The wisdom of equipotential bonding

[alexcarnes]

I'm probably displaying my ignorance here, but fortunately I'm not one of those people who can't stand to be laughed at!

I've been thinking a bit about one of those things I've tended not to dwell on. The technical reason for equipotential bonding is obvious - if you create equipotential zones, such that everything rises to the same potential in the event of a fault, then we reduce the risk of shock from touching simultaneously accessible conductive parts.

Very well. But doesn't it run the risk of simply making lots of conductive parts live to Earth instead of just a few? So instead of a bit of cable tray over there being live, they all are. So unless you're wearing thick rubber shoes...!

Doesn't it risk solving one problem and creating another?

 

[littlespark]

Back in the day, we had EEBADS… which specifically was EARTHED equipotential bonding and automatic disconnection of supply…. So if it wasn’t earthed, there was something wrong.

 

[Rockingit]

Back in the day, we had EEBADS… which specifically was EARTHED equipotential bonding and automatic disconnection of supply…. So if it wasn’t earthed, there was something wrong.

Do you have ANY idea of how much therapy you've just undone?!

 

[Dustydazzler]

There was a time everything would seem to get a green yellow wire. from the kitchen sink , the bathtub , all the pipes in and around the boiler and the immersion tank pipes etc. if it looked metal plumbers and builders would attach a green yellow wire to it.

 

[littlespark]

It’s supplementary, my dear Watson….


Just a little joke there…..

If all the metal parts are the same, at a level…. Is that a Flat Earth?

 

[Dustydazzler]

It’s supplementary, my dear Watson….


Just a little joke there…..

If all the metal parts are the same, at a level…. Is that a Flat Earth?

depends how hard you smack it with a hammer

 

[Pretty Mouth]

If your ADS does its stuff then happy day, but if it doesn't then absolutely everything rises to the supply voltage relative to Earth.

An extraneous part can bring in a voltage from outside your installation. Eg, sharing a lead water pipe with a neighbour whose earthing system has failed, and has a L-E fault on their boiler.

 

[oscar21]

Inside the equipotential zone pretty much the only things that are capable of introducing and earth path are the things that will be bonded anyway. Even if little Timmy touches something that is at 230V in bare feet it doesn't matter because the floor is pretty much insulated, it might be carpet or wood or whatever, the walls are insulated, usually plasterboard or similar, there is literally nothing other than the bonded stuff that can cause harm.

I remember years ago, before RCD's on lighting circuits and we were working in a house where the plumber was doing bits and pieces and he was putting plastic Tees into copper pipe work for new radiators and taps etc. The house owners dad (its always the dad isn't it) wanted me to bond across the plastic tees to keep the continuity of the pipework. I tried to explain that having a radiator isolated from the earthed plumbing system was a good thing rather than bad but he just wouldn't have it.

Just to add as well, I wonder whether many people realise that every time they touch anything that is earthed they are actually introducing 230v to their body via the N-E link somewhere, its only because the neutral side of the power is "bonded" to earth keeping it at the same voltage that they don't notice.

 

[Mark Wright]

Inside the equipotential zone pretty much the only things that are capable of introducing and earth path are the things that will be bonded anyway. Even if little Timmy touches something that is at 230V in bare feet it doesn't matter because the floor is pretty much insulated, it might be carpet or wood or whatever, the walls are insulated, usually plasterboard or similar, there is literally nothing other than the bonded stuff that can cause harm.

I remember years ago, before RCD's on lighting circuits and we were working in a house where the plumber was doing bits and pieces and he was putting plastic Tees into copper pipe work for new radiators and taps etc. The house owners dad (its always the dad isn't it) wanted me to bond across the plastic tees to keep the continuity of the pipework. I tried to explain that having a radiator isolated from the earthed plumbing system was a good thing rather than bad but he just wouldn't have it.

Just to add as well, I wonder whether many people realise that every time they touch anything that is earthed they are actually introducing 230v to their body via the N-E link somewhere, its only because the neutral side of the power is "bonded" to earth keeping it at the same voltage that they don't notice.

