Dean Forest Railway Telecoms


Posted on Feb 5, 2014

Each line key has three positions. Centralised it connects the line circuit to line to await a call. Downwards (KK) it operates a K relay which switches the line through to the common equipment. Upwards (KL) the line is connected to the line tester. The tester normally sends an earth on both legs to line, therefore any line with the key up is norm


Dean Forest Railway Telecoms
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ally earthed both legs and can be left in that state if it should go faulty. The lamp is normally out. If the SPT calls then it is flashed at a rapid rate. When the call is answered the lamp goes out. Should the call not be answered after a timeout period (2min 30sec) the lamp remains permanently lit. If the signaller calls the SPT, he operates the line key and then momentarily operates a ringing key. This causes ringing to be sent to line and the line lamp flashes slowly in synchronism with the ringing cadence. The common equipment has a standard transmission bridge which is connected between any SPT and the signalman`s telephone circuit by the operation of the SPT speak key and the operator`s key. The PHA relay circuit is of interest. It was derived from the IRSE text book on Railway Signalling and consists of a relay connected across a bridge circuit. If no speak key is thrown, then no current flows and the PHA relay remains released. If one speak key is thrown, then 450 ohm earths are connected to each side of the PHA relay which is balanced and remains released. If more than one key is thrown then 450 ohms is connected to the right hand side of the PHA relay and 225 ohms or less on the left hand side. This unbalances the circuit and current flows through PHA to operate it. PHA1 disconnects the earth from the SC wire thus preventing any K relay operating in the line circuits. PHA2 also operates the main warning light...




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