Testing What You Wired: Continuity, Polarity, Then Power

Set the meter to continuity and check every connection end to end. It should beep from the pin at one end to the pin at the other, and the reading should be a fraction of an ohm. A reading of a few ohms on a short wire means a bad joint — a poor crimp, a cold solder joint, or a screw terminal that is not actually gripping. It will work today and warm up later, so chase it now. Wiggle each joint while the probes are on it. An intermittent connection will break the beep, and finding that on the bench is a completely different proposition from finding it in a finished assembly.

Now check between things that must not be connected: supply to ground, each signal to its neighbours, and everything to the metal chassis. This is the test that catches a whisker of stray copper bridging two terminals, a screw pinching a wire against the case, or a conductor pushed too far into a terminal block so it touches the one beside it. None of those is visible. Do it before every first power-up. It takes two minutes and it is the difference between a mistake you correct and a mistake that releases the smoke — the exact situation the fault-finding rung then has to deal with.

Check polarity at the load end, not at the supply. Everyone gets the supply right and it is the swap somewhere in the middle that does the damage. For the first power-up, use a bench supply with the current limit turned down to just above what the circuit should draw. If something is wrong, the supply limits instead of the wiring burning — and the current limit light coming on tells you immediately. With no bench supply, a fuse of the right size does a cruder version of the same job, and an inline ammeter tells you whether the current drawn is what you predicted. A circuit drawing far less than expected is as informative as one drawing far more.

With power on and everything working, touch each connection. Anything noticeably warm is a joint with resistance it should not have, and it is only going to get worse. Warmth at a crimp means a poor crimp. Warmth at a screw terminal means it needs tightening — or that stranded wire went in without a ferrule and has crept. Warmth along a wire means the wire is undersized for the current, which the wire rung sizes properly. Check again after an hour of running. Some faults only appear once everything has reached temperature, and an hour on the bench is much cheaper than discovering it in service.