Touch voltage 2-pole short circuit

[hhhansen]

Hello
At the utility grid at 10 kV I have made a model of this for simulating touch voltage. When experiencing a 2-pole short circuit to ground the touch voltage exceed 3 kV related to ground potential because of the high short circuit current and the resistance to ground. Maximum (worst case) breaking time is 0,5 sec. Looking in EN50522 the permissible touch voltage is Utp= 240 V at 0,5 sec. From the design rule (EN50522 figure 5) the maximum step voltage must be less than 2xUtp, which is 480 V.
It is not possible to reduce breaking time less than 0,5 sec due to selectivity. Also from the permissible touch voltage curve in EN50522 it is simply not possible to fulfill Ustep_max < 2xUtp regardless of breaking time.

I believe other must have faced the same challenges and I remember somthing about an exception stipulated in the standards when dealing with e.g. a 2-pole short circuit. However I am not able to locate where this is written in the standards. Anyone, who can help on this topic ?


Best Regards,
Hans-Henrik

 

[waross]

That is why gradient control mats are found available on many service trucks. Gradient control mats are permanently installed at switch operating positions on many systems.

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Ohm's law
Not just a good idea;
It's the LAW!

 

[IrishPower]

Hans-Henrik,

The notes under Table 1 of EN50522 mention “if there is no automatic disconnection of earth faults, the need to consider double earth faults depends on operational experience”. This might be relevant if your 10kV system is operated with isolated neutral or resonant earthing. The expected touch voltage you mentioned seems very high and so as waross mentions, some form of improved equipotential control would also be worth looking at (better-meshed earth grid or gradient control mats).