Electrical safety and lockout/tagout

A control panel holds more than enough energy to kill a person, and most accidents are caused not by a lack of knowledge but by haste: «it will only take a minute», «I have already switched it off», «it surely is not live». The five safety rules (often called the five golden rules) and lockout/tagout exist so that working dead does not depend on memory or luck. They are few, they are learned in a day and they protect a whole working life.

What current does to the body

What causes harm is not the voltage itself but the current flowing through the body and how long it lasts. At supply frequency, roughly:

Besides current through the body there is the arc flash: an accidental short circuit, for example with a tool, can produce an explosion of heat and light that causes severe burns without anyone touching a conductor. That is why a short circuit in a power panel is as dangerous as direct contact.

The rules behind it

Every country has its own law, but the ideas are the same everywhere. In Europe, the standard EN 50110-1 on the operation of electrical installations sets out the five safety rules, and each country's regulations build on it. In the United States, OSHA requires the control of hazardous energy (lockout/tagout, 29 CFR 1910.147) and safe electrical work practices, and NFPA 70E details how to apply them. The common principles:

• As a rule, work is done dead (de-energised), applying the five rules.
• Live work is the exception: only qualified (skilled) persons may do it, with a written procedure, specific training, a risk assessment and specific equipment. It is never improvised.
• Switching, measuring, testing and inspection, such as measuring with a multimeter, have their own conditions and also require training.
• Companies must decide who is authorised to do each kind of work, and only people with the right knowledge and training may be.

The five safety rules

1. Disconnect

Open every device that can feed the part you will work on, with an effective break: an isolator (disconnect switch), a switch with a reliable position indicator, or fuses removed. Every source: besides the main supply there may be a UPS, a second incomer, a generator, a control supply coming from another panel, or capacitors still charged.

2. Secure against reconnection: lock out and tag out

Make sure nobody can switch back on while you work. The method is lockout/tagout: a personal padlock on the isolator or on a lockout device, and a tag with your name, the date and the reason.

Lockout rules that are not negotiable:

• Everyone who works fits their own padlock, even if they are the last to arrive.
• Only the person who fitted a padlock removes it. Nobody removes someone else's lock «because they must have finished».
• A switch with no physical lock, only a sign, is not locked out: a sign does not stop someone operating it without seeing it.

3. Verify the absence of voltage

Never assume there is no voltage because «it has been switched off». Test at the point of work, on every live conductor and also between them and earth, with suitable equipment: a voltage tester or a multimeter of the right measurement category.

And the procedure is always the same: test the tester on a known live source, test the point of work, then test the tester again (live-dead-live). If the tester failed along the way, a «0 V» reading means nothing.

4. Earth and short-circuit

The conductors at the point of work are connected together and to earth with equipment designed for it. That way, if someone switched on by mistake or voltage appeared by induction, the current would take that path and trip the protection instead of passing through the person. It is always required at high voltage; at low voltage, when the installation could become live accidentally, for example through induction or backfeed.

5. Protect against adjacent live parts and mark the area

If live parts remain near the point of work, cover them with insulating screens or shrouds, and mark out and sign the work area. Standards set approach distances that depend on the voltage (NFPA 70E calls them limited and restricted approach boundaries, and adds an arc flash boundary); follow the ones that apply where you work.

Re-energising

Restoring power is done in reverse order and with the same care: finish the work, warn everyone and check nobody is left in the area; remove the tools and the earthing equipment; each person removes their own padlock; and only then switch on.

Equipment and tools

• Insulated tools rated 1000 V, marked with their symbol and in good condition.
• Insulating gloves of the class suited to the voltage, inspected before use.
• Face protection and clothing suited to the arc flash risk when working near live parts.
• Voltage tester or multimeter with the right measurement category for where you measure.
• Lockout kit: personal padlocks, hasps for several locks, breaker lockout devices and tags.

24 V deserves respect too

24 V control circuits are not usually dangerous to touch, but the same panel has 230 V and 400 V (or 120 V and 480 V) a few centimetres away. Working «on the control side» with the panel live is working near live parts. And a short circuit on a powerful 24 V supply can melt cables and cause burns.

Common mistakes

• Trusting that «it is already off» without verifying.
• Testing with a device without checking it before and after.
• Locking out with only a sign, no padlock.
• Using a single padlock for a team of several people.
• Removing a colleague's padlock.
• Forgetting other sources: UPS, control supply from another panel, capacitors, compressed air.
• Using uninsulated tools near live parts.

Summary

• As a rule, work is done dead, following the five safety rules.
• Disconnect every source, lock out with padlock and tag, verify absence of voltage, earth and short-circuit, and protect and mark nearby live parts.
• Each person fits their own padlock and only they remove it.
• Always verify, testing the tester before and after.
• To re-energise, undo the steps in reverse order.