The Power Supply vs the Load: A Decision Tree

Why this matters

A dead or weak device has two possible stories: the device failed, or what powers it failed. They look the same from the outside, and guessing wrong is expensive. You replace a good motor when the supply was low, and the new motor underperforms too. You chase a supply problem when the device shorted. The split is the same in every trade where something draws power or pressure: separate the supply (what feeds it) from the load (what uses it). This tree keeps you from condemning either side without proof.

Safety first: prove it dead before you open it

Most of this work touches energized circuits, so set up safely before anything else. De-energize the circuit at its source, lock it out where required, and verify dead with a meter you have just proven on a known live source. Assume every conductor is live until you have measured it dead. When a reading genuinely requires the circuit energized, plan the measurement, keep one hand clear, use rated tools, and re-de-energize before making or breaking connections. A reading is never worth a shock.

Start here: is the supply present and correct at the load?

The whole tree turns on one measurement: what is actually arriving at the load's input terminals.

• Measure the supply right at the device, under its real operating condition. A supply can read fine with nothing connected and sag the moment the load tries to draw. Measure where it matters, at the load, ideally while it attempts to run.
• If the supply is absent or out of spec at the load, the problem is upstream. Stop testing the device. Go trace the supply.
• If the supply is present and correct at the load and the load still does nothing, the problem is the load. Now the device is fair game.

That single comparison, correct supply versus device response, splits supply faults from load faults cleanly.

Branch 1: the supply is the problem

When the input at the load is wrong, the fault lives between the source and that point.

• No supply at all. Trace back toward the source and find where it reappears; the break is between the last good point and the first dead one. Suspect an open: a tripped breaker, a blown fuse, an open switch, a loose or burned connection, a broken conductor. A protective device that opened did so for a reason; understand the cause before resetting.
• Supply present but low or weak. The load gets some input, just not enough, so it runs sluggish or strains. Look for a loose or corroded connection adding resistance, an undersized or overloaded source, or a brownout condition. Voltage that drops sharply under load is a classic weak-supply tell.
• Supply present but wrong. Reversed polarity, the wrong leg, a crossed connection, a missing neutral or reference. The load receives a valid-looking but incorrect input and responds wrong.

Branch 2: the load is the problem

When the supply at the load is proven correct and the device still misbehaves, the device is the fault.

• Correct supply, no response. Good input arrives and the device sits dead. Suspect an open inside the load: a burned winding, an open element, a failed internal connection. A load that draws nothing with correct supply is open internally.
• Correct supply that collapses when connected. Hook up the load and the supply crashes or the protection trips. That is a shorted or grounded load pulling far more than it should. A load that drags the supply down or trips protection is shorted or grounded. Isolate and test the device by itself.
• Correct supply, weak response. The device runs but underperforms on good input. Suspect a partially shorted winding, a tired starting component, a binding mechanism adding drag. Confirm by measuring the device's draw and resistance against spec.

The deciding test: measure at the load under load

Take the supply reading at the device's own terminals, ideally while it tries to operate, and the table points to the side at fault. The key habit is measuring under load, not at rest, because a no-load reading hides the very weakness you are hunting.

Working it cleanly

Confirm the supply at the load before condemning the load, and isolate the load before condemning the supply. Never replace a device on a hunch when a two-minute reading at its terminals would settle it. Re-energize only with everything buttoned up, and document which side failed and the reading that proved it.

References

• See related: Upstream Problem, Downstream Symptom: A Decision Tree
• See related: Proving a Circuit Dead the Right Way
• NFPA 70E (electrical safety, de-energize and verify dead before work)
• Manufacturer documentation for required supply specs and device resistance/draw values