Overhead power lines and limits of approach

Overhead power lines are everywhere: along streets, across parking lots, behind buildings, over farm fields and into construction sites. Most are bare conductors, not insulated cables. The coating you might see on some distribution lines is weather protection, not insulation that will protect you. Contact with overhead lines is one of the leading causes of electrical death in both Canada and the United States, and most of the victims are not electrical workers. They are painters, roofers, tree workers, truck drivers, crane and boom operators, labourers and farmers.

How power line contacts happen

Many incidents follow a few familiar patterns:

• Raising a dump truck box, boom, crane, concrete pump, excavator arm or aerial lift into or near a line.
• Carrying or raising a ladder, scaffold pole, eavestrough, pipe or long tool under a line.
• Working on a roof, sign, billboard or scaffold near a line.
• Trimming trees whose branches touch lines.
• Moving irrigation pipe, grain augers or tall farm equipment.
• Approaching a downed line after a storm or vehicle collision.

High-voltage electricity does not need direct contact. As voltage rises, it can arc across an air gap to a conductive object or person that comes close enough. That is why the rules set minimum distances, called limits of approach or minimum approach distances, rather than simply saying 'do not touch'.

The minimum distances

Each province sets its own limits of approach for workers and equipment who are not qualified electrical workers. The values below are the most common ones. They apply to any part of a person, tool, load, boom, rope or equipment.

When you do not know the voltage, assume it is high and use the largest distance that could apply, or call the utility to find out. Even low-voltage service drops to buildings, usually below 750 V, must not be touched; they are covered by general rules against contact with energized parts.

Planning work near overhead lines

Before work begins, the employer or constructor must identify every overhead line on or near the site, its voltage and its owner. The first choice is always to avoid the hazard:

• Ask the utility to de-energize and ground the line, or to relocate it, for the duration of the work. This takes time to arrange, so start early.
• Plan the work so that equipment and materials never come within the limit of approach: choose a different crane position, a smaller machine, or a different method.
• If work must happen near a live line, the utility may install insulating covers, sometimes called line hoses or blankets, but these reduce risk only for incidental contact and do not change the required distance unless the utility and regulation allow it.

When equipment could reach a line, more controls are needed:

• Assign a competent signaller or spotter whose only job is to watch the distance and warn the operator. The spotter must be able to see the line and the equipment and communicate instantly.
• Mark the limit of approach with warning signs, flags, barricades or range-limiting devices on the equipment.
• Use proximity alarms, range limiters or insulating links where appropriate, but treat them as backups to planning and spotting.
• Keep people away from equipment that could become energized, and keep the area around outriggers and ground contact points clear.
• Carry long objects horizontally, low and with two people when near lines, and never raise ladders or poles near them.

Ontario's construction regulation requires a written procedure, a competent signaller and warning devices when equipment could encroach on the limits of approach, and BC and Alberta have similar requirements. The utility must be contacted for assistance where work will be within the limits.

If equipment contacts a line

When a boom or vehicle touches a power line, the equipment and the ground around it may become energized. The ground near a grounded object carries a voltage that falls off with distance, like ripples spreading from a stone dropped in water. A person standing in this area can be shocked by step potential, the voltage between their two feet, or by touch potential, the voltage between their hand on the equipment and their feet.

If you are in a vehicle or machine that contacts a line:

• Stay inside unless there is fire or another immediate danger. The cab is usually the safest place.
• Try to move the equipment away from the line if you can do so safely.
• Warn others to stay back. Call 911 and the utility to de-energize the line.
• If you must leave, jump clear so that no part of your body touches the machine and the ground at the same time. Land with your feet together, then shuffle away with small steps, keeping both feet in contact with the ground, until you are well clear.
• Never return to the machine until the utility confirms the line is de-energized and grounded.

If you are on the ground nearby, do not approach the machine or the person in it, and do not try to rescue someone in contact with the line. Keep others back and call for help. Rescuers become victims when they touch an energized person or vehicle.

Downed lines

Treat every downed line as live, even if it is not sparking. Utilities in Canada generally advise staying at least 10 m, about the length of a school bus, away from a downed line and anything it touches, including fences, puddles and vehicles. Call 911 and the utility, keep others away, and wait for the utility to make it safe.

Working near lines at home and on farms

Many power line deaths happen outside traditional workplaces: a homeowner cleaning eavestroughs with an aluminium ladder, a farmer moving a grain auger, a student carrying a pole for a sailboat, a person flying a drone or kite near lines. The same rules apply. Look up before you raise anything, plan routes for tall equipment, lower augers and booms before moving, and keep ladders and poles well away from lines. Utilities offer free advice and, in many areas, will cover or de-energize lines for planned work.

Trees near lines are a special hazard. Only qualified utility arborists may trim trees within the limits of approach. Homeowners and general tree workers must stay clear and contact the utility.

Weather affects power line hazards too. Lines sag lower in hot weather and under heavy loads, and they swing in wind, so a clearance measured on a cool, calm morning may shrink in the afternoon. Ice and snow can bring lines and branches down. Wet conditions make arc-over and ground conduction more likely. Plan clearances with a margin and recheck them when conditions change.