Macurco CM-12 User Instructions page 11

In this application (above) the Fan or primary relay is used as a low level alarm relay. The Alarm or secondary relay is used as a
supervisory relay when utilized in the normally closed configuration. The GD-6 monitors all critical functions of the unit through software
diagnostics that continually test and verify its operations. If a problem is found, the unit will switch to a fail-safe/error mode or trouble
condition. In this error mode the Fan* and Alarm relays will be activated indicating the trouble condition at panel and the GD-6 display
will flash the error. *See the Trouble Fan Setting Option,
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