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