This is something I do not understand UNLESS it is that a lump of un-earthed metal ( such as a central heating radiator connected by plastic pipes ) is "safer" as it will not provide a circuit to earth for a person touching both the radiator and a live conductor.
That's exactly it. Don't think just about radiators - think also of things like toothbrush racks, toilet roll holders, hooks, metal door handles, saucepans, spoons - these are all metal items which are not e-c-ps, and connecting them to the cpcs of electrical circuits isn't an increase in safety.
That would make sense if there was no chance of the water in the plastic pipes being conductive enough ( inhibitors added, it is not pure water ) to carry a lethal current from the radiator to the earthed / bondded metal work of the boiler.
Precisely - the scenario is that there is no chance.
If any of the metal pipework connected to this radiator becomes live due to being squashed against damaged cables in a shared notch in a joist then the radiator can become live. Holding an earthed metal appliance and touching this radiator would be a potentially lethal shock hazard. In practise the conductivity of the water might be sufficient to trip an RCD when the pipe work and radiator became live but that is not 100% certain to happen.
Such a scenario is not what supplementary equipotential bonding is designed to deal with, and if a fault like that were catered for then what you'd be doing to the radiator would be earthing, not bonding.
Such a scenario is not what supplementary equipotential bonding is designed to deal with, and if a fault like that were catered for then what you'd be doing to the radiator would be earthing, not bonding.
Bonding to create equipotential area inside the property but it is not the same as earthing.
Isn't this distinction a hang over from the days of voltage operated ELCBs where the "protective" earth conductors ( CPC ) had to be prevented from making any contact with real earth ( ground ) as that would by-pass the coil in the ELCB and prevent it operating if there was current leaking from live onto the CPC somewhere in the property.
With modern current differential operated RCDs the CPC no longer has to be prevented from touching real earth as any leakage to CPC or real earth above the threshold of RCD will un-balance the RCD and trip it.
Or has the introduction of earths for the CPC derived from the neutral created another problem ?. The CPC has the same potential as the incoming neutral which may be forced several volts above true earth when the local network faults or is in significant phase un-balance.
So connecting CPC to real earth would create a circuit which has to carry a proportion of the current that should be flowing along the neutral back to the substation.
And I know that if you want to connect a metal item to a cpc because you fear it might become live in a fault situation then what you are wanting to do is earthing, not bonding.
Isn't this distinction a hang over from the days of voltage operated ELCBs where the "protective" earth conductors ( CPC ) had to be prevented from making any contact with real earth ( ground ) as that would by-pass the coil in the ELCB and prevent it operating if there was current leaking from live onto the CPC somewhere in the property.
I was saying that because the voltage operated earth leakage circuit breaker operated only when the voltage on the CPC compared to true ground was about 50 volts. Therefor any connection between the CPC and true ground was a short circuit across the coil of the ELCB which would reduce the efectiveness of the breaker. With the CPC connected by zero impedence connection a live to CPC short would not be detected by the ELCB and protection would only be from the fuse or MCB.
Agreed the voltage on the CPC would not get above 50 volts ( or whatever the operating voltage of the ELCB was ) but the earth leakage current could be very high.
If the impedance of the link ( fault ) between CPC and real earth ( ie in parallel with the coil in the ELCB ) is 10 ohms then a 5 amp earth leakage into the CPC will be needed before 50 volts appears across the coil to operate the ELCB.
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