The RCH (Railway Clearing House) 3-pin jumpers are used to interconnect between coaches as a method of " remote controlling" the lights all through the train from one switch in any carriage. It is also a convenient way of piping the PA system throughout the train.

The (C)MD Regulator box beneath the sole plate contains the two solenoids which switch the lights on (latched) and off (mechanical override from under the regulator housing). These solenoids have a DC resistance of about 14 ohms, so the impedance from the "1" line to the "2" line is 28 ohms. Or for a 10 coach train, 2.8ohms. This is the reason the wiring is heavy! The single operating switch has to sink about 20A to operate all the solenoids in the train simultaneously.

As you can see from the diagram, when the switch is not operated, the ON and OFF lines going to the regulator housing are in contact with the 1 and 2 RCH lines respectively. So any stimulus from another coach switch will operate the lights remotely. When the push buttons are pressed, the ON or OFF line is temporarily disconnected from the RCH line so the lights are switched locally when the ON or OFF line makes contact with the +24v supply.  When the twist switch in the middle of the controller is operated, the ON and OFF lines remain in contact with the RCH lines. The twist action connects +24v to either the ON or OFF line and therefore also to the other carriages via the RCH lines, remotely operating their lights.

RCH line 3 is connected to the 0v of each coach and acts as the return supply for the remote switching function.

The Ripper unit takes its input from the 1 and 2 RCH wires. This amplifier then drives the loudspeakers in the coach. Speakers are normally wired series/parallel to present a load of 3 to 6 ohms to the Ripper unit. The actual wiring depends on the number of speakers in the coach. The Ripper unit is normally situated close to the RCH controller switch and normally uses the same (fused) 24v supply for power.