If the power is cut, does the remote control command stay latched?
It does not: when the supply goes the relays release, contactors drop out, movement ends. The real measure is a self-holding circuit that blocks auto restart.
It should not. The moment the supply is cut the relays of the receiver release, the contactor coils drop out and the movement ends; there is no command left hanging. The real question is the second one: when the power comes back, does the machine start moving of its own accord? What prevents that is not the remote control but the restart interlock built on the panel side.
What happens at the moment of the cut
The relay coils lose their supply, the contacts open, the contactors drop out. On lifting machines the mechanical brake is designed to grip when the power is cut as well; that is, the brake holds in the de-energised state and the load stays suspended. IEC/EN 62745 addresses the behaviour on loss of supply in wireless control systems explicitly and expects a defined, safe outcome; the behaviour the standard expects of the control system is not a matter left to the preference of the manufacturer.
From the point of view of the operator the symptom is clear: there is no response at all when the button is pressed, and the status indicator on the receiver is dark. Faced with that picture, check the supply before the remote control. Taking the receiver supply from a fuse independent of the line that feeds the direction contactors makes it easier to tell which side has lost power after a cut, and shortens fault finding.
The real risk starts when the power comes back
If the control button is still pressed, or the circuit has been built as a straight-through one, the machine can start moving of its own accord; this is one of the most frequently overlooked dangers on site. Short dips are a separate trap: if the voltage recovers without falling to the drop-out voltage of the contactor, the contactor never drops out and the command carries on as though it had never been interrupted. The measures against this are standard:
- Build the direction contactors with a self-holding (latching) circuit; when the power goes the hold is released, and when it comes back the movement does not start until the operator gives a command again.
- On three-phase systems add a phase sequence and phase failure protection relay.
- Document the restart interlock on the circuit diagram; the prevention of automatic restarting is a function drawn in from the start, not one added later.
Which measure closes which risk
| Risk | Measure | Acceptance check |
|---|---|---|
| Movement of its own accord when the power returns | Self-holding contactor circuit | Switch the power on while holding the button pressed; no movement should start |
| Phase failure or a change of phase sequence | Phase protection relay | Disconnect one phase and see that the contactor does not pull in |
| A short voltage dip | A contactor with a suitable drop-out voltage | Try that the command does not carry on by itself after the dip |
| The operator being unable to cut the movement | Wired emergency stop chain | Open the chain and verify that all directions stop |
Where this arrangement is not enough
- Self-holding only makes sense on an installation with contactors. If the relay output is connected directly to a small DC motor there is no coil to hold in; you have to build the interlock with a separate relay.
- On motors driven by an inverter the restart behaviour is determined by a drive parameter. Even if the panel relay is right, the risk remains if the inverter is set to "run when powered".
- On machines with no mechanical brake, or with a worn brake, the load can slip at the moment of the cut; this is not a control matter but a maintenance matter.
- In a long cut the load stays in the air. Lowering it is done only with the machine's own manual lowering arrangement and by trained personnel; trying to make a movement by pressing the contactor in by hand is not an acceptable method.
- The stop button on the remote control does not take the place of the wired emergency stop circuit.
On the ST100 and SF100 sets, the manual button being able to work in parallel with the RF remote control makes a controlled restart easier when the power returns. Confirm the loss-of-supply behaviour of your set from the user manual, then carry out a single acceptance test: switch the main isolator on while holding the button pressed and see with your own eyes that no movement starts. This test takes a few minutes and is the most critical verification of the installation.
The full topic: Wireless Crane Remote Control Selection Guide