Arc flash basics

An arc flash is an explosive release of energy when electric current jumps through the air between conductors or to ground. It can cause severe burns, blindness, hearing loss and injuries from the blast, and it can happen in an instant during routine work on energized equipment. The most effective protection is to put equipment in an electrically safe work condition before anyone works on it.

What happens in an arc flash

When an arc forms, the air becomes a conducting path and current flows through it, limited only by the electrical system and how quickly protective devices clear the fault. In that fraction of a second the arc can produce:

• Intense heat, hot enough to ignite clothing and cause deep burns at a distance
• A bright flash that can damage eyes
• A pressure wave (arc blast) that can throw workers and send out molten metal and fragments
• A loud report that can damage hearing
• Toxic gases and smoke from vaporized metal and insulation

Common causes

• Tools, test leads or hardware dropped or slipped across live parts
• Testing with the wrong meter or damaged leads
• Racking breakers in or out, or operating switches on equipment in poor condition
• Dust, moisture, corrosion and contamination creating a tracking path
• Animals and pests inside equipment
• Failed insulation and loose connections
• Removing or installing covers on energized equipment

CSA Z462 and the law

In Canada, CSA Z462, Workplace electrical safety, is the main standard for protecting workers from shock and arc flash. It sets out an electrical safety program, risk assessment, approach boundaries, safe work practices and PPE selection. Provincial occupational health and safety laws, including the Alberta OHS Code and the regulations in BC and Ontario, require employers to protect workers from electrical hazards, and CSA Z462 is widely used to meet that duty. Provincial electrical codes and licensing rules also control who may do electrical work. Check the current edition of Z462 and your province's requirements.

Key terms

Incident energy

The heat energy a worker could be exposed to at a set working distance during an arc flash, usually expressed in calories per square centimetre. It depends on available fault current, voltage, equipment type, working distance and how long the arc lasts before a protective device clears it.

Arc flash boundary

The distance from exposed live parts within which a person could receive a second-degree burn if an arc flash occurred. Anyone inside this boundary needs arc-rated PPE suited to the hazard.

Shock approach boundaries

Separate distances, the limited and restricted approach boundaries, control how close unqualified and qualified people may come to exposed energized parts because of shock risk.

Electrically safe work condition

The state in which equipment has been disconnected, locked out, tested for absence of voltage and grounded where needed. Working in this condition is the primary control in Z462.

Arc flash studies and labels

An arc flash risk assessment, often called an arc flash study, is an engineering analysis of the electrical system. It models the system from the utility through each piece of distribution equipment, calculates fault currents and protective device clearing times, and works out incident energy and arc flash boundaries at each location. The results go on equipment labels, which typically show:

• Nominal voltage and shock approach boundaries
• Arc flash boundary
• Incident energy at the working distance, or the PPE category
• The date of the study and who prepared it

Studies need to be reviewed periodically and whenever the system changes, such as a new transformer, a change in utility supply or new protective device settings. A label that no longer matches the system is misleading.

Reducing the risk

Arc flash protection follows the hierarchy of controls, with PPE as the last line:

1. Eliminate the hazard by de-energizing and establishing an electrically safe work condition under your lockout program. Energized work should be the exception, justified and authorized in writing.
2. Engineering controls such as arc-resistant switchgear, remote racking devices, maintenance switches that speed up tripping during work, infrared windows and current-limiting devices.
3. Administrative controls such as energized work permits, job briefings, training, labels and restricting access to qualified workers.
4. PPE rated for the incident energy, including arc-rated clothing, face and head protection, gloves and hearing protection.

Who may work on electrical equipment

Only qualified workers, trained in the hazards and the specific equipment and authorized by the employer, should work on or near exposed energized parts. Building staff and other trades should not open electrical equipment or enter the arc flash boundary. In most provinces, electrical work also requires appropriate trade certification or licensing.

How ehZee can help

Our electrical system inspections look at the condition of distribution equipment, labelling and maintenance records, and point out where arc flash labels are missing, outdated or inconsistent with the equipment. Arc flash studies themselves are engineering analyses; we can help you scope one and coordinate it with your electrical engineer and contractor.

Common questions