Bonding Extraneous Conductive Parts

[John-]

hi. Question re bonding a metal where cold comes in as metal pipe ground into house. So cold will be bonded where service comes in. Read article in wiring matters 2019 about changes for part 18. Example given of faulty kettle where fault to earth exists at kettle, some one touches kettle earth and metal tap, pipe should be same so neggligeable difference exhists and no shock. Just made me think for a bit...

Questions...

Why didn’t the fuse or circuit protective device operate leaving no touch voltage?
I assume above is because current not high enough to rupture, leaving a potential high enough to kill but not high enough to damage cable- assumes rcd not used... which is wrong in itself given the example, but let’s go by their example.

So really when we are bonding like this aren’t we suplimenting the supplied earth into the building with the general mass of earth and not the other way. round? For the example above to be dangerous the General Mass of Earth has to be of an impedance lower than the earth connected to the kettle. Otherwise the fault current would not flow with no touch voltage.

So this means then that GME is lower than supplied cable and when you do you EFLI testing you should disconnect all bonds, otherwise the reading will include a supply pipe as an earth path that could change if it is converted to plastic which could then mean the overall ZS is increased, increasing disconnection times.

Thoughts?

 

[telectrix]

the purpose of main bonding is to raise the potential of extraneous metalwork ( e.g. water pipes) to the 230V for the duration of the fault current. i.e. until the appropriate OCPD reacts.

 

[telectrix]

why the dislike, pete?

 

[Pete999]

why the dislike, pete?

Sausage fingers Tel meant to be a like Apologies for the error, hope I didn't offend.

 

[telectrix]

thought as much. no offence taken. can you undo?

 

[Pete999]

thought as much. no offence taken. can you undo?

Done

 

[johnduffell]

the purpose of main bonding is to raise the potential of extraneous metalwork ( e.g. water pipes) to the 230V for the duration of the fault current. i.e. until the appropriate OCPD reacts.

That's one effect but the high level purpose is to create an equipotential zone which is basically preventing any potentially accessible conductive item introducing a different (from the installation earth) potential into the zone.

 

[telectrix]

That's one effect but the high level purpose is to create an equipotential zone which is basically preventing any potentially accessible conductive item introducing a different (from the installation earth) potential into the zone.

agreed. both exposed conductive parts and extraneous parts are equipotential, so when an exposed part rises in voltage due to a fault, the extraneous parts rise the same.

 

[Midwest]

thought as much. no offence taken. can you undo?

Least it wasn't a dumb.

 

[Deleted member 26818]

What happens when the tap and bonding conductor has a resistance lower than the R2 of the socket circuit the kettle is plugged into?

 

[telectrix]

What happens when the tap and bonding conductor has a resistance lower than the R2 of the socket circuit the kettle is plugged into?

then you hope that the touch voltage is <50V.

 

[johnduffell]

What happens when the tap and bonding conductor has a resistance lower than the R2 of the socket circuit the kettle is plugged into?

the fault disconnection time would be <400ms so it's deemed to be OK for that time.
But you are spot on and you illustrate exactly why supplementary bonding is essential in bathrooms - to remove the resistance between MET and the appliance CPC termination.
The only way around that is to have RCD protection on all circuits serving the bathroom which is a requirement for new installs anyway hence supplementary bonding going the way of the dodo.

 

[Deleted member 26818]

the fault disconnection time would be <400ms so it's deemed to be OK for that time.
But you are spot on and you illustrate exactly why supplementary bonding is essential in bathrooms - to remove the resistance between MET and the appliance CPC termination.
The only way around that is to have RCD protection on all circuits serving the bathroom which is a requirement for new installs anyway hence supplementary bonding going the way of the dodo.

How does that work if for instance the fault current comes into the location via the pipework?

 

[Wilko]

How does that work if for instance the fault current comes into the location via the pipework?

Interesting, but how could that be? (if the pipes are bonded to MET at entry to the Building...)

 

[Deleted member 26818]

Interesting, but how could that be? (if the pipes are bonded to MET at entry to the Building...)

What if the incoming supplies are plastic?

 

[johnduffell]

How does that work if for instance the fault current comes into the location via the pipework?

I think what you're meaning is a situation where there's some kind of conductive item, completely within the equipotential zone (i.e. not extraneous) and not an exposed conductive part. This means that it is unrelated to the electrical installation.

Since all live conductors must be double insulated or protected by earthed metalwork, there should be no touchable live conductors in a single fault situation (for more than the relevant disconnection time) This is why metal backboxes would be earthed as someone may thread a copper pipe down the stud wall or similar.

Of course in a double fault situation, *any* metal in the area is a hazard as it increases the chance of touching something that's live. However double faults are not considered and this is sometimes known as the "bonding teaspoons" mindset and is not necessary under bs7671.

Hope that essay makes some kind of sense!