What is the first step when you arrive at a machine that “won’t run”?

B. Get the symptom from the operator and see it for yourself, if it is safe to do so

A. Replace the most likely part

D. Reset every overload in the panel

Across an OPEN contact in an energized series circuit (with everything else closed), what does a voltmeter read?

B. About the full supply voltage

D. It depends on the wire colour

You have replaced a failed part and the machine runs. What is the last step?

B. Prove the fix — run it through its normal cycle — and find out why the part failed

C. Remove the lockout lock without telling anyone

D. Nothing; replacing the part was the fix

Why check your meter on a known live source before AND after testing for dead?

B. To prove the meter was working, so a zero reading really means zero

Rappel pour compagnons Refresher 5 of 10 · about 8 min

The troubleshooting method

What you’ll learn

The six-step loop every good troubleshooter runs, whatever the equipment.
Half-splitting, and why it beats walking a circuit end to end.
What a voltmeter reads across an open and a closed contact.
Why “prove the fix” and “why did it fail?” are part of the job.

The loop. If the fix does not prove out, you go back to the symptom — it may have changed, or there was more than one fault.

The six steps

Get the symptom. Ask the operator what it does and does not do, when it started, and what changed. See it yourself if it is safe.
Find known-good. Where does it still work? Supply present, control power on, other machines on the same feeder fine. Everything that works is off the suspect list.
Half-split. Measure in the middle of what is left. Good there? The fault is downstream. Bad? Upstream. Repeat.
Prove the fault. Before you change anything, prove the part is bad — voltage across it where there should be none, or a resistance test with it isolated and locked out.
Fix it. Locked out. Replace like with like, torque the connections, and restore anything you disturbed.
Prove the fix, then ask why. Run the machine through its normal cycle. Then find out why the part failed, or you will be back.

Worked: a coil that will not pull in

120 V control circuit: fuse, stop button, start button, three door interlocks, overload contact, then the coil. Start is held in. Coil is dead.
Known good: 120 V between X1 and X2 at the control transformer. The supply is fine.
Half-split: measure from X2 to the wire after the second door interlock. 0 V — the open is upstream of that point.
Split again: X2 to the wire after the stop button reads 120 V. The open is between the stop button and the second interlock — the start button or the first two interlocks.
Measure across each in turn: 0 V across the start button (closed), 120 V across interlock 1. That is the open. Four readings found it; walking device by device from one end could have taken seven or eight.

Sur le chantier

Tips

Use the drawings. Number the test points before you start, then split by number.
A voltage reading across a device: near zero means closed (or no current flowing anywhere), full supply means that device is the open.
Fault logs on PLCs and drives are free evidence. Read them before you open a door.

Common mistakes

Swapping parts until it works — the “shotgun” — and never knowing what was wrong.
Trusting a zero reading without proving the meter on a live source first.
Fixing the symptom (a burnt contact) and not the cause (an overloaded motor).
Skipping the lockout because “it’s only a quick check”.

Check yourself

0 right of 0 answered 5 questions · pick one answer each

1.A control circuit has 16 points you could test in series between the supply and a coil that will not pull in. With half-splitting, how many readings does it take, at most, to find a single open?
2.What is the first step when you arrive at a machine that “won’t run”?
3.Across an OPEN contact in an energized series circuit (with everything else closed), what does a voltmeter read?
4.You have replaced a failed part and the machine runs. What is the last step?
5.Why check your meter on a known live source before AND after testing for dead?

Answers

A. About 4 — Each reading halves what is left: 16 → 8 → 4 → 2 → 1. Four readings (log₂ 16). Walking point by point from one end could take all 16.
B. Get the symptom from the operator and see it for yourself, if it is safe to do so — Troubleshooting starts with an accurate symptom: what it does, what it does not do, when it started, what changed. Seeing it yourself often halves the search before a meter comes out.
B. About the full supply voltage — With the circuit open at that point, no current flows, so nothing else drops any voltage — all of it appears across the gap. A closed, good contact reads close to zero.
B. Prove the fix — run it through its normal cycle — and find out why the part failed — Prove the repair under normal operation, and ask why it failed: a burnt contact may be a symptom of an overloaded motor, and fixing only the contact brings you back next week.
B. To prove the meter was working, so a zero reading really means zero — Live-dead-live: a meter with a blown fuse, a broken lead or a flat battery also reads zero. Proving it on a known source before and after is what makes “dead” trustworthy.

Questions fréquentes

Should I troubleshoot live or dead?

As much as possible, dead. Some faults can only be found with the circuit energized — reading voltage across an open contact, for example — and that work falls under your employer’s energized-work rules (CSA Z462 in Canada, NFPA 70E in the US): the right PPE, the right meter, and a reason it cannot be done de-energized. Anything you can do with it locked out, do locked out.

What is half-splitting?

Measuring in the middle of the suspect section instead of at one end. The reading tells you which half the fault is in, and you split that half again. It finds a single fault in a long chain in a handful of readings.

What if I cannot reproduce the fault?

Intermittent faults are the hardest. Record what you can — time, temperature, what else was running — check connections for heat and looseness, and look at what the fault would need to happen (vibration, a hot afternoon, a particular sequence). A PLC’s fault log or a drive’s fault history is often the best witness you have.

Voir aussi

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