Electrical safety basics

Why it matters

Electrocution is one of construction's Focus Four hazards (falls, struck-by, caught-in/between, electrocution). It is invisible, silent and fast. Electricians are not the only people at risk: labourers touch overhead lines with ladders, mechanics reach into panels, and engineers open cabinets on site visits. Knowing how electricity injures people, and which rules keep you out of harm's way, is basic literacy for every trade and engineering career.

Awareness level only. This lesson explains the ideas so you recognise them on day one. It does not qualify you to do the work. Your employer, apprenticeship or school must train you on its own procedures and equipment, and you must work under supervision until they say you are qualified. Never work on or near exposed live parts unless you are a trained, qualified person following your employer's procedures.

How electric shock hurts you

Current, not voltage alone, does the damage. Your body becomes part of a circuit when it bridges two points at different voltages, for example a live wire and the ground. Ohm's law (I = V / R) tells you why wet skin and household voltages are dangerous: lower resistance means more current.

Approximate effects of 60 Hz AC current passing through the body:

Worked example: 120 V across a wet hand-to-hand path of about 1,000 ohms gives I = 120 / 1,000 = 0.12 A = 120 mA, well into the lethal range. Dry skin raises the resistance, but you cannot count on dry skin.

Shock also causes burns inside the body along the current path, and falls when the jolt throws someone off a ladder.

Arc flash and arc blast

An arc fault happens when current jumps through air between conductors, often because a tool slips, a part fails or dust and moisture build up. The result:

• Arc flash: intense heat and light that can cause severe burns several feet away and ignite clothing.
• Arc blast: a pressure wave that throws people and turns molten copper into shrapnel, plus a sound loud enough to damage hearing.

Arc flash can happen without anyone touching anything, simply by opening or operating equipment.

Approach boundaries (NFPA 70E)

NFPA 70E, the Standard for Electrical Safety in the Workplace, defines boundaries around exposed energised parts:

• Arc flash boundary: the distance at which incident energy drops to 1.2 cal/cm² (enough for a second-degree burn). Inside it you need arc-rated PPE.
• Limited approach boundary: unqualified people may not cross it unless escorted by a qualified person.
• Restricted approach boundary: only qualified people with the right PPE and a plan may cross it.

The distances depend on voltage and the equipment, and appear on equipment labels or in the employer's study. You do not need to memorise them; you need to recognise the labels and stay out.

Overhead power lines

Assume every overhead line is energised and uninsulated. OSHA's general rule is to keep yourself and any conductive object (ladders, scaffolds, booms, pipes) at least 10 feet from lines up to 50 kV, and further for higher voltages. Cranes and boom equipment have their own, larger clearances. Look up before you raise anything.

The rules that prevent most injuries

1. De-energise first. NFPA 70E's default is to establish an electrically safe work condition: isolate, lock out and test for absence of voltage before work. Working live is allowed only when de-energising creates a greater hazard or is infeasible, and it needs a written permit.
2. Qualified versus unqualified. A qualified person has the training and knowledge to recognise and avoid the hazards of that equipment. New apprentices are not qualified for live work.
3. Use GFCI protection for portable tools on construction sites and in wet locations.
4. Inspect cords and tools before use; remove damaged ones from service (lesson 8).
5. Keep panels closed and the 36-inch working space in front of them clear.
6. Never bypass a ground pin or use a cheater plug on a site tool.

Worked example: spotting an unsafe situation

On a site visit, an engineering intern sees a worker reaching into an open 480 V panel with a screwdriver, wearing a cotton T-shirt, to "tighten one loose lug". Problems: the panel was not put in an electrically safe work condition, no permit, no arc-rated PPE, and no test for absence of voltage. The intern should not intervene physically; they should stay back, tell their supervisor or the site safety lead immediately.

Common mistakes

• "It's only 120 volts." 120 V kills people every year.
• Trusting that a breaker is off because someone said so. Test before touch.
• Using a metal ladder near electrical work.
• Reaching into a panel "just to look" without PPE.

Official sources

• NFPA 70E: https://www.nfpa.org/70E
• OSHA electrical safety: https://www.osha.gov/electrical

Checked on 2026-10-23.

Recap

• Current through the body causes shock; a few tens of milliamps can be fatal.
• Arc flash (heat) and arc blast (pressure) can injure people who touch nothing.
• NFPA 70E: de-energise and test first, respect approach boundaries, use arc-rated PPE.
• Stay at least 10 feet from overhead lines up to 50 kV.