Disconnect before carrying out maintenance or repair
Instructs the reader to disconnect before carrying out maintenance or repair.
Understanding this sign
Blue circular signs communicate an action, such as wearing eye protection or following a specified procedure.
A pictogram does not select the grade of PPE or explain a complete procedure. Use the task-specific instructions.
What ISO 7010 M021 Requires
M021 orders that a machine be disconnected from all sources of power before maintenance or repair begins. The ISO register wording is precise about its scope: it applies to machines and equipment that are not connected to the mains by a plug — in other words, hard-wired installations where isolation means operating a disconnector, breaker, or valve rather than pulling a cord. For plug-connected equipment, the companion sign M006 applies instead.
The phrase all sources of power carries real weight. Industrial machines routinely combine electrical supply with compressed air, hydraulics, steam, and stored mechanical energy, and a fitter who kills the electrical feed but leaves a charged pneumatic line has not made the machine safe. M021 posted at the machine is the standing reminder that isolation is complete only when every energy path into the equipment has been cut and, where relevant, residual energy has been released.
The Signage Face of Lockout/Tagout
M021 is best understood as the visible tip of an energy-isolation program. In the United States that program is defined by OSHA's control of hazardous energy standard, 29 CFR 1910.147, which requires documented procedures, personal locks and tags, and verification that the equipment is actually de-energized before work starts. European workplaces reach the same practice through work-equipment and machinery safety obligations. The sign does not create the procedure; it points workers at the moment the procedure must be invoked.
A common audit finding is exactly this gap: M021 stickers on every panel, but no written isolation procedure and no locks issued. The reverse failure also happens — a mature lockout program whose isolation points are unmarked, so contractors cannot find the right disconnect. Effective facilities close both ends: procedures on file, and M021 plus equipment-specific isolation-point labels at the hardware itself.
Where to Post It
Place M021 at the locations where the maintenance decision is made and where isolation is executed: machine access doors and guard panels that technicians open, local disconnect switches and isolators, motor control centers, and the covers of terminal boxes. On packaging lines and conveyors, it belongs at each maintenance access point rather than once per line, because the person clearing a blockage mid-line will not walk back to read a sign at the head end.
Original equipment manufacturers also apply M021 directly to machines as part of their information-for-use obligations, particularly on equipment with remote or automatic starting, since the person servicing the machine years later may know nothing of its control architecture. For such machines, pairing matters: M021 alongside W018 (warning: automatic start-up) tells the technician both that the machine can begin moving without any local command and what to do about it before opening a guard.
Companion Signs in an Isolation Scheme
Three ISO 7010 signs form the core signage of an energy-isolation scheme. M021 commands disconnection before work. P031 — do not alter the state of the switch — is hung or posted at the isolator once it is locked open, prohibiting anyone from re-energizing while work proceeds. W018 warns that the equipment can start automatically, explaining why isolation is non-negotiable. Together they cover the before, during, and why of a lockout.
M021 should not be substituted where a more specific instruction exists. Cord-and-plug appliances take M006, since pulling the plug is the whole isolation. Machinery whose hazard is stored mechanical movement rather than powered movement calls for M068, which mandates mechanically locking or blocking moving parts against gravity and spring energy. Matching the sign to the energy type keeps each instruction actionable — a technician told exactly which step applies is far more likely to perform it than one shown a generic disconnect symbol.
Where you will see M021
It is posted at machine access doors and guard panels that technicians open, local disconnect switches and isolators, motor control centers, and terminal box covers; on packaging lines and conveyors it belongs at each maintenance access point rather than once per line. Equipment manufacturers also apply it directly to machines with remote or automatic starting, often paired with W018 warning of automatic start-up.
Common questions about M021
Is the M021 sign a legal substitute for a lockout/tagout procedure?
No. Signage alone does not satisfy energy-isolation requirements such as OSHA 29 CFR 1910.147, which demand documented machine-specific procedures, application of locks and tags by each worker, and verification of de-energization. M021 marks where and when the procedure applies; the procedure itself must exist separately.
What is the difference between M021 and M006?
M021 applies to hard-wired machines and equipment with no mains plug, where disconnection means operating isolators and typically locking them out. M006 applies to cord-and-plug equipment, where pulling the plug and keeping it under your control provides the isolation. The ISO register wording of M021 explicitly limits it to equipment not connected by a plug.
Which signs should be posted with M021 on machines that restart automatically?
Pair it with W018, the automatic start-up warning, so technicians understand the machine can begin moving without any local command. Once isolation is applied, P031 (do not alter the state of the switch) at the locked disconnect prohibits anyone from re-energizing the circuit while work is in progress.
Does disconnect before maintenance cover compressed air and hydraulics too?
Yes. The M021 referent requires disconnection from all sources of power, not just electricity. On machines with pneumatic or hydraulic supplies, isolation includes closing and locking the supply valves and venting or releasing trapped pressure, since a charged actuator can move violently even with the electrical feed dead.
Reference details
| Symbol code | M021 |
|---|---|
| Sign family | ISO Mandatory Signs |
| Standard reference | ISO 7010:2019 / ISO 3864-1 ISO Online Browsing Platform: M021 |
| Screen color reference | #0000FF. Screen colors are not a production color specification. |
| Download formats | SVG reference artwork; a 1024 × 1024 pixel PNG conversion on a white canvas. |
| Record date | · How to interpret our dates |
| Page updated |
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