Calculation of Touch and Step Voltage Limits using EN50522 2

[IrishPower]

EN50522 is the new European standard for earthing of power installations exceeding 1kV a.c.

The standard outlines the methodology to be followed to select appropriate touch and step voltage safety limits when designing high voltage substation earth grids (covered in Annex A and Annex B of the standard).

I would be interested in hearing from anyone using the new EN50522 standard regarding what touch voltage (and step voltage) limits are being used by different utilities, and how these are being calculated.

Although a 'default' plot of permissible touch voltages is shown in Figure 4 of the standard, showing touch voltage limits for a range of earth fault clearance times, it is not clear from the information presented in Annex A and Annex B of the standard exactly how this figure is derived. The standard suggests that "The curve in Figure 4, which gives the permissible touch voltage, should be used", although the Annexes also suggest that alternative limits can be derived using the methodology shown.

Figure B.2 (in Annex B) shows example touch voltage limits which have taken into account a range of different surface layer resistances (e.g. for different types of crushed rock) and footwear resistances. However, again it is not clear from the information provided exactly how these limits have been calculated. An "iterative process" is apparently used, but based on the limited information provided my colleagues have had difficulties in replicating the results shown in Figures 4 and B.2, let alone adapting these for different surface layer or footwear resistances.

I would welcome any comments or discussion based on the following:

(a) Are any utilities using the touch voltage limits based on the curve shown in Figure 4 of the EN50522 standard?

(b) Are any utilities using the touch voltage limits based on one (or more) of the curves shown in Figure B.2 in Annex B of EN50522?

(c) Are any utilities using alternative touch voltage limits calculated using the methodology shown in Annex A and Annex B of EN50522?

(d) Are any utilities using different criteria to determine step voltage limits, or are they simply assuming that the touch voltage limits provided in EN50522 also apply to step voltages (a conservative approach)?

Thanks in advance for any comments, and apologies for the length of the posting. It's my first post so I wanted to be thorough!

 

[ScottyUK]

Being just over the water from us you might want to look at BS 7430 which has been the basis of British earthing system design for many years. I'm not aware of any rush to change to the EN in the UK, so it probably falls into the category of 'alternative' methods.


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If we learn from our mistakes I'm getting a great education!

 

[IrishPower]

Thanks Scotty,

At present in Ireland the Transmission System Operator (Eirgrid)and the Distribution System Operator (ESB Networks) require use of the IEEE80-2000 standard for design of HV substation earth grids. Some colleagues of mine are presently investigating if (or how) we should move towards using the new EN50522 standard instead.

Northern Ireland (as part of the UK) uses BS 7430 as the basis for earthing design, as you've pointed out. However, a contact I have up there seemed to think that a move to EN50522 had taken place (or will take place), although they didn't seem to have any more detailed information. I will try to find out more.

I'd be interested to see if any engineering colleagues elsewhere in Europe are going through similar issues in deciding how / whether to use the new EN50522 standard?

 

[radug]

Hello,

I am also interested in the topic.

I do not have the final edition of EN 50522 but a draft. I am from Spain and here we are facing a very "interesting" situation concerning regulations: for HV lines permissible touch voltages have to be calculated using the principles of EN 50522, but for HV installations permissible touch voltages must be calculated using an equation with no information about how it has been obtained.

Currently there is a non-public draft version of the HV installations which I think will establish the same criteria as for HV lines, that is, EN50522.

In regards of EN 50522, its touch voltage calculations are derived from IEC 60479-1. If you want to study the topic in depth you should also see IEC 60479-1, 60479-5 and 61201.

Some time ago I also tried to obtain the values reflected in EN 50522 with partial success performing the interpolations in excel.

 

[IrishPower]

radug, thanks for the information.

Maybe by delving in more detail into the other standards you listed we'll be able to work out how the EN50522 limits were calculated. If I've any success I'll post the results here.

