Test before you touch is trade practice from NFPA 70E. It is not an NEC rule.
Check the tester on a known live source before the test and again after it. The first check proves the tester works. The second proves it did not fail during the test.
A tester can read zero on a live circuit with no warning. A broken wire inside a lead does it, and so does a dial left on the wrong setting. In this picture, the wire inside the black lead breaks.
The known live source here is a receptacle on breaker 2. Many electricians carry a small proving unit for this check instead.
Test every receptacle and wire you might touch. One box can hold more than one circuit. After the slots read zero, take off the cover. Pull the receptacles out by their straps without touching a wire. Then test every wire in the box to ground. This is NFPA 70E trade practice, not an NEC rule.
Panel labels can be wrong. The test tells you what is live, not the label.
A shared neutral carries current for two circuits. With one breaker off, it still carries the other circuit's current. If you open it, you can end up in that circuit's path.
In the shared neutral setup, the red wire from breaker 7 runs live through this box. It is spliced behind the receptacles. A test at the receptacle slots cannot reach it. With the heater running, the 1.2 volts from neutral to ground is a clue. With the heater off, only the test inside the box finds it.
The box is metal, so a ground pigtail runs to its green screw, 250.148(C).
Breakers on a shared neutral must turn off together, 210.4(B). A handle tie or a two-pole breaker does that. Here the white is spliced, with a short pigtail to each receptacle, so pulling a receptacle does not open it, 300.15(B).
The heater on breaker 7 is rated 1,500 watts at 120 volts, so it acts like 9.6 ohms. Its cable runs 50 feet to this box and 25 feet more to the heater.
12 AWG copper has 1.93 ohms per 1,000 feet, from Chapter 9 Table 8. So the heater draws 12.1 amps. That current loses 1.2 volts in the 50 feet of white wire to this box, and that is what neutral to ground reads.
The model treats a wire that is off as zero volts. A real digital meter can show a small ghost voltage on a wire that is off. Any reading that is not zero needs an answer before you touch.
Locking and tagging the breaker is NFPA 70E practice. Use your own lock and your own tag. Where the NEC calls for a disconnect that can be locked, 110.25 covers how it locks in the off position.
Testing a live circuit is a trained task. Wear rubber insulating gloves with leather protectors and safety glasses. Add arc-rated clothing and a face shield when the panel's arc flash label calls for them. This is NFPA 70E practice, not an NEC rule.
Use a meter rated CAT III 600 V or higher. The CAT rating comes from the IEC 61010 product standard, not the NEC. The red lead plugs into the meter's V jack and the black lead into COM.
The panel cover stays on. You turn off and lock a breaker without opening the panel.
The cables are 12 AWG copper on 20 amp breakers. Cable jacket colors show the wire size by maker custom, not by code. Yellow is 12 AWG. The wall is drawn cut open to show the cables.
The dots show how much current flows. They do not show how fast electrons move. Real electrons drift far slower than a snail, a fraction of a millimeter per second in house wiring.
Wall power is AC. It runs 60 cycles a second, so it reverses 120 times a second. That is too fast to see. The dots show the heater current going out on the red and back on the white.
Depending on the room, these receptacles may also need AFCI or GFCI protection. The breakers are drawn plain to keep the picture simple.