In residential air handler/fan coils it is common to use a high voltage interlock between the blower and the electric heat strips to ensure that the blower comes on whenever the heat is on.
The problem is that it CANNOT work the other way around where the heat comes on with the blower.
Heat strips are generally going to draw 20+ amps depending on the voltage and KW rating which means you CANNOT power them through a typical relay like a blower relay or board which are generally rated for 15 amps or less.
The way the interlock is wired is really quite obvious but is easy to forget because it’s the reverse of what we are used to with a relay.
In short, we connect the blower to the “common” terminal on the relay, L1 power to the normally open (n.o.) terminal and the load (out) side of the heat strip contactor /relay/sequencer to the normally closed (n.c.) terminal.
This diagram from Carrier shows the blower connected on the common terminal and constant power coming in on black to the normally open terminal from the right side of the transformer primary.
Using this 90-340 relay as an example, the blower would connect to 1, power to 3 and the heat strips to 2.
I made a video on it as well if you need it. The result is that the blower runs with the heat but the heat doesn’t run with the blower.