Backfeeding occurs when electricity flows in the opposite direction of its normal path. If a secondary power system (solar arrays, generators, or batteries) is connected to a building’s electrical system without a properly engineered isolation or transfer switch, it can feed electricity backward into the main panel and utility lines. This backfeeding turns an assumed-de-energized breaker panel, equipment, or service line into an active, high-voltage hazard.
Pre-work inspection
Never assume a facility has only one power source. Inspect the roof and surrounding area for solar panels, standby generators, battery storage cabinets, or portable generator plug-in stations (inlet boxes). Temporary trailers or offices are highly susceptible to non-standard wiring, so be sure to look for extension cords running from nearby buildings, portable generators, or small solar/RV-style hookups.
Look for manual or automatic transfer switches (ATS) near the main service entrance or subpanels indicating the presence of a standby power system. Check for exterior, rapid-shutdown solar switches or auxiliary disconnects labeled “Solar,” “PV,” “Generator,” or “Battery Disconnect.” Carefully read the breaker directory and look for labels indicating “Backfed Breaker,” “Solar PV Connection,” or “Alternative Supply.”
Tips for Working Safely
Individually lock out and tag out the main utility breaker, auxiliary solar rapid-shutdown switches, battery storage isolators, and generator transfer switches.
If a transfer switch cannot be locked out, physically disconnect the secondary power source (e.g., unplug the generator and pull the solar DC disconnect).
Treat all circuits as energized until you have personally tested them.
Use a calibrated, functioning, and properly rated voltage detector (using the “Live-Dead-Live” testing method) to verify the absence of voltage on all phases, buses, and neutral lines before starting work.