An alarm panel of a security system is additionally provided with an interference circuit for evaluating the possible presence of a large electromagnetic interference signal such as lightning. The earth ground connection of the alarm panel to a power source is used to provide an input to the interference circuit. The earth ground connection receives large transient signals caused by such naturally occurring events. The alarm panel uses this additional information to modify the reporting of alarm conditions. This has particular application for addressing problems associated with motion detectors falsely triggering when a large electromagnetic signal is received. The solution of the alarm panel sensing this condition in contrast to each detector sensing this condition is more reliable, allows combining of detector information and is more cost effective.