These are heavy-duty magnetic reed contacts for use on roller shutters (overhead doors) on shop fronts, workshops, etc. Below we see a half closed shutter. The magnet is concealed behind the shutter to reduce the risk of tampering. The contact is secured to the ground by self tapping screws and the cable is passed through a pre-drilled hole in the wall.

As you can imagine fitting these contacts is not an easy task so give yourself plenty of time and think it out cearfully before starting to install.

A few tips: Place the contact part on the ground or where you intend to fix it, then check where the magnet will fit on to the door. Then check to see if the door will fully retract upwards with the magnet in your chosen position.

Mostly problems with fitting these are not related to closing the contact, they are to do with making sure the door can rereact back all the way.

Belive me I've done it myself. Fitted the contact only to realise the magnet is getting in the way.

Also the wiring is protected by an armoured cable (see above) which is very hard to cut to size so it's easier to use the full lenght rather than try to cut it.

However they do have a lot of 'play' in them so it should work with a gap of up to 30mm ( over an inch). 

Make sure the shutter can retract to its full height when the magnet is bolted on.