Historically, Metro Manila carries a whopping 50 to 60 percent of the total load of the Luzon Grid in the Philippines. Five bulk power substations deliver power to the metropolitan, if a disruption in the transmission lines connecting the load center to any of these substations, a power blackout is imminent in the area. And when your demand is cut into about half, the tendency is for system frequency and bus voltages to swing throughout the system.

When the crane cut off the transmission line, this produces an open circuit that may divert power flows in the power system and may produce a high impedance as seen by protection devices set to guard against system faults. Normally, the protection devices act faster for short circuit and slower to open circuits, since open circuit produces less fault (overload) currents.