26.4.1 Electro-Magnets

If an electrical current is passed through a wire it will produce a magnetic field around that wire which will exist whilst the current flows. In most cases when the current is switched off the magnetic field will collapse. The strength of the magnetic field can be intensified by forming the wire into a coil containing a core made of magnetic material. The core will display the properties of a magnet for as long as current flows through the wire. The power can be supplied to an electro-magnet lifter from the mains electricity or from built in rechargeable batteries. Electro-magnet lifters can vary from small portable units to large multi-head units integrated into the crane.

Battery fed electric lifting magnets should provide a tear-off force corresponding to at least 2 times the working load limit under the conditions specified by the manufacturer. There should also be an indicator to show if the magnet is switched on or off. An automatic warning device should be provided which monitors the power supply and provides a warning at least 10 minutes before the supply reaches the level where the load will be released. The warning device can either be optical or audible. There should also be a safety device which, in the event that the low power warning is activated, prevents the magnet from being switched on again until the battery is recharged to the minimum level at which the low power warning is not activated.

Mains fed electric magnets should also provide a tear-off force corresponding to at least 2 times the working load limit working under the conditions specified by the manufacturer. There should be an indicator to show if the magnet is switched on or off and, for magnets with variable power, to distinguish between full and partial magnetism. An optical or audible warning device should be fitted to indicate mains power supply failure. A stand- by battery should be fitted to supply power to the magnet in case of mains failure. The battery should be capable of supplying enough power to hold the working load limit for at least 10 minutes.

The warning device and stand-by battery are not necessary if the unit is working in a ‘no go’ area or if the maximum height of lift at the magnet is restricted to 1.8m and the load is less than 20kg.

Magnets lifting loads such as plates, sheets or bars from the top of a stack, should have controls to allow the operator to reduce the power to shed any excess load and restore full power when the excess has been removed.