Submarine cable laying and repairing

THE LOCALISATION OF BREAKS AND FAULTS. 451
The resiatance r is taken approximately as the joint resis-
tance of R and /. That is,
whence /=t5^ •
(1)
Also the ratio of the distances D and d is proportional to the
resistances R and /. That is,
^ /
Substituting the value of / from (1) we have
D_ R(R-r) _R_.^
d R?- r
And as — = n, the correction is
r
d=^^ (2)
This correction must be added to or subtracted from the
observed distance, according as to whether the N.R.F. is on the
near side of the fault or beyond it. That is, expressing the dis-
tances along the cable conductor in ohms, it is obvious from
the diagram that to obtain the true distance of fault we must
Add the correction to the observed distance of fault if the distance
of the N.R.F. from the testing end Is less than the observed
distance of fault, or
Subtract the correction from the observed distance of fault
if the distance of the N.R F. from the testing end is greater
than the observed distance of fault.
For example, suppose the N.R.F. to be 3,200 ohms distant,
and the observed distance of the fault 2, 700 ohms (D = 500)
also that the insulation was 1"185 megohms at the time the
N.R.F. was determined and 01 megohm when the fault came
in. Then n = 11-85, and the correction is
=46 ohms.
11-85-1
In this case the N.R.F. distance is greater than the observed
distance of the fault, and therefore the correction must be sub-
tracted, and we have
True distance of fault = 2,700 - 46 = 2,654 ohms.
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