Troubleshooting: dead circuits, tripping breakers, flicker and the loose neutral, voltage drop, and multiwire branch circuits
In one paragraph
Five complaints make up most troubleshooting calls, and each has a tree: a dead circuit (find where the voltage stops), a tripping breaker (overload, short, ground fault, arc fault or a failed breaker, and each has a different test), flicker (almost always a loose or open neutral, and dangerous when it is the service neutral), voltage drop (a long run or a bad connection, measured under load), and the multiwire branch circuit with a lost neutral that puts 240 volts across 120-volt equipment. The lesson gives the test order for each, the code sections that decide the fix, and the one rule under all of it: voltage testing is done live with the safeguards the safety orders require, and everything else is done on a locked and tagged circuit.
The rule
Troubleshooting is the diagnostic method applied to a symptom: voltage testing is the one step performed energized, under the conditions of 8 CCR 2320.2(a), and every continuity, resistance and repair step follows a lock and tag under 2320.4(a). The code decides the fix: a multiwire branch circuit must have a means to disconnect all ungrounded conductors simultaneously and its conductors grouped at the panel (2025 CEC 210.4(B), (D)); a neutral may not depend on a device's terminals for continuity (300.13(B)); a modified circuit in an AFCI area gets AFCI protection (210.12(E)); and the code's informational guidance treats 3 percent branch-circuit and 5 percent total voltage drop as the design target (210.19 Informational Note).
Why it matters
Troubleshooting is where a new contractor either earns the customer for life or replaces three breakers and leaves the problem in the wall. The trees below are how experienced electricians think without knowing they are doing it, and the loose-neutral and multiwire sections are about the two faults that burn houses and kill electronics while looking like a nuisance.