I guess that's a whole other can of worms the fact that the Neutral is Earthed at some point.

I mean there are concrete floors and conservertries, garage floors etc

 

[pc1966]

It comes down to what are the risks you are looking to avoid and under what circumstances.

Equipotential bonding avoids the risk of two or more parts having significant difference in voltage between them, but does not deal with the issue of the (now single) conductive assembly being at a different potential to anything else, such as the Earth.

RCD protection helps in many cases as if you have an open E/CPC, or high-ish impedance earth (such as TT case) you still have a good chance of ADS happening with leakages (hopefully before you are the leakage path). However, in the open-PEN case with TN-C-S it makes no difference as the RCD sees the L-N differences, not a sign of any current flowing in the CPC network that would give away the situation.

This has been the dirty secret of modern electricity networks until recently (except for a few special areas like marinas, etc). Now the prevalence of EV has thrust the risks of an energised car being washed outdoors in to the spotlight and so we now see open-PEN protection devices as stand-alone, or in a charger, chargers demanding TT earths, etc.

Ultimately you have trade-offs, a TN earth allows OSCP disconnection in most cases, adding a 2nd protection to RCD that TT lacks, while TT has no issues of open-PEN faults, but often they have a "single point of failure" in the RCD/RCBO electronics. You pays your money, you take your chances...

 

[alexcarnes]

Euipotential bonding avoids the risk of two or more parts having significant difference in voltage between them, but does not deal with the issue of the (now single) conductive assembly being at a different potential to anything else, such as the Earth.

Yes, this is the problem I was referring to. In the event of ADS failure, you're simply increasing the chances of touching something with a potential difference to Earth, whilst obviously mitigating the risk of completing a circuit between simultaneously accessible parts. Solving one problem but creating another.

 

[pc1966]

Yes, this is the problem I was referring to. In the event of ADS failure, you're simply increasing the chances of touching something with a potential difference to Earth, whilst obviously mitigating the risk of completing a circuit between simultaneously accessible parts. Solving one problem but creating another.

Again it depends upon the circumstances as to which is a greater risk.

All class I products are earthed, so any with external metallic parts will be CPC bonded anyway and touchable. Under ADS failure, or open-PEN fault, they might become live and if there is a real possibility of them being simultaneously touched with another Earth-connected item then you have a shock risk.

Indoors that item-item touching is far more likely than the outdoor example of touch EV and the true Earth simultaneous, and typically building have a few obvious items such as metallic service pipes, significant building steelworks, etc, that can and usually do introduce a separate connection to the Earth that may become different from the Earth-referenced CPC system, so bonding makes sense as the added risk is tiny compared to the mitigated risk.

 

[brianmoooore]

I remember years ago, before RCD's on lighting circuits and we were working in a house where the plumber was doing bits and pieces and he was putting plastic Tees into copper pipe work for new radiators and taps etc. The house owners dad (its always the dad isn't it) wanted me to bond across the plastic tees to keep the continuity of the pipework. I tried to explain that having a radiator isolated from the earthed plumbing system was a good thing rather than bad but he just wouldn't have it.

I remember a case where an unbonded radiator in a kitchen had the lead from a radio permanently hanging down behind it. The lead deteriorated over time, as a result of the heat and sharp edges of the radiator fins and the radiator became live.
A child placed one hand on the radiator and the other on a nearby metal electric kettle, and was electrocuted.
This clearly wouldn't have happened if the radiator had been bonded to the kettle.

 

[oscar21]

I remember a case where an unbonded radiator in a kitchen had the lead from a radio permanently hanging down behind it. The lead deteriorated over time, as a result of the heat and sharp edges of the radiator fins and the radiator became live.
A child placed one hand on the radiator and the other on a nearby metal electric kettle, and was electrocuted.
This clearly wouldn't have happened if the radiator had been bonded to the kettle.

Thats a freak accident though, it could just as easilly been anything that killed him. It could very well have been a double insulated bench grinder that damaged the flex which then came to rest on the body of the grinder, that too would just as easily become live and complete the circuit if you touched it and the kettle together, not a fault of badly designed electrics.