The top event, the ellipse, contains the description of the system-level fault or undesired event. This symbol appears at the head or top of the tree and is included only once in any tree. The input to the ellipse is from a logic gate.
The fault event, the rectangle, contains a brief description of a lower-level fault. This description should be short without being vague. Fault events appear throughout the tree and have both their input and output from a logic gate.
Logic Gate inputs and outputs, except for the Inhibit Gate, which is addressed below, have similar connections. The output from a logic gate is to any fault event block or to a Transfer Out function. The input is from any fault event block or from a Transfer In function. The AND Gate is the logic gate in which the output occurs only if all inputs exist.
The OR Gate is the logic gate in which the output occurs only if one or more of the input events occur.