (I) in Amps (A).

Resistance (R) is in Ohms (Ω).

So if you have a 2kW electric fire (not a fan heater) designed for use on a 240V AC supply, the current is 2000W divided by 240V = 8.33A

So the resistance must be 28.8Ω

If you now go to the USA and connect the very same 2kW 240V heater to their 120V AC supply, you will be very disappointed at the heat output.

Now we have 120V divided by 28.8Ω which results in only 4.16A flowing. Hence the actual power is now only 500W!

 

[bishdunster]

It’s the current flow that does the work (and damage if talking about being electrocuted).
With static electricity, you can have a very high voltage. But if there is no conducive path for the current to flow, it’s harmless.

But for a current to flow, there has to be a voltage. Also power cannot be ‘developed’ if there is no resistance to the flow of the current. You can’t make a very good electric fire using thick copper wire, as it does not have enough resistance.

Hence in simple terms, you need both a voltage to drive the current, and the current flow. Together this results in electrical power.

The formula for power (P) (in Watts, W) is P = V x I where V is voltage in volts (elsewhere U is used to mean voltage, with V still used as the symbol for Volts) and current (I) in Amps (A).

Resistance (R) is in Ohms (Ω).

So if you have a 2kW electric fire (not a fan heater) designed for use on a 240V AC supply, the current is 2000W divided by 240V = 8.33A

So the resistance must be 28.8Ω

If you now go to the USA and connect the very same 2kW 240V heater to their 120V AC supply, you will be very disappointed at the heat output.

Now we have 120V divided by 28.8Ω which results in only 4.16A flowing. Hence the actual power is now only 500W!

Its the volts wot jolts and the mils (milliamperes) wot kills, as i was taught !

 

[alxndr]

You sure its the NR part off the feeder station and not. The DNO/National Grid part? I know a couple of Grid Feeders so protected but no NR feeder buildings, even those co located on the same site

It could be, I suppose. It’s not my area of expertise, all I know is there’s feeder station there, its on railway land, and it’s got an electric fence. I’ve never looked at the signs closer than to know it’ll hurt if I touch it!

 

[Gathursty]

Some of these posts have reminded me of my physics lessons from schools. Still as baffling.

 

[D821]

What stops these fences being defeated by someone attaching a wire to the fence and earthing it?

 

[najaB]

What stops these fences being defeated by someone attaching a wire to the fence and earthing it?

The fact that it's high-voltage, low-current pulses makes me think that even if it was earthed there would still be a bit of a sting to it.

 

[furnessvale]

What stops these fences being defeated by someone attaching a wire to the fence and earthing it?

A tamper warning circuit perhaps?

 

[D821]

A tamper warning circuit perhaps?

I would imagine that high-security places have the fences alarmed, it must be easy enough to detect.

 

[AM9]

The fact that it's high-voltage, low-current pulses makes me think that even if it was earthed there would still be a bit of a sting to it.

Not really. Low current means that it has a high resistance between the source and the load (i.e. the intended victim). The resistance of a human body is between 5000 and 20000 ohms, meaning that an 8000v pulse could have a peak current between 8000/20000=0.4A to 8000/5000=1.6A. Likely to be lethal to many and certainly way above safety allowed levels. An electric fence will have a power source with a very high source resistance that would limit the maximum possible current to, say, below 1mA (1/1000A). That would be acheived by an 8MOhm resistor. Therefore placing a low 'ish resistance to ground (as high as several hundred ohms) would reduce the current to 1/8000 th of a mA (125nA), which I would challenge any normal human to feel.

 

[Spaceflower]

What stops these fences being defeated by someone attaching a wire to the fence and earthing it?

Nothing. They're probably deployed to prevent pests getting in and damaging critical infrastructure (rabbits). Those protecting strategic infrastructure are alarmed but the primary method of preventing unauthorised access is the rather large spiked metal fence and CCTV.

Even low tech, agricultural fences can have some kind of alarm/signal to alert the operator to a circuit break which is particularly useful for large, inaccessible perimeters

 

[Annetts key]

What stops these fences being defeated by someone attaching a wire to the fence and earthing it?

It’s like many forms of security or protection. It’s a deterrent. If you know what you are doing, everything can be defeated.

 

[XAM2175]

Even low tech, agricultural fences can have some kind of alarm/signal to alert the operator to a circuit break which is particularly useful for large, inaccessible perimeters

Yep, that was always a highlight of my time on the farm as a wee lad - seeing the big red "GROUND FAULT" indicator lit up on the energiser was the cue for a ride around the boundary fences.

 

[Pigeon]

Not really. Low current means that it has a high resistance between the source and the load (i.e. the intended victim). The resistance of a human body is between 5000 and 20000 ohms, meaning that an 8000v pulse could have a peak current between 8000/20000=0.4A to 8000/5000=1.6A. Likely to be lethal to many and certainly way above safety allowed levels. An electric fence will have a power source with a very high source resistance that would limit the maximum possible current to, say, below 1mA (1/1000A). That would be acheived by an 8MOhm resistor. Therefore placing a low 'ish resistance to ground (as high as several hundred ohms) would reduce the current to 1/8000 th of a mA (125nA), which I would challenge any normal human to feel.

It's not plain DC. It's a very short duration pulse. So the plain resistive potential divider approach is not adequate; you have to consider the load (fence wire and short to ground) as (most likely) a transmission line. Without going into technical detail the point is that the voltage does not rise simultaneously all the way along the wire; instead the voltage pulse travels at a particular speed, and if it gets to you before it gets to the short to ground, it doesn't know it's been shorted to ground yet, and it can still find you.

The dangerousness of the pulse is limited not by having an especially high source impedance, but by the amount of energy delivered to each pulse. This is not only a more dependable method, it pretty much "comes for free" without calling for any additional parts or any additional design effort.

 

[najaB]

It's not plain DC. It's a very short duration pulse. So the plain resistive potential divider approach is not adequate; you have to consider the load (fence wire and short to ground) as (most likely) a transmission line. Without going into technical detail the point is that the voltage does not rise simultaneously all the way along the wire; instead the voltage pulse travels at a particular speed, and if it gets to you before it gets to the short to ground, it doesn't know it's been shorted to ground yet, and it can still find you.

Thanks for putting into words what was in my head.