Backfeeding is the delivery or flow of electric power in the reverse direction of the "normal" flow of power, which is from power stations, through electric power transmission and electric power distribution, to homes and buildings, to electrical appliances. It includes feeding power into the "load" side of a distribution panel, rather than the "line" side.
One example of backfeeding is a portable generator supplying electrical power into a house or building via an electrical outlet or a load-side circuit breaker (instead of "feeding" power through a correctly wired electrical inlet and a transfer switch). Backfeeding into a building is typically improvised during a blackout, in order to quickly restore electrical power to freezers, heating, ventilation, and air conditioning equipment, lighting, communications, pumps, and business operations, to prevent damage (by thawing or freezing), inconvenience, loss of business, etc., until power is restored. It is vital to disconnect from the local power grid, typically by turning off all circuit breakers or fuses that connect to the external mains, or pulling the electric meter. Failure to disconnect from the external mains can be expensive, due to unintentionally powering neighboring buildings and homes. Failure to disconnect from the external mains is illegal. It could energize downed power lines, potentially killing or injuring people who assume that the lines are de-energized. It could cause unexpected fires.
Another example is a house or building supplying electrical power into the local power grid. This is usually unintentional, and it is dangerous. It can be caused by feeding power into an electrical outlet of a building (as above). It can be caused by feeding power into the main power inlet of the building without disconnecting it from the external mains. Wires, fuses, and almost all circuit breakers carry power in either direction.
By definition, backfeeding causes electrical power to flow in the opposite direction from its usual flow. When studying backfeeding, engineers must understand the transfer of electrical power, and not confuse this with momentary AC voltages or current flows viewed in isolation from the overall situation.
Power grid generators normally pump energy into the grid, making it available for others to use. A power station will typically backfeed (and thus consume power) when it is shut down, due to its own local loads (e.g. lights or repair equipment).
Power grid loads may backfeed if they also have distributed generation installed, such as a grid-connected photovoltaic solar power system or a microturbine-based power generator. It is also possible for an electric motor to temporarily backfeed if it is mechanically driven (see Regenerative braking).
For cost reasons, many of the circuit (overcurrent) protection and power quality control (voltage regulation) devices used by electric utility companies are designed with the assumption that power always flows in one direction. An interconnection agreement can be arranged for equipment designed to backfeed between an electric utility customer with distributed generation and their power company. This type of interconnection can involve nontrivial engineering, and possibly equipment upgrade costs to keep the distribution circuit properly protected. Such costs may be minimized by limiting distributed generation capacity to less than is locally consumed, and guaranteeing this condition by installing a reverse-power cutoff relay that opens if backfeeding occurs.
Because it involves transfer of significant amounts of energy, backfeeding must be carefully controlled and monitored. Personnel working on equipment subject to backfeeding must be aware of all possible power sources, and follow systematic protocols to ensure that equipment is fully de-energized before commencing work, or use special equipment and techniques suitable for working on live equipment.
When working on de-energized power conductors, lineworkers attach "temporary protective grounding assemblies" or "protective ground sets", which short all conductors together and to ground. This ensures that no wires can become energized, whether by accidental switching on of the power company's switches or by someone in a home or building starting up an incorrectly wired generator that backfeeds the lines.
Feeding power into an electrical outlet energized the blades of a male connector. If the power cord is unplugged from the outlet while the generator is running, the exposed blades of the male connector present a risk of shock, fire, and damage to the generator.
When the power comes back on, the backfeeding connection must be undone in the correct order. The generator must be disconnected before the main breaker is turned on, or else the generator could be damaged.
- "What is Backfeeding? – Electricians Library". www.electricianslibrary.com. Retrieved 2018-07-04.