I had noticed that EN50522 (approved/dated November 2010) specifically states that it "does not apply to the design and erection of earthing systems... of overhead and underground lines between separate installations".

Radug, in Spain are you applying EN50522 to the earthing design of overhead line towers or to substations ("installations")? I'm not sure from your post what you intended to say - are there two versions of the standard? Any clarification much appreciated.

 

[radug]

Hello,

In Spain we have a "Code" for HV lines (aerial and underground), another for HV substations and power plants, and another one for LV (everything). They are called Royal Decrees (Real Decreto). The LV Code is similar to the British wiring regulations, as it is derived from IEC 60364.
Each Code gives a list of standards to use, so that you can check the standards to analyse each topic in more depth.
For HV lines, the standard to check is EN 50341 (for aerial lines) and in that standard, when dealing with earthing of HV lines, similar curves of EN 50522 are included, all based in IEC 60479-1.
This is for me the right thing to do, same criteria for permissible touch voltages and not different things.
If you have to perform railway installations, and you go to EN 50122, similar curves are also there, again based on IEC 60479-1.

The problem for me is that there should be more guidance on the criteria to use. For example EN 50522 says that the figure 4 curve was obtained by:
"NOTE 1 Different touch voltage conditions, e.g. left hand to feet, hand to hand, lead to different tolerable touch voltages.
Figure 4 of this standard is based on a weighted average taken from four different touch voltage configurations. Touch voltage left hand to feet (weighted 1,0), touch voltage right hand to feet (weighted 1,0), touch voltage both hand to feet (weighted 1,0) and touch voltage hand to hand (weighted 0,7)."

By the way, I revised my calculations and was able to obtain figure 4 and Table B.3 results applying the method explained in Annex A. Later I will try to obtain the curves in figure B.2.

 

[IrishPower]

radug,

Thanks again for the useful information. I wasn't aware that there were similar touch and step voltage criteria listed in standard EN50341 (for overhead lines) and in standard 50122 (for railways). As you say, it makes sense that similar criteria should be used for other types of high voltage 'installation' - whether substations, lines or railways.

I agree that there should be more guidance provided on how to adapt the touch and step voltage limits shown in EN50522. In the absence of such guidance, I guess it's left to us engineers to read through the standards and figure it out for ourselves!

It's good that you were able to replicate the Figure 4 and Table B.3 results; at least it shows that it can be done. I'll ask my colleagues to try to do the same, or have a go myself when I have some more time. I'll post back here if/when we've any success. If you have any luck with obtaining the curves in Figure B.2 please do let us know about it here.

Anyone else out there have any luck with this issue yet?

 

[radug]

IrishPower,

I reproduce the values I was able to replicate. As you can see of the 8 values of the table, I was able to obtain 6 exact rounded values but 2 values were obtained with some error. I find it quite odd to be able to reproduce 6 values perfectly and then have an error in reproducing the others.
By the way, as design criteria, I almost never use ground-fault clearing times below 0,5seconds to account for backup relay trippings, so those errors would be not important.

time(s) My values/Rounded/EN50522
0,05 715,86 716,00 716,00
0,1 613,25 613,00 654,00 (error=6,27%)
0,2 510,14 510,00 537,00 (error=5,03%)
0,5 220,17 220,00 220,00
1 117,09 117,00 117,00
2 96,04 96,00 96,00
5 85,78 86,00 86,00
10 84,61 85,00 85,00

 

[IrishPower]

radug,

Thanks for sharing the values you've been able to replicate from EN50522. It does seem strange that two of the values are different. I occasionally need to use touch voltage limits calculated using faster earth fault clearance times (in cases where there is duplicate fast-acting/reliable protection), so I'll have to see if I've any more luck than you in getting the 'correct' answers for 0.1s and 0.2s! Unfortunately I'll be working on something else for the next few weeks, but I will post back my own results when I get a bit of time to go through this in more detail - thanks again.

IrishPower