IEC 60479.1 Time/Current curves for Touch Voltages

[Shotty]

I'm doing a study of the EPR around some existing 330kV towers and am using various standards to determine the acceptable touch voltages. One of them is IEC/AS/NZ 60479.1 which shows a statistical set of regions for allowable body current vs duration of current flow in Figure 20. Rather than refer to the log-log graph and interpolate to get an approximate figure, i'm wondering if anyone has derived a formula for curves c1, c2 and/or c3 that i could plug into excel?

 

[7anoter4]

I have not this standard with me.However in the past I did an excl file according to VDE 141.I hope could be useful.

 

[Shotty]

Thanks anyway.

I put together a table yesterday which seems to be doing the job well enough. The local authority requires IEC 60479, so no substituting.

 

[LiteYear]

FWIW, I've done some study in this area and for my purposes also manually created a table from the figures. You could then curve fit to allow interpolation. For some of the work I used the MATLAB script called "Graph Picker"[1] to digitize the figures, which can help a lot, otherwise just using a transparent image and manually building an Excel chart can be quite effective. However you quickly realise the graphs are generally not to be considered pixel perfect, which probably reflects the underlying uncertainty in the numbers.

Remember if your goal is acceptable touch voltages, you need to consider body impedance as well as body current vs time. It gets complicated very quickly and requires judgement, which is why 60479 is not generally used in applications. Rather it forms the basis of more applicable standards that have done that extra legwork taking into consideration the environment at hand. For example, in the AS/NZS world you'll see that AS/NZS 2067, 3000 and 4871 all derive from 60479, yet they arrive at very different thresholds for acceptable touch voltages that are suitable in their respective environments of concern.

Interested to hear how you go.

[1]

http://www.mathworks.com.au/matlabcentral/fileexchange/41313-graph-picker

or the alternatives (which I haven't tried) GRABIT and Graph Digitzer.

 

[Shotty]

Sorry about the delay, I've just come back from holidays.

The DNSP was requesting a worst case allowance under IEEE80 or IEC61936 (which refers to 60479). Otherwise I would have used the generalised curves from ENA-EG0. This is now the 2nd time in a year that a DNSP has requested the standard to be applied and I'm thinking that creating a standard process is probably justified. I know other consultancies have this in place.

In the end, I used the transperant graph in excel to verify the table. I'll try downloading the link on the weekend and see what formula Matlab will produce for the C1 curve.

 

[LiteYear]

If the customer specifies IEEE80, why not use the worst case from that standard? I'm not very familiar with it, but I guess it would be the 50kg touch potential limit given by this formula:

E = 0.116*(1000+1.5*C*p)/sqrt(t)

where:
E - permissible driving touch voltage
C - reduction factor
p - earth resistivity
t - exposure duration

If you take reduction factor -> 0 (ie. no crushed rock underfoot) then you get worst case, which kinda lines up with other conservative interpretations of 60479.

Are you concerned IEC61936 will be even more conservative? It's possible, I'm not familiar with it.

Note that graph picker will just give you the points - curve fitting C1 might be a difficult exercise in itself. It's certainly not as mathematically simple as the IEEE80 curve.