Zs values and 50v

[Jako13!]

Hi all I’m doing some work on my NVQ and came across this on the internet. Can someone explain this picture to me and if it is correct.

 

[pc1966]

The basic idea is you should not see a dangerous voltage for longer than an acceptable time. Both 'dangerous' and 'acceptable' depend on the circumstances but for something like a sub-main not in a wet/agricultural area it would be your 50V and 5s.

So if you know the current that will disconnect in no more then 5s (say the 170A for a 40A fuse) you can compute the maximum earth impedance such that it is not exceeding the 50V danger level during that period (50V / 170A = 0.294... ohms).

Under a harder fault you would see more than 50V, but the disconnection time would drop rapidly so again you are still keeping within an accepted window of risk.

For something like an RCD of course it might be 30mA (or 100mA for an incomer) trip level with very little time-dependence in most cases, hence the 1667 ohm (or 500 ohm) limit often seen. But there you would still want lower so it is likely to be reliable, which is why the guidance is 200 ohm max for an earth rod.
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Just to add, if you have measured or computed Zs then typically it contains some of the supply impedance (the R1 part) as well as the earth impedance (R2 + Ze) so if Zs is no more than your computed max Ze value from the above steps then:

1. You can always disconnect in under 5s
2. During that period your earth voltage must be less than the 50V (as the voltage drop on any finite R1 part serves to reduce the voltage seen over R2 + Ze)

 

[Ian1981]

Ultimately for TN arrangements regulation 411.4.4 shall be satisfied for ADS , the formula
Is
Zs x Ia shall be equal to or less than Uo x Cmin

 

[pc1966]

As Ian1981 says, the current requirements are driven solely by the overall fault impedance Zs to disconnect under hard fault conditions at minimum supply voltage (Uo x Cmin) sufficiently quickly.

I don't see any mention of maximum CPC impedance (i.e. separated from overall Zs) any more in the 18th edition, but there are still requirements on minimum CPC size.

I suspect it was dropped after the 16th edition (anyone know for sure?) when the use of RCD protection for socket outlets, etc, became the norm so prolonged high fault currents and so high touch voltages on earthed appliances would be very, very unlikely.