Engineering notes
Interposing relays and dry contacts on a BMS panel
When a DDC needs an interposing relay before a motor circuit. Dry contacts, load, 24 V coil.
A DDC digital output is not a breaker
Many controllers offer a dry contact or a small 24 V transistor, a few hundred milliamps. Feed that straight into a pump starter or a 230 V coil and you weld the output. An interposing relay sits in between: the DDC drives a 24 V coil; the power contact sits on the motor circuit.
- Coil voltage matches the panel: 24 V DC or AC
- Contact rating at least the real load
- Usually one normally open; sometimes a normally closed for status
- Socket and label — do not solder it to the board
Incoming dry contacts (generator, breaker, float, door) must not carry 230 V into a digital input. 24 V supply · BMS equipment