Earth fault loop impedance testing is necessary to ensure circuit protection operates quickly enough in the event of a fault. It measures the impedance of the path fault current would take between the phase conductor and earth. Testing includes the Ze test at the distribution board and the Zs test to also include installation resistance. Proper protection against electric shock hazards requires that the product of maximum protective device current and earth fault loop impedance remain below 50V. Prospective short circuit testing checks that protective devices will operate safely by measuring fault current between phase and neutral.