Short Circuits No. 4. - Common Return is really Common.

Common return in electrical circuits is really more common that you may have thought. After all, planet Earth is by far the largest common return conductor of all and it does seem to work rather well.

On model railway layouts it is an ideal way to reduce the number of interconnecting wires required between the control panel and your layout. In theory the minimum number of interconnecting wires may be defined as one wire per individual device or track section you wish to control, or two wires to each point motor, plus one single, and only one, common return wire. Even point control, whether it is solenoid or motor driven, can be reduced to one wire plus a common return. Using route control for points will also substantially reduce the amount of wiring too. More detail of those subjects though in subsequent articles.

However, common return does have an inherent downside especially for the complacent and the unwary. As the sum of the return currents of
all the circuits is passing along this one single conductor, it is absolutely essential that this conductor wire is of as low a resistance as possible. Using heavy gauge wire such as solid copper mains earth return wire is one solution to reducing this resistance. It is essential that all devices connected to this earth common are terminated with secure connections. This is not the place to have sloppy and twisted dry joints, bare wire connections, or poorly terminated and soldered joints. Bad, resistive or even broken connections result in some really bizarre effects and can be time consuming to localise and fix. Not the situation you want to find yourself in within an exhibition environment.

Common return may be used for positive and negative earth common DC and also AC feeds to different devices and also for train control to track sections simultaneously. It is imperative, nay mandatory, that every power supply connected to this common return is derived from a fully independent secondary transformer source.

There you are, You have almost halved the amount of wiring you have to install on your layout. Can't be all bad, can it?