TT Earth and Neutral Voltage.

[steptoe]

High integrity although not sure of the expression. The RCBO's are protecting different circuits to the RCD's on the main board. Three separate circuits are protected by three separate DP RCBO's.The rest of the house is protected by a Wylex dual RCD board.

I read a horror story about the live incommer touching the earth bar on a TT and realised that this fault is as bad as a lost neutral on PME IE everything attached to earth would become live and no safety device would detect it and it could remain live and undetected for quite some time hence the need to buy a 100ma time delayed RCD as the main switch to protect the tails and main board. Nearly

 

[Zeespark]

3 DP RCBO's would take up 6 modules in total, could you have installed 3 SP RCBO's at all?

AndyGuinness

 

[steptoe]

3 DP RCBO's would take up 6 modules in total, could you have installed 3 SP RCBO's at all?AndyGuinness

not in all boards Zee,

some will fit a single way.

 

[Zeespark]

I apologise if thats the case, its just what I have seen thus far as I have been on the look out for single module DP RCBO's as you well know Steps.

AndyGuinness

 

[la poste]

BTW, if it did happen then I would think the main incomer protective device would blow, Ra/Ze wont be that high to prevent the 80/100A incomer from blowing on a direct short

Ze is 15 ohms (combination of earth rod and bonding), the mains voltage is 240V so a current of 16 Amps will flow if the live incommer touched the earth within the board. In my understanding this will not cause a main 100A fuse to blow and would leave the metalwork in the house live until someone touches it. Correct me if I'm wrong please because it would save me

 

[Lurch]

Ze is 15 ohms (combination of earth rod and bonding),

If the bonding is connected then that isn't Ze.

 

[la poste]

If the bonding is connected then that isn't Ze.

Ok.

The existing earth rod is quite high about 400 ohms.

The house gets a reading of 15 ohms off of the water and gas pipes.

I was not sure what earthing system to use and so I left it to the last job while I read and asked questions about earthing.

I realise that bonding should not be used as a main earth but while in the process of re-wiring this place I have felt secure in the knowledge that the earth fault loop impedance is 15 ohms and everything works fine, all RCD's trip in their correct time. I mean the guy has lead water pipes running all the way under his property, up the driveway and to the road and is not going to have them changed ever.

The new earth rod is going in this week, I have decided against TNC-s.

 

[Andy™]

the guy may not plan to change them, but water & gas utilities are constantly replacing metallic pipes with plastic (either as planned maintenance, of as emergency repair). if that happens, then there goes your low earth loop. i would not be leaving a job like that

 

[la poste]

[quote name='Andy

 

[Andy™]

but customer may not think 'there replacing the gas pipe. better call the sparky to check the electrics'

 

[Lurch]

I'm there most days and have no intention of leaving it permanently, just for a couple of weeks. If the utility companies decided to change the pipes the whole driveaway would haveto come up, we would be given notice, it would not be an overnight thing.

That's not how it works. As I mentioned above, Ze should not be calculated with bonding in place. You are looking for a worst case scenario. You cannot leave an installation with a high Ze but good parallel paths and mark it off as done\fine\finished\safe\sorted.

What happens in 3 weeks\6 months when you're not there?

 

[la poste]

That's not how it works. As I mentioned above, Ze should not be calculated with bonding in place. You are looking for a worst case scenario. You cannot leave an installation with a high Ze but good parallel paths and mark it off as done\fine\finished\safe\sorted.What happens in 3 weeks\6 months when you're not there?

As I said I am fitting a new earth rod this week.

I had/have no intention of leaving the place as it is and I am there most days.

In all the TT I have done I have never seen or heard of an incomer touching the earth bar, so I think someone is on the stir with that story.

It's something I read.

http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=25879&STARTPAGE=2&FTVAR_FORUMVIEWTMP=Linear

I'm tempted to try it to see what happens but I know this is a very bad idea.