potential difference

[Wilson12]

hello im struggling to get my head around potential difference,i understand about bonding but
when it comes to say birds on a wire or not getting a shock if your not grounded confuses me.
how does the current know we are not grounded and not conduct through us or a bird as normal ?

 

[Richard Burns]

You must have a complete circuit for current to flow.
i.e. there must be a continuous conductive path to and from the supply.

A bird on a wire will only be touching one conductor so no continuous path to compete the circuit.
The bird will rise to 230V but there will be no current flow to cause a shock.
If the bird bridged two conductors then there would be current flow.

Because the electricity supply neutral is connected firmly to earth any contact from a live conductor to earth will also be a complete circuit, so if someone is touching earth and a line conductor then this is a complete circuit and current will flow.

 

[Darkwood]

Don't worry, this is one of those stepping stones that alot take a while to get then it just clicks.

Potential Difference is really just the voltage that exists between 2 chosen points, so it can be the Live wire and Neutral or the Live and earth. The example of a bird on a live overhead wire is simple to explain, as the bird is not bridging 2 points with a potential difference then the voltage will not pass through the bird, the voltage will take the path of least resistance so if both its feet are on the wire then their exists no potential difference between it left foot and its right foot so no voltage will flow throw the bird, if however the bird had one foot on live wire and one on an earth point then because their exists a PD between these too points then provided the voltage is high enough which is usually the case on overhead lines the the current will flow through the bird and fry it.

With regard to the pylons when you see birds on the top wire then this is an earth wire that ensures the pylons remain the same earth potential and no step voltages are created between pylons which could see currents flowing through the ground between pylons, birds tend not to use the phase wires to land on for other reasons like the massive fields present around the wire and possible line hum.

 

[Wilson12]

touch voltage is 50v if im correct so the bird being at 230v dosnt have any effect then? without current flowing in a incomplete circuit is that the answer

 

[Wilson12]

oh okay cheers

 

[Richard Burns]

The touch voltage of 50V is only relevant if there is a current path (generally for touch voltage a current path to earth).

The method of electrical separation is a protective measure that ensures the supply is separated from earth and so does not present a shock risk.

The protective measure non conducting location is set up to ensure that there is not anything in the area earthed (i.e. everything around is non conductive) and so prevent a shock risk by preventing there being a complete circuit.

In both the above cases touching both line and neutral will still give a shock but touching only one conductor will not complete a circuit and so will be safe.