Another take on bonding for TNCS

[Apache]

Yes, you are right but considering the chance that it will happen, and do not forget that the Main bonding is a parallel to the main earth conductor and! it is probably connected to the gas supply. Your body resistance is much higher than the 10mm2 of the main bonding.At the end of the day it is only an insurance policy, do not want it but hope when you need it it will work for you.

Think I'll wear me wellies to water the garden - just incase

 

[Andy™]

Yes, you are right but considering the chance that it will happen, and do not forget that the Main bonding is a parallel to the main earth conductor and! it is probably connected to the gas supply. Your body resistance is much higher than the 10mm2 of the main bonding.At the end of the day it is only an insurance policy, do not want it but hope when you need it it will work for you.

the chance of it happening is almost 0, but it could happen

As ive said before, PME is all about money over safety

 

[Andy™]

Think I'll wear me wellies to water the garden - just incase

stick a bit plastic pipe between internal pipework and tap

 

[Apache]

stick a bit plastic pipe between internal pipework and tap

mm - water is conductive

Were you here when Proffessor Location proved it by experimentation?

 

[Andy™]

mm - water is conductiveWere you here when Proffessor Location proved it by experimentation?

water is a very good insulator. its the impurities in water the conduct

 

[davetheglitz]

Many thanks for the diagram - all becomes clear! Nice!

 

[AK-UK]

Think I'll wear me wellies to water the garden - just incase

Very good idea, what about rubber gloves?

 

[ianmacd]

Just turn the power off before you go out there

 

[sparkyork]

does anyone have any info of when this situation has arrisen? obviously its gonna be very rare and just goes to show, that dispite the regs and lots of complicated equations, the above situation would be catastrophic. realistically the with carpets etc on most houses your inlikely in the event of the above, to acrually "earth" out unless your in the garden! as apache says, perhaps all houses and there sourrounding land should be built on rubber?!

 

[Andy™]

i dont have any links to any deaths caused by it, but im sure there has been neutral failures which have resulted in a shock at least

 

[Apache]

water is a very good insulator. its the impurities in water the conduct

quite right Andy

see here

http://www.talk.electricianforum.co.uk/showpost.php?p=14729&postcount=33

took a bit of finding

 

[davetheglitz]

I have a cunning plan!

In the attached diagram courtesy of andy-c - imagine putting another RCD before the existing RDC, and this time connecting the house earth to the neutral between the neutral connections (sorry I haven't got a scanner!).

This time - if neutral went open circuit before the first RCD then there would be an imbalance, so that RCD would trip - removing the live - so no problem. Within most boards this could be achieved by replacing the main switch with an RCD and a bit of rewiring.

Plus point - removed problem with any DNO neutral connection

Minus point - a touch of DIY PME? Relies on good joint to earth between the main switch RCD and subsequent RCDs

What do you reckon!

 

[sparkyork]

would that not allow for circuit descrimination tho ? as evrything else is soley protected by this rcd

 

[Andy™]

I have a cunning plan!

In the attached diagram courtesy of andy-c - imagine putting another RCD before the existing RDC, and this time connecting the house earth to the neutral between the neutral connections (sorry I haven't got a scanner!).

This time - if neutral went open circuit before the first RCD then there would be an imbalance, so that RCD would trip - removing the live - so no problem. Within most boards this could be achieved by replacing the main switch with an RCD and a bit of rewiring.

Plus point - removed problem with any DNO neutral connection

Minus point - a touch of DIY PME? Relies on good joint to earth between the main switch RCD and subsequent RCDs

What do you reckon!

whilst in theory it could help, that would then be classed as a TNC supply, and the red comic also states RCD's MUST be after a combined earth is split. but then on safety reasons you could note a deviation

overall though, i dont think it would be practical to do

 

[Andy™]

would that not allow for circuit descrimination tho ? as evrything else is soley protected by this rcd

given that this RCD would be a final backup, you could use a 100mA TD

 

[ianmacd]

Isn't 100mA well outside the death range (50-80mA)?

 

[Andy™]

Isn't 100mA well outside the death range (50-80mA)?

Not as far out the death range as a BS 88 or 1361!

 

[sparkyork]

if you went to the trouble of making it totally safe then it'd have to be 30 ma surely, for the same reasons everything in the house is now 30ma

 

[Andy™]

if you went to the trouble of making it totally safe then it'd have to be 30 ma surely, for the same reasons everything in the house is now 30ma

if i were to go to the trouble of making it totally safe.... it would be getting TT'd!

 

[sparkyork]

fair point! lol but i thought we are talking pme