A meter placed across a single open fuse in an otherwise complete, energized circuit can display supply voltage even though the load is stopped. Across a sound closed contact carrying current, the drop should be small. The distinction is about where the probes sit and whether a complete path exists, not a magic voltage that identifies every failed part.
Why does the open point show voltage?
Imagine a source, one fuse and a motor in series. If the fuse is the only open point, one side remains connected to the source and the other side has a path through the load to the return. Nearly the full supply potential can therefore appear across the break while essentially no load current flows. Fluke's HVAC electrical-testing note explicitly limits this interpretation to a circuit complete except for that one open fuse or switch.
A zero reading across the fuse does not, by itself, prove the fuse is good: an absent supply, another open point or an unsuitable reference can change the result. In a study sketch, mark both probe points and trace the entire return path before naming the fault.
What changes when the contact is closed?
When a contact closes and carries load current, a healthy low-resistance path has only a small voltage drop across it. Fluke says voltage drop across motor contactors or relays should be negligible and suggests comparing line-side and load-side phase voltages. If a closed contact under load has an unexpectedly large drop, the contact or connection may be resistive; interpret the reading against the equipment documentation rather than a universal threshold.
Contrast this with the open fuse: the full voltage appears at the break because current cannot pass, whereas a resistive closed point drops some voltage while current flows. For related motor-control reasoning, see overload relay versus contactor and why an ohmmeter can mislead in circuit.
How can you practise the reasoning safely?
Draw a source, protective device, control contact and load. For each proposed meter placement, label what is energized, which contacts are open and whether current has a return path. Then ask: “Am I measuring across one component, or between a point and a reference?” Change one condition at a time and predict the direction of the reading.
This is a paper exercise, not instructions to probe live equipment. Energized testing requires an authorized, qualified worker, a suitable instrument and the equipment-specific safe-work procedure. For service work, CCOHS explains hazardous-energy isolation and verification.
Where does this fit in 313A preparation?
It builds the habit of tracing a circuit before interpreting a meter reading. Use the Ontario 313A exam guide and the current-standard note to place the concept in the applicable exam outline, without treating this example as a recalled exam question. The free sample is original, unofficial draft practice awaiting certified-mechanic review; it is not a pass prediction.
Sources
- Fluke: HVAC applications for electrical testersPages 2 and 4, contactor voltage drop and single-open-point fuse example; checked 2026-10-03
- CCOHS: Lockout/Tag outAuthorized hazardous-energy isolation and verification; checked 2026-10-03
- Red Seal: Refrigeration and Air Conditioning MechanicCurrent-exam standard status; checked 2026-10-03
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.