Should I wire the limit switch to the remote control or to the contactor?
The limit switch goes in series in the contactor coil line: relay output, normally closed contact, coil. The protection then works independently of the radio.
To the contactor. A limit switch is always wired in series into the power circuit, that is into the contactor coil line. The order is this: receiver relay output, then the normally closed contact of the limit switch, then the contactor coil. That way the limit protection works independently of the radio link and of the state of the receiver; when the limit switch opens, the contactor cannot pull in under any circumstances.
Why it is not wired to the receiver input
The basic principle of limit protection is that the protection must not depend on communication. Wiring the limit switch to the receiver input and expecting a cut-off in software removes the protection as well when the radio link or the receiver fails. For the same reason the contact type is not chosen at random either: using a normally closed contact keeps you on the safe side when a cable breaks or a terminal works loose.
The current the switch will carry should not be overlooked either. If the circuit feeds a contactor coil, the current through it is small and a standard limit switch is enough. In installations where the relay output is connected directly to a small motor, however, the switch carries the motor current; in that case the contact current rating has to be chosen according to the starting current of the motor.
The correct wiring order
- The receiver relay contact closes.
- The current passes through the normally closed contact of the limit switch for that direction.
- The contactor coil is energised and the motor turns.
- When the limit position is reached the NC contact opens, the coil drops out, the movement stops.
A separate switch for each direction
In this arrangement a separate limit switch is used for each direction: the upper limit switch cuts only the hoisting direction and the lower limit switch only the lowering direction; movement in the opposite direction stays free. Otherwise the machine hits the limit and locks up and the operator cannot recover the load. If more than one switch is to protect the same direction, they are wired in series; when any one of them opens, the movement stops.
On lifting machines with high inertia a two-stage arrangement is common: a slowdown switch that comes in before the limit reduces the speed, and the limit switch proper cuts the movement. That way the load does not coast past the limit after the switch has opened. In a single-stage arrangement the limit switch has to be placed early enough to cover the stopping distance.
The connection point and its consequence
| Connection | When the limit switch opens | Assessment |
|---|---|---|
| Relay output → NC limit → contactor coil | The coil drops out, the movement stops | Correct arrangement |
| Limit switch on the receiver signal input | It stops if the receiver is sound, it does not if the receiver has failed | Inadequate |
| Wiring with a normally open contact | When a cable breaks the protection is lost silently | Wrong |
| A single switch cutting both directions | The machine locks up at the limit | Wrong |
Where a limit switch is not enough
- Limit protection and the wired emergency stop circuit of the machine are different functions; neither takes the place of the other.
- A limit switch protects position, not overload. A separate arrangement is needed for load protection.
- A limit switch does not prevent the forward and reverse contactors pulling in at the same time; electrical and mechanical interlocking is built for that.
- A switch whose mechanical adjustment slips, freezes or gets dirty drops out of service silently. That is why a periodic function test is obligatory.
When you have finished the installation, run the machine with no load and approach both limits slowly; verify by eye the point at which the switch opens and that movement in the opposite direction stays free. Then write the position of the switch and the test date into the maintenance record. Protect the switch cable from vibration and add terminal tightness to the periodic check list; most of the limit faults met on site come not from the switch itself but from a terminal that has worked loose or broken off.
The full topic: Wireless remote control installation: pairing, relays, safety