Patent ID: 8259929

Claim:
An apparatus comprising: a network interface device (NID) located outside a building; a power supply unit located inside the building, the power supply unit having one or more ports to provide a power signal to the NID; and a direct current (DC) back up power unit located in the NID, the DC back up power unit to provide back up power to the NID, wherein the NB) is configured to periodically switch between receiving the power signal from the power supply unit and the DC back up power unit and when the power signal is received from the DC back up power unit, the apparatus enters a fault detection mode and, when in said mode, the power supply unit monitors its outgoing power to determine if there is a fault condition and wherein said switching between receiving the power signal from the power supply unit and the DC back up power unit occurs on a periodic basis even when no fault condition exists in the power supply unit.