This document discusses fault loop impedance testing, which is required by IEC 60364 standards to verify protection in electrical installations. It measures the impedance of the path fault current would take to ensure it is low enough for protective devices to operate safely. The Fluke 1650 Series Installation Testers can perform earth loop impedance measurements and calculate the prospective fault current, displaying results to verify protective devices are adequately rated. The testers use innovative technology to prevent RCDs from tripping during testing for consistent, safe measurements.