Industrial electrical work exposes people to shock, arc flash, arc blast and fire. An arc can release intense heat, light and pressure in a fraction of a second, and the resulting hazard is not limited to the electrical contact itself. The only reliable protection is to de-energise, isolate, lock, tag and prove dead before work begins.
Lockout-tagout sequence
Lockout-tagout is a controlled procedure, not a single action. It identifies every energy source, isolates it, applies a personal lock and tag, releases stored energy and verifies the absence of voltage. Only the person who applied a lock should remove it, under the site procedure.
- Identify all sources, including control supplies, backfeeds and stored energy.
- Notify affected people before isolation.
- Isolate, lock and tag each source with a personal lock.
- Release stored energy: capacitors, springs, elevated loads and pressurised systems.
- Prove the tester on a known source, test the conductors, then re-prove the tester.
- Verify the isolation at the point of work before touching anything.
Stored energy in a DC-bus capacitor
Problem. A 1000 µF capacitor is charged to 400 V. What energy does it store?
Solution. E = ½ × 1000 × 10⁻⁶ × 400² = 0.5 × 1000 × 10⁻⁶ × 160 000 = 80 J. Energy of this order is hazardous and must be treated with the site’s stored-energy procedure.
Arc hazard awareness
Arc flash risk depends on the available fault current, the protection settings, the working distance and the equipment condition. Arc-rated clothing and a formal risk assessment are used where the assessment shows an arc hazard; the required boundaries and PPE categories are defined by the site’s electrical safety programme and local regulation.
- De-energise, isolate, lock, tag and prove dead.
- Stored energy must be released and verified.
- An arc flash is a heat, light and pressure hazard.
- Arc boundaries and PPE are defined by assessment and local rules.
Worked example
Before working on a motor feeder, the isolator is opened, locked and tagged, the control supply is isolated, the drive’s DC-bus energy is allowed to discharge per the manufacturer’s instructions, and the feeder is tested dead at the point of work with a proved tester.