ZRA holds for "Zero Resistance Ammeter". It allows you to measure the short-circuit current without introducing a series resistor (load) in the circuit. The operation principle is that of the Operational Amplifier, OA, (https://en.wikipedia.org/wiki/Operational_amplifier). The two electrodes of interest are connected to the input terminals of the OA, that are self-set virtually to the same potential (thus chort-circuit). To maintain that condition the OA will supply a current, the galvanic current, measurable as the output terminal voltage of the OA (Ig= Vout x Gain).