For example, a voltage can be applied to the array by adjusting the timing of a three-phase bridge or by connecting a precharge circuit to the array to cause the array's voltage to increase while some or all the modules are disabled. In one embodiment, one particular example is to change the frequency of the pulse width modulated signal driving the switches in the inverter's main power conversion stages. By varying these control signals in a unique pattern, the pattern can be transmitted as a change in the fairly high frequency noise coupled from the inverter throughout the modules of the array. A safety shutdown device, located in the field, can measure these current and/or voltage differences and reconstruct a shutdown, startup, or heartbeat signal, and take the appropriate action to enable/disable the array, the subset of the array, or the module under its control.

In embodiments, circuitry 110 can detect the normal operation of the inverter, including the current drawn during normal operation or small variations in the current due to MPPT or the inverter's tracking of the AC power grid frequency. Circuitry 110 can detect current with a current-sensing circuit or by inferring the current, or a change in current, from the voltage across the module or part of the module. Accordingly, circuitry 110 likewise can detect variations in the normal operation of the inverter in embodiments in which the heartbeat indication is a ripple or other variation to the normal operation.

As another example, time-varying light can be applied to the module's cells causing a change in the voltage or current measured by sensing circuitry in the module such that the indication can be received in the form of the change in voltage or current. Another example is a vibration transmitted through the module's mounting frame and detected by a sensing device, such as sensor 150.