Which switch is which?

You see switches advertised with numerous descriptions such as single pole single throw (SPST) and double pole double throw (DPDT). Do you understand what all this jargon actually means? If so then there is no benefit in you reading on. If not then please continue.

Switches are used to connect or disconnect a circuit or to divert a circuit from one location to another. It is important to use the right switch for the right function. The simplest function is to switch on and off a circuit and this requires a single make contact unit described as Single Pole Single Throw (SPST). There are two versions of this switch, locking or non-locking. Locking stays on until you switch it off (used for isolating sections or station lights) and non-locking (used for operating point solenoids) which returns to the previous setting on removal of your finger from the switch. If you add another contact unit or pole to the switch to independently switch another circuit this becomes Double Pole Single Throw (DPST). Also available are triple and quadruple pole switches.

Change over switches do just that. This time a centre contact switches between two different circuits. These are also available in multi-pole options, for example if there are two change over units it is a Double Pole Double Throw Changeover (DPDT C/O). Also available in non-locking format. Another option here is to have a centre off position and this becomes a Double Pole Double Throw Centre Off (DPDT C/Off). Also available is non-locking in either one position or both positions. Uses for the first option is changing a point and connecting a track feed and the second is to set and reset a point solenoid.

Something else to remember is the voltage and current rating of a switch. Switching on but more so, switching off a higher current will cause arcing within the switch and will burn the switch contacts, especially if the item being disconnected is inductive such as a point solenoid. Use a switch rated for the job intended.

As always it is best to handle the switch before you buy. That way you will see the functionality (locking or non-locking), see how many poles are installed and check the current and voltage ratings.

Yes, buying model railway electrical components is not as glamorous as buying a new locomotive or some new scenic items, but if you want reliable operation of your layout there is only one option. Buying cheap is just that. It is false economy. It is always best to use good quality switches, as that way you will only have to do your electrical wiring job once.