Solution Summary: Transfer Switch for Generators
A transfer switch is an engineering control that can prevent inadvertent energization of utility power lines. This device is installed next to a structure's main panel by a licensed electrician and switches the electrical power source from utility to generator. The purpose is to eliminate a simultaneous connection of power source between utility and generator which ultimately prevents the generator power to backfeed into utility power lines.
In the event of a power outage, backfeeding is a practice that involves connecting a portable generator to a structure's outlet with a heavy power cord. This allows electrical power to enter the main panel through the outlet's branch circuit breaker and distributes the power to the structure. However, this panel can also feed electrical power out through the main breaker to the transformer, which then converts it back to high voltage and attempts to energize the connected utility lines. To prevent this from happening, a transfer switch is recommended.
Types of Transfer Switches
There are two types of transfer switches. A manual transfer switch allows for the selection of which circuit to turn on and off for the load management on the generator. An automatic transfer switch (figure 1), as the name implies, automatically adjusts the power needs without the need to toggle circuits. In addition, an automatic transfer switch actively monitors the status of utility power which can disconnect a generator when utility power is restored from an outtage.
Figure 1. APC Universtal Transfer Switch is an automatic transfer switch (Source: http://www.apc.com/shop/us/en/products/APC-Universal-Transfer-Switch-10-Circuit-120-240V/P-UTS10BI)
The type of transfer switch required will depend on the type of generator in use. Please consult the manufacturer on selecting the appropriate unit device.
Utility line workers repairing power lines may unexpectedly encounter high voltage and suffer a fatal shock if portable generators do not have transfer switches installed. Contact with electric current is a major cause of injury and death among construction workers (Janicak 2008). In 2012, the Census of Fatal Occupational Injuries (CFOI) data produced by the Bureau of Labor Statistics (BLS) indicated that contact with electric current was the fourth leading cause of work related deaths—after falls, transportation incidents, and contact with objects and equipment (BLS 2012). Electricity can cause electric burns, electrocution, shock, arc flash/blast, fires and explosions. One of the major causes of an electrocution hazard is contact with overhead power lines and energized sources. Overhead power lines are particularly hazardous since they carry high-voltage electricity. Although electrocution is considered the main risk, other important hazards of working near or on high-voltage lines include electrical shock, burns, and falls to lower levels. These types of hazards happen when the body becomes part of the electric circuit, whether by direct contact with an energized source or by contact with a conductive material that has become energized. The severity of the electrical hazard depends on several factors, including the length of exposure time, energy deposited into the body, the pathway through the body, wetness or dryness of the surface and the amount of current.
How Risks are Reduced:
A transfer switch is a device that switches a load between two sources and prevents each backfeeding into the other.
Le, Jean Christophe, MPH - CPWR - The Center for Construction for Research and Training