A Quick Theory Recap
A DPDT relay has two poles, each having a Common (C), Normally Open (NO), and Normally Closed (NC) contact. When the coil is energized, both poles switch at the same time - this allows the same signal to control or isolate two separate circuits simultaneously.

That’s what makes DPDT different from SPDT:
SPDT: Controls one circuit -> chooses between two outputs.
DPDT: Controls two circuits -> performs mirrored switching (ideal for interlock, reversal, or isolation).

In short - SPDT decides; DPDT synchronizes.

Real Application: Fan–Damper Interlock Objective:
a. Fan should run only when the damper is fully open.
b. Fan should stop when the damper closes.

The damper actuator’s end switch energizes the DPDT relay coil once the blade reaches the “open” position.

Now both poles act in sync:
Pole 1 (NO contact): Starts the fan contactor coil -> allowing fan operation.
Pole 2 (NO or NC): Sends DDC feedback or triggers secondary safety interlock.
Thus, one relay bridges mechanical safety and digital logic - ensuring reliability and control integrity.

If You Skip DPDT
Fan might start before damper opens -> pressure surge, noise, or duct damage.
Actuator gears may fail under pressure.
DDC may read false “fan running” status.
Control panel loses feedback coordination between mechanical and electrical sides.

"DPDT isn’t optional… it’s essential"
Have you used DPDT relays for fan-damper or pump-valve interlocks in your projects?