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?

 

[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.