Relay Output Chain
It is the series of elements running from the receiver's dry contact to the motor: the relay contact, the limit switch NC contact, the interlock contact of the opposite-direction contactor, the contactor coil, and finally the power contacts. Every element in the chain can interrupt the movement independently; for this reason fault finding has to proceed in order.
Building the chain correctly is a precondition for the remote control behaving safely. In a typical crane installation the order is as follows:
- Receiver relay contact (NO) — closes when the command arrives.
- Limit switch NC contact — opens when the limit is reached and cuts the movement independently of the radio link.
- NC auxiliary contact of the opposite-direction contactor — electrical interlocking.
- Contactor coil — this is the real load on the relay.
- Contactor power contacts — carry the motor current.
This ordering has two benefits. First, the safety functions (limit and interlock) sit in front of the control electronics rather than behind them; even if the receiver gives a faulty command, the limit is protected. Second, fault diagnosis becomes linear: if the relay pulls in but the motor does not turn, you look for the fault in the links of the chain after the relay.
When building the chain, make sure the only load on the relay is the coil; the motor current must be carried by the contactor. Treat the coil load as inductive too and size the contact with derating.
The context in which this term is used: Wireless remote control installation: pairing, relays, safety