Rule 36 of 40 · Chapter VI — Tools, Habits, and Judgement
Change one thing at a time
Why this rule exists
When you alter several things before testing, and something goes wrong, you cannot tell which change did it. In electrical work that ambiguity is dangerous: you might energize a mistake, chase the wrong fault, or fix something by accident and never understand your own circuit. Methodical one-change-at-a-time work keeps a clear cause-and-effect thread, so every result is traceable and every fault is bounded to what you just touched. It is slower per step and faster overall, because you never unwind a tangle of simultaneous changes to find which one is trying to hurt you.
In practice
Work in small, verifiable steps: make one change, de-energize and verify as needed, test the result, then move on. Label wires before you disconnect them and photograph the existing arrangement so you can retrace it. When troubleshooting, change one variable and re-measure before changing another. Keep notes on what you have altered, especially across a multi-visit job. Restore and verify each connection as you complete it rather than leaving a pile of half-done work to sort out under time pressure later.
When it doesn't apply
Some tasks are genuinely atomic and must be done as a set, but those are fewer than impatience suggests. Emergency shutdowns are the opposite case: kill everything at once, sort it out after. The principle applies to construction and troubleshooting; it is not a reason to hesitate on cutting power when cutting power is the answer.