Idle question about TNCS systems

[westie101]

There are two issues: - the loss of the neutral in the 3 phase part of the system which leads to the risk of higher than normal voltages between live and "floating" neutral.
The loss of the neutral between the 3 phase main and the property which can result in all bonded metalwork being live at supply voltage. This can also occur in the first scenario.

The aim of the bonding is to produce one of these
http://en.wikipedia.org/wiki/Faraday_cage
so that within the house all should be safe.
Of course stepping outside the "cage" could put folk in the position of receiving a shock and as related by touching outside taps etc.

As regards petrol stations AFAIK the pipe work and tanks are metal so if bonded to a PME earth will turn into a superb earth rod following the loss of a neutral, this increases a risk of heat and or sparks near a rather explosive liquid!

Caravans are made usually of aluminium, so it the neutral comes live the whole caravan is live and a huge risk to anyone that touches it, same applies to metal hulled boats.

 

[kai]

In this photo, the neutral apparently failed, and the resulting explosion blew a hole in the pavement, luckily nobody was injured at that time.

Unfortunately hundreds of households lost power for a couple of hours as a result of the fault!

Aftermath of a cable fault underground.

• kai
• 19 Apr 2008
• 1

 

[Paul_C]

That's the case in the U.S., where all regular residential supplies are arranged as TN-C-S and the NEC (National Electrical Code) requires a local earth electrode to be connected to the incoming neutral at the service entrance.

That certainly makes sense to me. Indeed, if the supplier installed an earth electrode for each service entrance, that could presumably be the 'PME', couldn't it?

It certainly could, although in American practice the local earth rod at the premises is installed by and is the responsibility of the homeowner rather than the power company.

There's also the point to consider that in the typical American residential neighborhood the tendency is to employ a larger number of smaller transformers for a given number of homes than here, each transformer feeding only a few houses, so apart from the local earth rods at each service entrance, there may be only the one earth connection to the neutral at the pole carrying the transformer. That's similar to what can happen with TN-C-S in the U.K. where a transformer is feeding a couple of relatively isolated houses in a rural area, of course.

 

[westie101]

American practice

http://www.emfs.info/Sources+of+EMFs/distribution/USA/

This leads to greater NET currents
Discussed here
//www.diynot.com/forums/viewtopic.php?t=276476