Earth Loop Impedance Table 17th Edition

[pevvers]

Just come across this post through a strange journey.

If anybody is using this table, please be aware that the BS3871 values in this table have been calculated at 240V and not 230V as is needed to conform with BS7671:2008.

 

[the bootneck]

hi guys can some one point me in the right direction for the correct Zs readings or where I can find details of where they might be in the regs for a gen set on a construction site with an RCD fitted

 

[Guest 004]

A generator will not have a fixed earth, so whatever values you record will be much higher than is permitted. (see table in above post). However since it is on RCD, which of course is mandatory then is theory the values can be as high as 1766 ohms. Just record the values, and in the comment box state that it is a generator with an earth spike, thus giving high readings, but acceptable as is protected by an RCD.

 

[chiplard]

When taking a Zs reading, we cannot use the direct measurement to compare with the Zs earth loop tables in the Regs.

We have to take into account ambient temperature (at the time of test) and the conductor operating temperature. Both have an effect on the resistance. There is therefore a 0.8 rule to apply for your max Zs reading.

Example:

Regs book Page 49, Table 41.3, Type B BS EN 60898, 6A Zs = 7.67 ohms

7.67 x 0.8 Rule of Thumb = 6.13 ohms

17th Table - BS EN 60898, 6A Zs = 6.13 ohms

So the table gives you a direct comparison for your Zs readings.

Hope this helps!

Warren

Am I right in saying that this is for an ambient temp of 10 deg C?

If so and the ambient temp at the time was 20 deg C then you could apply a correction factor of 1.04 to the OSG (0.8) figure. Then your Zs reading would have to meet the corrected figure.

 

[Guest 004]

@chiplard No! ambient temperature is for cable calculations.

 

[chiplard]

@chiplard No! ambient temperature is for cable calculations.

So what is Table 2E in the OSG for then?

 

[pevvers]

For why we use the 80% value, have a look at appendix 14 (or thereabouts) in the BRB, and it will explain all.

Table 2E is the corrected values to 80% for the reason above, but the 100% value in the regs still has to be stated on any paperwork.

 

[chiplard]

Quote from Appendix 2 of OSG - first paragraph.

The tables in this appendix provide max permissible measured earth fault loop impedences Zs............... .............The values are those that must not be exceeded in the tests carried out under 10.3.6 at an ambient temp of 10 deg C. Table 2E provides correction factors for other ambient temperatures.

What is that trying to explain then?

 

[mattmoo1974]

Quote from Appendix 2 of OSG - first paragraph.

The tables in this appendix provide max permissible measured earth fault loop impedences Zs............... .............The values are those that must not be exceeded in the tests carried out under 10.3.6 at an ambient temp of 10 deg C. Table 2E provides correction factors for other ambient temperatures.

What is that trying to explain then?

OSG is the mini BS7671
and GN3 tells the truth in plain old english

 

[chiplard]

So let me get this straight then.

BS7671 gives max Zs for normal conductor operating temperature

OSG applies the correction factor for an ambient temp of 10 deg C

GN3 states that the figures are only to be used if the ambient temp is 10 - 20 deg C - any lower or higher than this range then the correction factors must be applied.

GN3 also states that the tabulated figures only apply if the nominal voltage is 230V

If the variables you have differ from the above then you must calculate to ascertain your max allowable Zs.

Is that right?

 

[Guest 004]

lol, welcome to the art of testing, BS7671 is the bible. I think what is important here is the actual reading obtained, if the reading is close to the max Zs allowed, then I would suggest doing some calculations. Otherwise I would just stick to what BS 7671 states.

 

[chiplard]

So my first post in this thread was right then!!!!!!!

 

[brizospark]

Always go by the BRB and like previous posts say if the reading you are getting is close to it then you will have to carry out calculations to see if it passed the 80 percent figure. This figure is used for to compensate for various variables including temperature and the way I se is is the IEE's way of erring on the side of caution