Friday, August 08, 2014

YAGI Attack on GFCI

GFCI stands for Ground-Fault Circuit Interrupter. This problem is relegated to the non-threatening urban legend category. Let me tell you this is very real. GFCIs are designed, to serve as a safety measure. The modules work like a circuit breaker. s, cutting the power to devices if they detect that a gadget’s current is flowing through an unintended path.. like a body of water or a human body. It does this by measuring the current leaving the hot side of the power source and comparing it to the current returning to the neutral side. This device has a critical weakness. The ARRL describes it here in the following soft but accurate language:
 "Ground-Fault Circuit Interrupters (GFCI) circuit breakers are occasionally reported to “trip” (open the circuit) when a strong RF signal is present, usually a ham’s HF transmissions. GFCI circuit-breakers operate by sensing unbalanced currents in the hot and neutral conductors of an ac circuit. In the absence of RF interference such an imbalance indicates the presence of a fault somewhere in the circuit, creating a shock hazard. The breaker then trips (opens) to remove the shock hazard."
But point of failure is the tiny transformer most of these devices contain. A tiny transformer isn't much more than a ferrite doughnut wound with copper wire.. in other words.. a toroid. That wire can be induced by the right frequency of radio waves to carry a current. If the attacker can find the right frequency, and the amplitude of that current is strong enough, it can fry the device instead of just tripping the GFCI. Below you can see the results of a two YAGI attacks on household appliances.