difrips, this is what the Australian regulations say, I'd bet there are similar rules in your own country.
6 GROUND OPERATION OF AIRCRAFT RADAR EQUIPMENT
6.1 The requirement of this subsection shall apply to all radar equipment with a nominal peak power output rating in excess of 25 kW.
6.2 During all ground operation, including testing and maintenance of aircraft radar equipment, the operator and person in charge of such equipment shall ensure that:
6.2.1 The equipment is not energised in its normal mode (antenna rotating) unless the sector area scanned by the radar beam is clear of the following objects to a distance of 37 metres (120 ft) from the antenna:
(a) aircraft being refuelled or defuelled;
(b) fuel tankers, fuel tanks or fuel storage areas;
(c) persons or cargo;
(d) any other aircraft or aircraft hangar.
If you consider that the spacing between runway edge lights is normally 60 metres you'll realise that 37 metres isn't very far but it's probably unwise to just turn the thing on without thinking about what's around you. Our company procedure is to turn it on, if required, when entering the runway with the intention of taking off (ie. not when just crossing a runway.)