Kitchen Garden Forum

Someone must have the answer to this one.

While watching birds for the RSPB Big Garden Birdwatch yesterday I got to wondering yet again how birds manage to perch on the high voltage cables which cross our field without barbecuing themselves. There are yellow signs warning us off the wires - I know birds can't read.
I have asked people who know about electricity, but I have never been able to understand their explanations.
Can someone explain it it terms I can understand. I really would be very grateful.

Alan

Alan

An electric shock occurs when current (amps not volts) flows.

The birds are not conducting electricity to anywhere.

If the bird had legs long enough to go between the earth and the wire, it would conduct electricity to earth, and get fried. If they bridged between 2 wires, the voltage difference between would make current flow - hence if a branche falls across wires, we get power cuts due to short circuits tripping the substation.

Hope this is helpful

Thanks Mole
I'm getting there. But what if the birds touch each other? And why doesn't the current go up one leg and down the other? Or is that a daft question

Alan

Hiya,

See if this helps. Its like water pressure. If you compare the pressure 200m down with 1m down the difference is significant. The difference in 'pressure' across the bird's feet is insignificant. I guess that, even if they were hungarian hugging starlings the same would apply given their close proximity.

The difference in 'pressure' or voltage (AKA potential difference) between the wire and the ground is significant and hey presto one fried angler clutching his 18m carbon fibre pole (or portable lighting conductor whichever you prefer).

Not sure if this helps or just confuses. Lecky is funny stuff having pressure and flow and other stuff which makes it confusing. High voltages are not necessarily that dangerous, it might just make your hair stand on end or stick a baloon to the ceiling.

Loz

if the bird has both feet on the same wire both feet are at the same voltage so no current flows through the bird.
if you had a large bird with one foot on the steel pylon and the other on the wire,one foot would be earthed at zero volts and the one on the wire at maybe several thousand volts , current would flow through the bird frying it.

These explanations may help.

Electricity is the potential difference between two places, this is expressed in volts and amps, which multiplied together gives watts.

A 240Volt 3Kilowatt electric fire is 3000/240=12.5 Amps and you use a 13Amp fuse.

When you look at the bare metal wires outside they are connected to the pole by either a stubby glazed "ink-cap mushroom" shape or a big "stack of upside-down bowls each with a couple of crumpets in the base" shape.
These things are insulators, through which electricity cannot pass.
Thus the bare wire is looped across the poles without the electricity leaking away down the poles into the ground or in electrical terms "earth".

The power in the cables may be (lets say) 750Volts and some enormous amparage, but a bird, or (and this does happen) a workman lowered from a helicopter, sat on the cable effectively becomes part of the cable.
The electricity has to flow through you to somewhere with a lower electrical potential in order to hurt you. But you are only touching the cable which has the same potential, so the electricty cannot flow.
Therefore you would need to stand on the ground and touch the wire with a long electrically conductive stick. Alternatively climb up the pole and keeping firm hold of it put out a hand and hold the wire.
You then become part of the circuit and fry.

Now in some parts of the world, large bird have a problem, their wings are long enough to bridge the gap between a pair of wires (positive and negative) thus making the bird become the element in the lightbulb.

Have seen on TV and american black bear frightened in suburbia, climb the nearest "tree" for safety. Sadly this tree was a mains eletric supply pole feeding a tansformer strapped to the top (steps the electric down from supply network voltage to domestic level) and poor old bruin tried to climb a bit higher. She put a paw on the wire and got blown off the top of the pole, survived.

Got it

Many thanks. Peter,I particularly liked the reassurance that you could be lowered onto the wire from a helicopter. Not that I am thinking of trying it.

Thanks again

Alan