The multimeter worth owning is rated CAT III 600V

The one

A handheld digital multimeter, auto-ranging, with CAT III 600V printed on the case and listed on the maker's spec sheet. Palm-sized, with a fused current input and probes that have finger guards.

The spec profile:

• Safety rating: CAT III 600V, stated by the maker. Look for a mark from an independent safety lab on the meter too (UL, CSA, ETL and similar).
• Ranging: auto-ranging. You turn the dial to AC volts and read the number.
• Functions you'll use: AC volts, DC volts, resistance, and a continuity setting that beeps.
• Fusing: the amps input should be protected by a fuse, ideally the ceramic kind. The maker lists the fuse type, and you replace it with exactly that.
• Leads: thick, flexible insulation, finger guards, and probe tips that are shrouded or have a cap you can slide on.
• Small extras worth having: backlight, data-hold button, auto power-off.

That's the whole list. Nothing on it is exotic.

Why this one

The CAT rating is the part that matters. It tells you where in a building the maker designed the meter to be used, and how big a voltage spike it's built to survive. In plain terms, CAT II covers things you plug in. CAT III covers the fixed wiring behind your walls, your breaker panel and your branch circuits. CAT IV is the service entrance, where the utility's line meets your house.

You'll mostly stay at outlets and switches. But a dead outlet leads to a breaker, and a breaker leads to a panel. A meter rated for that is the one you want in your hand.

A voltage number with no CAT category next to it tells you very little. A meter that says "1000V" and nothing else could be a serious tool or a toy. Say the rated one costs $70 and the unrated one costs $20. That's an example, not a quote. The difference buys you a meter whose maker will stand behind the claim on the case.

Auto-ranging cuts down on mistakes. Fewer settings means fewer wrong ones.

Now the checks, in the order you'd actually do them.

First, prove the meter. Before you trust a "zero volts" reading, test the meter on something you know is live, like a working outlet. Test the suspect circuit. Then test the known-live source again. Pros call it live, dead, live. A meter with a dead battery or a bad lead reads zero all day and lets you believe it.

Dead outlet. Press reset on any GFCI outlet in the house first. Many a "dead" kitchen outlet is just a tripped one in the garage. Check the breaker next. Then set the meter to AC volts, hold the probes by the insulated grips with one hand, and touch one to each slot. A standard U.S. outlet should read around 120 volts. Low or zero with the breaker on is information, not a repair. It's also where this piece stops being a DIY guide, which we'll get to.

Switches, cords and fuses. Shut off the power at the breaker first. Then use continuity. Touch the probes to each end of a lamp cord, an appliance cord or a fuse. A beep means the path is good. Silence means it's broken. You never test continuity or resistance on a live circuit. That one mistake is how meters get ruined and people get hurt.

Low-voltage stuff. This is the safest use of the tool. A car or mower battery, a doorbell transformer, the transformer that runs your thermostat. If your furnace won't fire, the first three checks start well before you open a panel. A meter on the thermostat wires tells you whether the problem is the control side or the heat side. A cordless mower with an electric start can be checked the same way, and so can the battery on any mower that won't start. A healthy 12-volt lead-acid battery sits at roughly 12.6 volts at rest.

When to stop and call a licensed electrician. Anything inside the panel. Anything at the service entrance. 240-volt circuits like a dryer or range. Breakers that trip repeatedly. A burning smell, a warm outlet, scorch marks, flickering across several rooms. Aluminum wiring. Any reading that doesn't make sense to you. A meter tells you what's happening. It doesn't make you the person who should be working on it.

Codes and permit rules vary by state, county and city. In many places, replacing fixed wiring needs a permit or a licensed electrician. Call your local building department before you start anything bigger than swapping a device.

What to skip

• A big voltage number with no CAT rating. See above.
• Bluetooth and app logging. Fine for a lab. Not what you need while standing in a basement with a flashlight in your teeth.
• Meters with forty functions. Capacitance, frequency, temperature, diode test. You'll never turn that dial past voltage and continuity, and every extra position is one more place to land by accident.
• Manual-ranging only. Cheaper, and slower, and easier to misread. Skip it unless you already know how to use one.
• Paying extra for true-RMS to check outlet voltage. It matters for certain electronic loads. For a wall outlet, it's not the reason you're buying.
• Fifty-piece accessory kits. You need good leads. Alligator clips are the only extra worth a look.
• Extended warranties. Read the maker's own warranty instead.

If your situation is different

You work on furnaces or air conditioners. Then a meter with capacitance and a microamp range earns its place. Capacitors can hold a charge after the power's off, so learn to discharge them properly or leave that to a pro.

You want a quick "is this wire live?" answer. A non-contact voltage tester is a cheap pen-style tool and handy as a second opinion. But it can give false alarms or miss shielded wiring. It's never a replacement for a meter, and never your only check before touching a wire.

You only ever test batteries. Almost any meter will do. The CAT rating matters because of what you might do next, though. You'll be glad it's there the day you test an outlet.

You're nervous. Good. It means you're paying attention. Start on batteries and cords with the power off, and hire a pro for anything past that. Nobody gets a medal for doing it themselves.

Before you buy

• CAT III 600V is printed on the meter itself, not only on the box.
• The maker's spec sheet lists the same rating and the type of fuse.
• An independent safety lab mark appears on the meter.
• Probes have finger guards and tip shields.
• Read the one-year-later owner reviews and look for complaints about the dial, the leads and fuse access.
• Read the actual warranty document: length, whether registration is required, who pays shipping.
• Buy from a seller that sells the maker's replacement leads and fuses.
• Check the current price.

When you lend it out, put in a fresh battery first.