13-11] PRESSURE 711 



13-11 PRESSURE 



The operation of electrical equipment located in nonpressurized areas of 

 airplanes and missiles may be adversely affected by the change in the 

 ambient atmospheric pressure induced by changes in the flight altitude. 

 One of the most serious effects of the low pressure existing at high altitudes 

 is the reduction in dielectric strength of air. This reduction in the strength 

 of the dielectric medium can cause a corona discharge. Corona discharge 

 occurs when the dielectric strength of the air is exceeded by the potential 

 gradient created by a conductor in a high-voltage circuit — for example, a 

 power supply unit. It can be detected visually by the purplish discharge 

 around or between the conductors or by the radio noise level that is 

 produced. 



Low ambient pressure conditions also allow a condition known as 

 arc-over to take place between switch contact points, breaking points, bare 

 terminals, uninsulated wire, etc. Erratic operation or the partial or 

 complete breakdown of the equipment may result. Increase in the wear 

 rate of the contact points will occur because of the increase of intensity of 

 the arc. To eliminate induced radar noise, decrease in the insulating ability 

 of material, and reduction in the useful life of equipment, the designer of 

 electrical equipment must include this environment in the basic design 

 criteria for the equipment. 



Pressurization of high-voltage equipment is a positive method of combat- 

 ting the adverse effects of low-pressure conditions. However, this method 

 for elimination of this particular electrical problem complicates the overall 

 mechanical problem. For example, pressurized containers, in order to 

 withstand required differential pressures, must have a certain degree of 

 structural strength and stiffness, thus adding a weight penalty to the overall 

 system weight. Sealing problems along with the possible use of a pressure 

 pump to hold a fixed pressure level within the container add complexity to 

 the problem. The cooling system is made more complex since the heat- 

 dissipating electrical units cannot be cooled directly by the ambient air. 

 Instead, an air-to-air heat exchanger must be used in order to remove the 

 heat from the recirculated air within the pressurized container. 



For nonpressurized units, the use of proper detail design techniques will 

 allow the equipment to operate satisfactorily in low-density air. Some 

 recommended methods follow. Since the breakdown voltage between 

 electrical conductors is a function of the distance between conductors, the 

 correct spacing between conductors will materially aid in the solution to 

 the problem. For some voltage levels, the required distance between 

 conductors may be greater than the available space. In this case, replacing 

 the air by some other medium, gas or liquid, with a higher dielectric 

 strength is a feasible solution to the problem. This technique, however, 



