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 The purpose of this article is to explain the common terminology involved when referring to switches and push buttons. Each of these devices are generally available in numerous configurations and knowing “which switch is which” can greatly simplify quoting, installing, and servicing electronic locking systems. The switching device of a door control system can be used to simply unlock a door or, with the right accessories, unlock the door, generate a system log, notify a guard station, and relock the door after an adjustable time delay. Hopefully this will make adding these features into a door installation much easier to understand.
Switch Contacts
Basic switches are classified by form; A-Form, B-Form, and C-Form are the most common. These three groups are explained below.
a. An A-Form switch is often referred to as a Normally Open (N/O) Single Pole Single Throw (SPST) switch. There should be no resistance across the terminals when the switch is in its normal state. When the switch is activated these terminals will “short” creating a path for current flow.
b. A B-Form switch is often referred to as a Normally Closed (N/C) Single Pole Single Throw (SPST) switch. In its normal state this switch will display a “short” condition across its terminals. When the switch is activated these terminals will “open” removing the path for current flow.
c. A C-Form switch is a basic example of a Single Pole Double Throw (SPDT) switch. This implies that there is one Common (C) terminal, one N/C terminal, and one N/O terminal. When this switch is activated the “Pole” moves from the “N/C” terminal to the “N/O” terminal. This means that the current path from the “C” to the “N/C” terminal is removed and a current path from “C” to “N/O” is created.
Of Poles and Throws
Simply put, the pole is the conductor that current flows through and the throws are the number of points that the pole can be switched to. For example a SPDT switch will have a common terminal at the base of the pole and, being a Double Throw, it has two points that it can switch between. In this case the two points are typically a N/C and a N/O. The diagram above explains the connectivity between the most common switch configurations.
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