PME and main bonding

[Albert]

Albert, Is what you describe as NC-S what 16th edition calls TN-S?

Yep, sorry I call it TN-S too, I have a customer named NCS; easy mistake to make and make me look stupid...

 

[bernardgreen]

Our area is supplied by over head cables and lose of neutral or a bounced neutral due to loss of a phase creating serious un-balance and significant neutral current and volt drop is not un-known. Our supply is PME.

With a PME supply the loss of continuity in the neutral in the local distribution network can present (in my opinion) a risk of electrocution when a person touches at teh same time an "earthed" item and the real earth. I therefor have a substantial earth rod in sunk ground that is permanently wet (adjacent to and extending below the bed of a stream) and all circuits are on 30mA RCDs.

In the design stage (25 years ago ) I did give some thought to an automatic disconnect if the voltage on the PME provided earth went more than 25 volts above true earth. This was not included in the final design.

 

[Spark123]

Not sure that's 100% correct, if you have PVC incoming water main and gas main, how does this earth all the pipework ???

What you have if you don't bond is a resistance of all pipework if not bonded, you then have Volts/Resistance = Amps (current will flow), should you touch both.

Also if not bonded and the pipework becomes live then touching anything earthed will allow current to flow.

It isn't just the water and gas which which require bonding, any metalwork which can introduce a potential into the equipotential zone requires bonding. The reason for the IEE to require us to bond where a plastic service becomes metal does seem a bit daft, the only logical reason I can think is if the IEE think the service may be replaced with metal in the future.
Generally if an metallic item is 23K> ohms from earth in a normal location and a hand to hand contact current of 10mA is permittable then it may not need to be considered an extraneous conductive part.

 

[Steve3948]

Spark123

Nicely Put

 

[Podooser]

.....If you don't bond the incoming water and gas then the neutral and earth conductors will rise to full live potential,........

Is this under fault conditions.

 

[Spark123]

The only fault which needs to be apparent is the open circuit PEN conductor (combined neutral and earth). A normal load on a normal circuit may be enough to drive the potential on the PEN conductor up to mains voltage - you will also be competing with everyone else on the other phases downstream of the fault and any installed earth electrodes. It really depends where the fault is.