Fire extinguisher five categories. In the UK the use of Halon gas is now prohibited except under certain situations such as on aircraft and in the military and police. Fire extinguishing performance per fire class is displayed using numbers and letters such as 13A, 55B. EN3 does not recognise a separate electrical class - however there is an additional feature requiring special testing (35 kV dielectric test per EN 3-7:2004). A powder or CO extinguisher will bear an electrical pictogramme as standard signifying that it can be used on live electrical fires (given the symbol E in the table). If a water-based
Cold gas thruster movement and activating the extinguisher. Because the gas is usually unheated, speed at the throat is low and very low performance is achieved; in a vacuum with nitrogen gas a specific impulse of 73 seconds can be achieved. The maximum theoretical specific impulse for nitrogen gas is 76 seconds. In the simplest approximation, the specific impulse is modeled as proportional to the square root of the (absolute) gas temperature, so performance rises as the gas temperature is increased. A thruster in which the performance is increased by heating the gas by an electrical resistance is known as a resistojet. Cold