Mike PleziaInside service technician, Clark Associates · Commercial Equipment, HVAC, and Refrigeration professional

Power Off First, Every Time #hvac #electricalsafety #tradetips

4 Sept 2026 · 1 min

Watch on YouTube

1:26

In this concise trades-tip monologue, the speaker walks new technicians through the fundamentals of measuring resistance with a multimeter, emphasizing the critical safety rule to always power off a circuit before testing. He explains the omega (Ω) symbol on the dial, how the meter injects a small test current, and why external voltage can corrupt readings or damage the meter. The speaker then details how to interpret resistance values: near‑zero ohms indicates a good conductor, "OL" (open loop) signals a broken path, and any resistance to the metal chassis reveals a grounded winding. Using a compressor as a practical example, he describes testing the three terminals (Common, Start, Run), identifying the highest reading (Start‑Run), confirming the common terminal by additive relationship, and spotting failed or grounded windings. He also shows a quick fuse test by probing both ends, distinguishing a good fuse (low resistance) from a blown one (OL). The overall message is to equip newcomers with safe, reliable resistance‑testing techniques they can pass on to others.

Chapters

Hey, are you new to the trades? Here's something you may not know about measuring resistance. On the dial, look for the omega symbol— the Greek letter omega. Before you touch a single probe to anything, power off first. Every time. No exceptions. Here's why: your meter sends its own small current through whatever you're testing. If there's still outside voltage in that circuit, it fights your meter. Bad reading, possibly a damaged meter. So what does resistance actually tell you? It tells you how much something opposes the flow of current. A good wire close to zero ohms. A broken path shows something different. OL means open. The path is broken. No connection at all. On a compressor, you'll test three terminals: common, start. Run. C, S, R. The highest reading you find is between start and run. Common is the terminal where the other 2 readings add up to that one. Any terminal reading OL? That winding failed. Any reading between a terminal and the bare metal shell? That's a grounded winding. Dangerous. Doesn't belong there. Same idea on a fuse. Pull it. Touch a probe to each end. What's the difference between a fuse and a relay? Low reading? Good fuse. OL? It's done. Now that you know, you can help someone else know.