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steepnesses and front times is smaller than the
measured dispersion rm-
r = ]/r m* — ta* — tg*
For simplicity when determing ra two coils having
the indication limits 1/J!a and ]/« are considered.
With logarithmic scale and rectangular distribution,
the following applies:
in V7T
J x2 dx
— In <fa 1 . ,
T" ––= l2lnr’
J d*
- In
With a = 3 this gives ra = 0.32. The gap dispersion
of 15 % gives a standard deviation in logarithmic
scale of about 0.14 and thereby a total standard
deviation of the measuring device of
yo.322 + 0.142 = 0.35
As can be seen from fig. 4 a the measured
logarithmic standard deviation rm for front times is
about 2.9. The correction for the dispersion of the
measuring device therefore is negligible:
r = \J 2.902 — 0.352 = 2.88
It has been reported that such indicators give false
indications when mounted on the overhead earth
wires of power lines2, probably due to corona
discharges. However, the consistency of our gap
indications and especially the fact that no gap
indicated unless all the gaps for lower steepness indicated
simultaneously, make it improbable that corona has
falsified the results. A reason for the different
opinions could be that our indicators were mounted
close to the ground side of the lightning rod whereas
in the quoted tests2 they were at the tops of the
power line poles.
The magnetic links
The magnetic links — constructed by Blomberg3’4
—• contain actually four small magnetic bars
mounted in a plexiglassrod, fig. 3 a. Each bar is made of
a 5 mm long piece of a magnetic steelband 0.1 X 3
mm used for recording purposes in the beginning
of the magnetic recording era. One of the bars
was used in the conventional way for recording
crest values while the three others were attenuated
with surrounding small coils having different time
constants.
The remanence of an attenuated coil is a function
of the maximum field strength Hrmax of the field
penetrating the coil. This field is dependent on the
time constant L/R of the coil, the shape of the
lightning current Is, and the distance r of the coil from
the conductor. Therefore the relative magnetisation
p of an attenuated coil, compared with the
unatte-nuated one at the same distance gives information
about the time parameters of the lightning stroke.
„ Hrmax
p = 2n r —-
Is max
The results of calculations of the function x for
three different cases, an exponential, a half
sinusoidal and a rectangular current, having the time
above half value Th are shown in fig. 3 b.
Th
Experimental checks4,5 shows that the error of the
links under favourable conditions is about ± 10 %.
Favourable conditions mean that the current should
be well below the saturation current and well above
the disturbances at low remanences. Further the
relative magnetization p should be between say 20 and
70 %. In practical use, however, none of these
conditions are fulfilled. Consequently the accuracy of
the links may not be overestimated.
The links were supported by a holder at a distance
of 100, 750, and (as a later addition) 1 500 mm from
the conductor. The coils were rated 0.69 L/R = 1,
10 and 100 (is. Therefore the total measuring range
was about 0.5—150 k A and 1—500 (.is.
The measurement of the links was made by the
conventional remagnetization method. For practical
reasons the links were inserted in a holder and
0.69 L/R
= f(p)
Fig. 3. Magnetic links for recording of current amplitudes
and i.uration
a The link
b Calculated relation x between the relative
magnetization p and the times above half value Th
c Record from Hellefors Bruks AB, Fredriksberg,
1957
d Record from Eslövs Yllefabrik AB, Eslöv, 1957
ELTEKNIK 1959 1 115
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