TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas

10. Impulse Ratio Check on Selective Signalling. (See Fig. 23.)
It is important that the impulses due to line battery reversals sent out
to line on selective ringing circuits should be of equal duration. This can be
checked as follows:
Insert test plugs into line under test. Operate Line Cut Off Key (LCO)
(to avoid calling all stations). Operate Impulse Ratio Key (IRK) and transmit
a "General Call."
Selector Circuit Impulse Ratio Test.
Fig. 23.
The voltmeter needle should move a short equal distance on either side
of the zero marking, and at the end of the code give a full swing to the right.
This will only apply if the impulse ratio is correct, that is 50% operated,
50% unoperated. If, however, the pointer vibrates on either side of, say,
the 10 volt marking to the right on the 70 volt scale, then the ratio would be
incorrect at approximately 60% operated and 40% unoperated. The function
of the Relay IRR is to ensure that the voltmeter is not connected
across the lines when the line battery is applied to line before impulses are
sent out. If this were not done the voltmeter needle would receive an initial
deflection and might not settle down to the correct indication before the
end of the General Call code.
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