Step 00a.3 — Buy a meter that is rated for the job¶
Step 3 of 12 · Chapter 00a — Mains safety: read before Chapter 09
(no image — see text)
Parts: none.
Do: Buy a meter with CAT III 600 V printed on its face, and leads rated at least the same category. CAT III leaves margin if you ever probe at a panel.
Check
Meter on the bench with a printed category rating, leads rated to at least that category, fresh battery, and all four functions tried once.
What you're looking at
No parts: the meter you will buy. A digital multimeter measures continuity, resistance, DC volts and AC volts. The CAT rating on its face says how large a surge it can survive where you use it, which is not what voltage it can display.
A meter's CAT rating says what size transient it can survive where you are using it, not what voltage it can display; a meter with no category rating, or the wrong one, can fail violently. Fluke's definitions: CAT II is "Single-phase receptacle-connected loads such as appliances and portable tools"; CAT III is "3-phase distribution including single-phase commercial lighting and equipment in fixed locations such as switchgear and polyphase motors." This printer is a receptacle-connected appliance, so CAT II is the category; CAT III is simply the common rating on decent meters. The four functions in the table are the ones Ch 10's Checkpoint #1 asks for.
| Function | Why this build needs it | Used at |
|---|---|---|
| Continuity / beeper | every Checkpoint #1 measurement | 10.17–10.21 |
| Resistance (Ω) | bed heater, thermistors, PE bonding, 24 V shorts | 10.13, 10.74, 10.76, 10.77 |
| DC volts | confirm dead — 0 V across the PSU's +V/−V, held ten seconds, before the DC side is touched; never a live reading | 10.24, 10.73 |
| AC volts, 600 V range | the meter self-test in 00a.4; never used inside the machine | this chapter only |
Two details worth getting right. Resistance range: Ch 10's tool list asks for "at least 2 MΩ", but 10.13 wants to see the bed heater read "well above 10 MΩ" to earth, and 10.76 reads thermistors up to 162 kΩ. A 2 MΩ meter answers the pass/fail (anything higher shows OL, which is the pass), but a meter that ranges to 20 MΩ or more turns a pass into a number you can write in the log. Leads: they carry their own CAT rating and it is the lower of meter and lead that governs. Do not put unrated leads on a rated meter.
Source: Fluke — IEC category ratings · Ch 00 Step 00.8 · Ch 10 — Tools