Flagpole safety: wind, lightning and the power grid

A flagpole is a tall, metal and exposed structure. It bears the wind load on the flag, may stand near power lines and, in a country with 100 to 150 million lightning discharges a year according to INPE, is a high point on the plot. This guide brings together what the authorities and distribution companies say about these risks.

Wind and INMET warnings

The National Institute of Meteorology (INMET) issues warnings with three levels of severity. In storm warnings, the forecast winds are:

In its warnings, INMET advises against sheltering under trees and parking near transmission towers and billboards, and refers to the Civil Defence (199) and the Fire Brigade (193). Law 5,700 sets no wind limit for the flag; during an orange or red warning, it is worth bearing in mind that a raised flag increases the load on the pole and that it can be lowered.

The structural design

Brazilian manufacturers design poles to the ABNT NBR 6123 standard on wind forces on buildings. According to the sheet published on AECweb, Metalsinter’s poles are designed for winds of up to 160 km/h; B&B Iluminação cites NBR 6123 and NBR 8800, on steel structures. The size of the flag is part of the calculation: in B&B’s guide, each flag size has a minimum and a maximum pole height. The guide to flagpole heights reproduces that guide. Under article 10 of São Paulo’s Building Code, the city does not take responsibility for the stability of buildings and equipment.

Lightning

According to the National Institute for Space Research (INPE), Brazil recorded on average over the last 10 years about 100 to 150 million lightning discharges a year, depending on the monitoring technique, and the trend is rising. A tall metal pole is an exposed point for these discharges. São Paulo’s Building and Construction Code provides, in article 11(2), that the use-safety design follows the standards for the lightning protection system and for electrical installations. Raising and lowering the flag during thunderstorms exposes the person at the pole; for antennas, Neoenergia Elektro advises against installing or removing them on days of rain or strong wind.

The power grid

Law 5,700 does not deal with the distance between a flagpole and the grid. Distribution companies give references. Neoenergia Elektro states:

• that walls, windows, balconies and roofs may not be built less than 2.5 metres from the power grid;
• that the upper part of the poles carries lines of 13,800 volts or more, and that merely approaching these wires can cause a high-intensity electrical discharge;
• that antennas, whose supports are usually metal, are installed at a safe distance so that, if they fall, they do not touch the wires, and that nobody tries to hold or recover them if they fall towards the grid;
• that long metal materials, such as reinforcing bars and aluminium straightedges, are handled without tilting them towards the wires;
• that, if a wire comes down, people keep away and call the distribution company.

The same logic applies to a flagpole, especially during assembly, when the tube is raised.

Buried networks

A flagpole’s foundation requires digging. In Comgás’s area, in São Paulo, the company offers a gas network search and requests for records and technical guidance, and gives a safety margin of 1 metre on each side of the pipe for using machinery. The guide to installing a flagpole covers these points.

Corrosion and fatigue

Over time, corrosion may attack the metal, and the repeated movement of the flag in the wind puts stress on the base, the joints and the fixing. Hot-dip galvanising protects the steel, and B&B Iluminação notes that durability varies with the environment. The guide to flagpole maintenance covers inspecting these parts.