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Discuss Earth Loop Impedance Table 17th Edition in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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WarrenG

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Hi Guys,

Here is a handy 17th Edition Loop Impedance Table.

This table gives you a direct comparison with your Zs tests results and is calculated @ the 0.8 Rule of Thumb figure.

Enjoy! :)

Warren
 
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D

DanBrown

  • Thread Starter Thread Starter
  • #2
Hi Guys,

Here is a handy 17th Edition Loop Impedance Table.

This table gives you a direct comparison with your Zs tests results and is calculated @ the 0.8 Rule of Thumb figure.

Enjoy! :)

Warren

What do you mean by the 0.8 rule of thumb figure?? What is that??
 

jeremy

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Mentor
Arms
DO you have to point 8 of your thumbs at something?? I've got 2 and am willing to lend tham out!
 
W

WarrenG

  • Thread Starter Thread Starter
  • #4
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
 
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DanBrown

  • Thread Starter Thread Starter
  • #5
Yeah, thanks for that warren..
Where does the figure 0.8 come from? is it in onsite guide or big regs book?.
 

jeremy

-
Mentor
Arms
OSG. The big one quotes the max allowed under perfect conditions then you have to adjust it by allowing for max operating temp etc. The 0.8 is allowing for worst case scenario but if your Zs is greater than the 0.8 adjusted figure it doesnt necessarily mean it has failed there are longer calcs that can be done to take into account each individual installation. Hope this is of use.
 
H

hugo

  • Thread Starter Thread Starter
  • #7
Warren

You posted a table with max eli details earlier this year. Im new to the forum, can you tell me how to get a hold of the table.

Cheers

Hugo
 
B

BigD62

  • Thread Starter Thread Starter
  • #9
hi warren
Sorry cannot find the table you describe
regards
BigD62
 
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hugo

  • Thread Starter Thread Starter
  • #10
Thanks. I've got the tables in the Regs, but I thought that someone had taken the time to put them into another format; Word or Exel.

Just lazy mate!:)

Regards

Hugo
 

old dog

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Arms
the tables in the on site guide and guidance note 3 give the values after the 0.8 calculation
bs 7671 gives the max before calculation
have a look on page 111 table b.4
osg page 103 table 2 d
for examples
 
G

Guest 004

  • Thread Starter Thread Starter
  • #12
Hi Guys,

Here is a handy 17th Edition Loop Impedance Table.

This table gives you a direct comparison with your Zs tests results and is calculated @ the 0.8 Rule of Thumb figure.

Enjoy! :)

Warren
where is it??
and does anybody have 100% values?
These 80% values are also in the NICEEIC test book
 
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.
 
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
 
G

Guest 004

  • Thread Starter Thread Starter
  • #18
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.
 
C

chiplard

  • Thread Starter Thread Starter
  • #19
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.
 
G

Guest 004

  • Thread Starter Thread Starter
  • #20
@chiplard No! ambient temperature is for cable calculations.
 
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.
 
C

chiplard

  • Thread Starter Thread Starter
  • #23
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?
 
M

mattmoo1974

  • Thread Starter Thread Starter
  • #24
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 :)
 
C

chiplard

  • Thread Starter Thread Starter
  • #25
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?
 
G

Guest 004

  • Thread Starter Thread Starter
  • #26
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.
 
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chiplard

  • Thread Starter Thread Starter
  • #27
So my first post in this thread was right then!!!!!!!
 
B

brizospark

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  • #28
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
 
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