MI 3290 Earth Analyser
Appendix F - The balancing procedure
159
Appendix F
– The balancing procedure
1. Repositioning of the S probe when using the Selective Flex method with In-Line
probe set-up
On the pylons, which have local earthing as well as overhead grounding wire, the current
through the local earthing
I
f_sum
and the auxiliary test probe
I
H
is not equal. The consequence is,
that in case of in-line probe set-up the equation C.9 from IEEE 81, which defines the position of
the S rod on 62% of the distance between the centre E of the tested pylon and the auxiliary test
probe H, is not valid any more.
In such case, the local earthing current I
f_sum
has to be measured separately with the method as
described in IEEE 81-2012 Ch. 8.2.2.6 and implemented in
MI 3290 Earth Analyser
(measurements with multiple flex clamps).
The following relations are valid:
𝐼
𝐻
= 𝐼
𝐸
= ∑ 𝐼
𝑓_𝑖
4
𝑖=0
+ 𝐼
𝐺𝑊
= 𝐼
𝑓_𝑠𝑢𝑚
+ 𝐼
𝐺𝑊
Therefore, the formula C.9 must be corrected with the coefficient
k
i
:
𝑘
𝑖
=
𝐼
𝑓_𝑠𝑢𝑚
𝐼
𝐻
where I
f_sum
is the summary of all earth currents measured with flex clamps and I
H
is the test
current generated by the instrument.
The corrected formula C.9 for the S probe positioned between E and H (S
B1
position) is:
1
(𝑅 − 𝑥)
−
1
𝑅
−
𝑘
𝑖
𝑥
= 0
and for the S probe positioned on other side of the H probe (S
B2
position) is:
1
(𝑥 − 𝑅)
−
1
𝑅
−
𝑘
𝑖
𝑥
= 0
R is the distance between E (centre of the tested object) and earth rod H (auxiliary current test probe).
This results in a changed position of the S probes
– the distances r
1
and r
2
of the probe
positions S
B1
and S
B2
:
𝑟
1
= 𝑑
𝐸_𝑆𝐵1
= 𝑅 ∗
−𝑘
𝑖
+ √𝑘
𝑖
2
+ 4𝑘
𝑖
2
𝑟
2
= 𝑑
𝐸_𝑆𝐵2
= 𝑅 ∗
2 − 𝑘
𝑖
+ √𝑘
𝑖
2
+ 4
2
