Consumer unit - Neutral bar

[steptoe]

its not perhaps quite as funny as first appears though.!

in that case the neutral bar may well NOT be at 0v .

answers on a postcard please.

 

[ADS]

Hi Digger,

As the others have pointed out, you shouldn't go poking around in a 'live' CU, but I've a feeling you want the 'principle' and 'understanding' that your question asks for, without actually doing anything stupid.

When any part of the installation is under load, the neutral bar will have current flowing through it.

The amount of current depends on what is on (i.e. the load)

If you were to touch the neutral bar without any 'load' on the system, the neutral bar will be at 0v with no current flowing through the neutral and you should be perfectly safe.

If you touch the neutral bar whilst the installation is under 'load', you are, in effect, introducing a neutral-earth fault/leak.......or, if you like, a parallel path.

Some current will flow through you to earth, although this will be tiny due to your body's resistance and the resistance between you and earth.

The supply neutral will still be the easiest path by far

As for the neutral being at 0 volts - this isn't necessarily true.

ohms law tells us that there must be a potential difference to earth at any point along the neutral.....think about it:

If the installation is under load and drawing 40 amps.

If the resistance of the supply neutral from service head to transformer star point (0 volts) was 0.3 ohms, then the potential difference (at the service head) between neutral and earth would be 40 x 0.3 = 12 volts - not huge, but still a voltage to earth.

As someone pointed out - disconnect a neutral from the neutral bar and touch that (under load) and your looking at 230v difference.

Hoped I've helped clear things in your mind a bit......probably made things worse, to be honest

 

[sellers]

make sure you follow safe isolation, could have the wrong polarity, dodgy main switch, missing neutral, anything! Assume nowt princess.

 

[Digger]

Hi Digger,As the others have pointed out, you shouldn't go poking around in a 'live' CU, but I've a feeling you want the 'principle' and 'understanding' that your question asks for, without actually doing anything stupid.

When any part of the installation is under load, the neutral bar will have current flowing through it.

The amount of current depends on what is on (i.e. the load)

If you were to touch the neutral bar without any 'load' on the system, the neutral bar will be at 0v with no current flowing through the neutral and you should be perfectly safe.

If you touch the neutral bar whilst the installation is under 'load', you are, in effect, introducing a neutral-earth fault/leak.......or, if you like, a parallel path.

Some current will flow through you to earth, although this will be tiny due to your body's resistance and the resistance between you and earth.

The supply neutral will still be the easiest path by far

As for the neutral being at 0 volts - this isn't necessarily true.

ohms law tells us that there must be a potential difference to earth at any point along the neutral.....think about it:

If the installation is under load and drawing 40 amps.

If the resistance of the supply neutral from service head to transformer star point (0 volts) was 0.3 ohms, then the potential difference (at the service head) between neutral and earth would be 40 x 0.3 = 12 volts - not huge, but still a voltage to earth.

As someone pointed out - disconnect a neutral from the neutral bar and touch that (under load) and your looking at 230v difference.

Hoped I've helped clear things in your mind a bit......probably made things worse, to be honest

Thanks very much ADS, that really helps a lot I had no intention of going anywhere near a live exposed busbar, I was just playing devil's advocate by asking the question to get an understanding of the science behind the concept. So just to confirm i've got it right, when the neutral is under load, there will always be a PD between neutral and earth?

 

[Andy™]

not always consider PME. neutral & earth are bonded together at service head. now why would you get a shock from neutral bar & earth terminal?

TNS is the same but bonded at star. but due to distance, there is a possibility of a small voltage difference between neutral & earth

 

[ADS]

[quote name='Andy

 

[Digger]

This is true - between neutral bar and earth terminal - but there will be a PD between neutral bar and 'true' earth.......this is one of the reasons that the discussion always arises about running a PME supplied earth to an outbuilding - the difference in potentials between 'system' earth and 'true' earth.Digger, to go one step further and explain what 'Steps' was saying - if the system were under load and you were to lose continuity of the supply neutral for whatever reason, in a TN-S system everything would cease to operate - although the neutral bar would rise to 230 v.

If this were to happen on a PME supplied TN-C-S system, then the neutral bar and all earthed and bonded metalwork could rise to a dangerous potential, because the earth and neutral are combined at the incommer and the current will seek alternative paths via the bonded 'extraneous conductive parts'.

This is why the emphasis on 'equipotential bonding' is so great - to limit potential differences.

Thanks ADS, that does make sense