Abstract: A diagnostic instrument distinguishes primary arcs from other electrical discharges in an electric power system based on pulse time density of radio frequency noise caused by the discharges. The instrument counts a maximum number of noise pulses in any small time window over a period, and identifies the discharges as primary arcs if the pulse time density is in a range characteristics of primary arcs.

Abstract: A dimming circuit (10) for use with an illuminated dual display (12) of an aircraft instrument is provided. A voltage divider (14) divides a variable supply voltage, V.sub.S, produces an input voltage, V.sub.IN, and applies V.sub.IN to one input of a switch (16). A threshold voltage circuit (15) receives the V.sub.S voltage and produces a reference voltage, V.sub.REF, and a lower magnitude supply voltage, V'.sub.S. A reference comparator (OA3) compares the V.sub.REF and V'.sub.S voltages and produces a threshold voltage, V.sub.T. The V.sub.T voltage is applied to a second input of the switch (16). A first comparator (OA1) in the switch (16) produces a low output voltage when the V.sub.T voltage exceeds the V.sub.IN voltage and causes a first lamp (L1) to brightly illuminate a first indicia (24). A second comparator (OA2) in the switch (16) produces a low output voltage when the V.sub.T voltage is less than the V.sub.IN voltage, which causes a second lamp (L2) to dimly illuminate a second indicia (26).