Shock hazard and approach boundaries

Shock is the oldest electrical hazard and still a leading cause of electrical fatalities among electrical workers. This lesson looks at what determines how much current flows through a body, how the standards turn that physics into approach boundaries around exposed energized parts, and how the boundaries differ for AC and DC systems. It also connects the workplace boundaries with the separate limits of approach used for overhead power lines.

What drives current through the body

The current through a person depends on the voltage across the contact points and the total resistance of the path, which includes the skin at each contact, the internal body, any gloves or footwear, and the surface the person is standing on. Internal body resistance is low, often taken as about 500 Ω hand to foot. Skin resistance varies enormously: dry, calloused skin may present tens of thousands of ohms, while wet or broken skin, or skin under a contact that has burned through, may offer almost none. For safety calculations, a total of about 1,000 Ω hand to hand or hand to foot is a common and deliberately pessimistic assumption.

Above a few hundred volts, skin breaks down quickly and the current is limited mainly by internal resistance. That is one reason why shocks at 347 V, 480 V and 600 V are so much more dangerous than at 120 V: the voltage is higher and the resistance falls.

Direct current is generally less likely than AC to cause fibrillation at the same magnitude, because it does not repeatedly stimulate the heart during its vulnerable period. Even so, DC above about 300 mA can cause fibrillation, the onset and interruption of DC current produce strong muscle reactions, and DC causes deep burns and electrolytic tissue damage. DC let-go thresholds are less well defined because DC tends to produce a single violent contraction at contact and release rather than sustained clamping.

Exposed, energized and the three boundaries

The approach boundaries apply to exposed energized electrical conductors and circuit parts, meaning parts that can be inadvertently touched or approached nearer than a safe distance because they are not suitably guarded, isolated or insulated. Closing a panel cover makes the parts unexposed; removing it makes them exposed. Two shock boundaries and one arc flash boundary surround every exposed energized part:

• The limited approach boundary is the distance at which a shock hazard exists. Unqualified persons may not cross it unless they are continuously escorted and advised by a qualified person.
• The restricted approach boundary is the distance at which there is an increased likelihood of shock, because of electrical arc-over combined with inadvertent movement. Only qualified persons may cross it, and only with shock protection such as insulating gloves and tools, or with the part insulated or guarded.
• The arc flash boundary is the distance at which incident energy falls to 1.2 cal/cm² (5 J/cm²), the level at which a second-degree burn just becomes possible. It is independent of the shock boundaries and may be larger or smaller than either. Lesson 5 explains how it is determined.

Older editions of CSA Z462 and NFPA 70E also defined a prohibited approach boundary. It was removed because, in practice, crossing the restricted approach boundary already requires the same protection as contact.

The AC boundary table

CSA Z462-24 and NFPA 70E-2024 Table 130.4(E)(a) give the shock boundaries for AC systems. The values below are the common low and medium voltage rows. The 2024 editions aligned the restricted approach boundary with the minimum approach distances used for utility work and added a note that the values assume an elevation of up to 900 m; at higher elevations, such as parts of Alberta and British Columbia, the restricted boundary must be increased. Metric values are rounded slightly differently in different documents, so always use the table in the edition your employer has adopted.

The limited approach boundary has two values. The larger one applies to an exposed movable conductor, such as an overhead conductor that can sway or a cable being pulled, because it can move towards the person. The smaller one applies to fixed circuit parts, such as bus inside a switchboard. In most indoor equipment work, the fixed-part value is the one that matters.

The DC boundary table

DC systems are now common in battery rooms, uninterruptible power supplies, solar arrays, electric vehicle chargers, rail transit, data centres and telecommunications. The DC table uses different voltage bands because the shock hazard thresholds differ.

Crossing the boundaries

Crossing the limited approach boundary is not prohibited for qualified persons, but it means a shock hazard is present and the shock risk assessment applies. Crossing the restricted approach boundary requires all of the following:

• The person is qualified for the task.
• An energized work permit is in place, unless the task is exempt (Lesson 6).
• The person uses PPE suitable for working near exposed energized parts and rated for the voltage, such as rubber insulating gloves with leather protectors, or the energized parts are insulated or guarded from the person.
• No uninsulated part of the body crosses the boundary. Insulating gloves protect hands only; leaning a shoulder or head into the space requires insulating sleeves or barriers.
• The person minimizes the time spent inside the boundary and plans the work to avoid inadvertent movement, such as stepping back into a live part or reaching across one.

Conductive articles such as watches, rings, metal frame glasses, key chains, metal pens, necklaces and body piercings must not be worn inside the restricted approach boundary. Conductive tools and equipment, such as metal ladders, steel tape measures and uninsulated tools, must not be used there either.

Overhead lines and limits of approach

The CSA Z462 approach boundaries apply to workplace electrical equipment. Work near overhead power lines is governed by separate provincial limits of approach, which apply to people, equipment and materials. For example, O. Reg. 213/91 s. 188 in Ontario requires a minimum distance of 3 m for lines of 750 V to 150 kV, 4.5 m for 150 kV to 250 kV and 6 m for over 250 kV, unless the line is de-energized or specific precautions are in place. British Columbia's OHSR Part 19 sets limits of approach in Table 19-1A, and Alberta's OHS Code Part 17 sets safe limits of approach for overhead lines. Utility workers follow their own utility safety rules, such as the IHSA Electrical Utility Safety Rules in Ontario.

These overhead line distances are not interchangeable with the workplace boundaries. A forklift operator in a yard near a 27.6 kV line is governed by the overhead line rules, while an electrician opening a 600 V switchboard in the same building is governed by the workplace boundaries.