TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas

the particular line to the earthed point is then read on the potentiometer
scale.
5. Earth Fault Location by Simplified Murray Earth Test. (See Fig. 20).
When there is a single earth or resistance earth connection on one line,
it can be located by means of the earth fault location test.
Have line short-circuited at distant end. Operate WBK and MEK.
Operate meter key (MK) and adjust potentiometer to bring meter near
zero.
Operate sensitivity key (SK) and make final adjustment of
potentiometer to bring meter to zero.
The percentage indicated on the inner scale of the potentiometer represents
the distance of the fault from the control station, stated as a percentage
of the total line.
In order to obtain the best advantage from this test, it is desirable to
prepare a chart showing the measured percentage to convenient test points
on all lines. For example :
If the total circuit loop resistance is 1,000 ohms, and at test points A,
B and C, the loop resistance is recorded as, A = 250 ohms, B = 500 ohms
and C = 750 ohms, then
At test point A the percentage of the total circuit would be
250 X 100 = 25%
100 1
and at Test Point B, 500 X 100 = 50%.
1000 1
and at Test Point C, 750 X 100 = 75%.
1000 1
An earth fault actually at these points would be recorded by the Murray
Test as 25%, 50% and 75% respectively. An earth fault tested to be say
40% would be localised between test points A and B. To obtain the full
value of this facility, as many loop readings as are possible should be
obtained and recorded.
If the Main Circuit has a branch or branches, the loop resistance should
be recorded to test points on both the Main, Branch or Branches. The
chart can then be made up to cover these Branches.
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