What is a switch sensor?

A switch sensor uses relays that are either Normally Open (NO) or Normally Closed (NC) and can only be on one of these two states. The states can be used to indicate normal running and alarm conditions. When the set condition being monitored changes, the switch sensor changes from NO to NC or NC to NO. The opening or closing of the relay determines whether a voltage is present and it is this that an environment monitoring device monitors and records or uses to invoke an action in a detector i.e. a Smoke Alarm, Air Flow, Fuel Tank level or Water level monitor.