(f)the use of ground power supply units, air-conditioning units, tractors and similar equipment shall be permissible subject to compliance with the following conditions:--(i)flexible training cables suitable for use in hazardous areas shall be used;(ii)they shall be located outside the Danger Zone. "Danger Zone" is defined as the area within the largest polygon obtainable by joining point 3 metres away from the wings and the fuelling vehicle;(iii)the units, including the associated electrical equipment, shall be flame-proof and of a type approved by the Chief Inspector of Explosives; otherwise they shall be stationed at a distance of not less than 15 metres, in the case of gasoline or wide out fuels and 6 metres in the case of straight kerosine, from the aircraft and the fuelling vehicle;(iv)they shall not be switched "ON" or "OFF" during fuel transfer;