On BAC 1-11 production a very long time ago, power was first applied with all CBs pulled. Then each CB was pushed in one at a time to a schedule and the systems checked for correct operation before progressing to the next one.
In 1976 I did a radio re-fit on a Viscount and checked the supply on the connector to each box before fitting the box and turning on. After about 6 boxes were powered up we got a burning smell, so quickly turned the power off. On inspecting the radio rack by the aircraft door we realised the burning smell was from the first of many forest fires around Bournemouth that long hot summer. Panic over!