Allocation of Protection against the Effects of Lightning on Electronic Systems

Abstract : A procedure for determining how much protection is needed between the external lightning environment and the circuits inside equipment is developed on the basis of nested shields or electromagnetic barriers. Part of this protection is usually placed at the system level (at the aircraft skin or at a facility shield) and part may be placed at the equipment level. Four distinct allocations of protection are defined and their implications on electromagnetic compatibility and protection against lightning and other external sources are discussed. In two of these, all of the protection is allocated to one level and none is allocated to the other. When all protection is allocated to the system level, all internal cabling and equipment is protected, and no configuration control or lightning requirement on equipment is needed inside the system-level barrier to maintain the protection. When all protection is allocated to the equipment or box level, however, unprotected cabling and other system structure may remain outside the protected space (i.e., outside the boxes). When the protection is disturbed between the system-level barrier and the equipment-level barrier, all of the system inside the system-level barrier has some protection. The implications of these allocations on system protection design and maintenance are discussed in detail.