TT with low ze when bonding connected

[Tetris1985]

TT System with 31 ohm ze that drops down to 0.31 ohms when the bonding is all connected

Am I right in thinking that's because a house nearby has been converted to a tncs and they share a common bonded gas supply?

 

[JBW175]

It’s not a Ze with the bonding connected, it’s a Zdb

Parallel Paths.

 

[Tetris1985]

It’s not a Ze with the bonding connected, it’s a Zdb

Parallel Paths.

Does it matter that the gas supply provides a better earth than the earth rod though.

In a fault would it just use the gas pipe as opposed to the rod

 

[JBW175]

Electricity is like humans, it’s lazy and will always take the path of least resistance to get where it wants to go.

From what you’re saying, if the properties do share a common bonded gas supply, I’d imagine it’s using the neighbours tnc-s earthing arrangement via the gas bond when it’s connected.

 

[LastManOnline]

Does it matter that the gas supply provides a better earth than the earth rod though.

A metallic service pipe will always provide a better earth than a rod due to its much greater physical contact with true earth.

In a fault would it just use the gas pipe as opposed to the rod

In a fault situation electricity does,nt use one path as opposed to another.It uses EVERY path that's available.We are all accustomed to hearing "the path of least resistance" when in reality its "every path "

 

[westward10]

Typical TT the earthing conductor is doing little more than earthing the electrode.

 

[pc1966]

Does it matter that the gas supply provides a better earth than the earth rod though.

No, but service pipes cannot be relied upon as next plumbing job, etc, could change it to plastic.

So even if the bonded Zs is low enough for OCPD-based disconnection, you still need the RCDs and a rod capable of reliable ADS without the bonding in place.

 

[UNG]

No, but service pipes cannot be relied upon as next plumbing job, etc, could change it to plastic.

And this is the problem that is often forgotten on installations like this

The other problem is the simplistic approach many take to earth rod installation without having any understanding of how to optimise the installation to get the best result. The general approach of a 4ft rod will do doesn't really cut it in all cases and there are times when an 8ft or 12ft rod or multiple linked rods gives the optimum result, but this all comes back to training and when the approach of some is that domestic is only a few wires the reality of complex earthing issues hits and then it takes some skill to pull the resistance down

So even if the bonded Zs is low enough for OCPD-based disconnection, you still need the RCDs and a rod capable of reliable ADS without the bonding in place.

Metal gas and water pipes were never intended to be an earth and the bonding was always intended to protect those carrying out maintainance on the external pipework from stray voltages / currents from properties on the network

 

[pc1966]

The other problem is the simplistic approach many take to earth rod installation without having any understanding of how to optimise the installation to get the best result. The general approach of a 4ft rod will do doesn't really cut it in all cases and there are times when an 8ft or 12ft rod or multiple linked rods gives the optimum result, but this all comes back to training and when the approach of some is that domestic is only a few wires the reality of complex earthing issues hits and then it takes some skill to pull the resistance down

Very much so!

Getting a good and reliable earth "rod" is far harder than most folks realise. I am luck at our site in that it is fairly moist clay all year once you are down by the 30cm or so tarmac-ish layer (site is only 10m above nearby river) but even for that I used 2m rods as the top layer drying out in summer, or freezing in winter, has a massive impact on conductivity.

Once you discount the top 2 feet or so as unreliable for moisture you see why a 4 foot rod is asking for trouble.

 

[LastManOnline]

Metal gas and water pipes were never intended to be an earth

Valid point.The idea of currents flowing along a gas pipe never sounds right.Risk of arcing.

and the bonding was always intended to protect those carrying out maintainance on the external pipework

That will work when carrying out maintenance on internal metalwork.Such as changing a metal gas boiler.The Cross bonding serves that purpose.

On external pipework however the housebonding is of no value when it comes to working on metallic services.For protection in these circumstances maintenance crews need to "cross bond" wherever the metal services are been cut by connecting a copper loop across the cut