Hook up & down safety switch
Prevents the hook from being run into the boom or driven into the ground. The hook stops automatically — even if the operator has not let go of the control — one metre below the boom and one metre above ground level.
Trolley in & out limit switch
Stops the trolley 20 centimetres before it reaches either end of the boom.
Overload limit switch
If a load beyond the crane's rated capacity is lifted at a given boom length, the alarm sounds and the hoisting circuit stops.
Moment cut out switch
Guards the combination of load and radius. If the maximum load is lifted at the permissible length and then trolleyed outward, the result without this switch is a severe accident.
Slewing limit switch
Restricts the slewing system so the boom cannot be swung into another crane or into the building within its radius.
Our recommendation
Fit a phase failure relay
Because we are concerned with the trouble free and safe working of your machines at all times, we recommend inserting a phase failure relay into the existing circuitry.
What it does
It cuts the power supply when one phase is not reaching the equipment through the distribution board. That saves the motors from single phasing, which is what burns them out.
It also protects against a reversed phase. If the phases arrive at the machine in the wrong order, none of the safety features built into the equipment will function as intended.
The failure worth understanding
The control transformer is fed from two of the three phases. If the third phase — the one not connected to the transformer — is lost, no power reaches the motors, but the two phases feeding the transformer still energise the control circuits.
Those circuits release the brakes on the hoisting motors. The power reaching the motors is not enough to raise the load, so instead of lifting, the load falls free, accelerating under its own weight. If the operator is not paying full attention at that moment, it causes a serious accident.