IEC 61557-3:2019

Overview - IEC 61557-3:2019 (Loop impedance)

IEC 61557-3:2019 is the third edition of the IEC standard that defines requirements for equipment used to measure loop impedance in low-voltage distribution systems (up to 1 000 V AC and 1 500 V DC). The standard covers instruments that determine loop impedance between a line conductor and protective conductor, between line and neutral, or between two line conductors by applying a load and measuring the resulting voltage drop. This edition updates the previous (2007) version with new requirements for measurement category, operating instructions, and aligns the structure with the rest of the IEC 61557 series.

Key topics and technical requirements

• Measurement method: Loading method - apply a controlled test current and measure voltage drop to calculate loop impedance (ZL).
• Operating uncertainty: Maximum percentage operating uncertainty shall not exceed ±30% of the measured value within the instrument’s specified range, determined under specified rated operating conditions.
• Rated conditions: Requirements apply when the circuit is unloaded, system voltage is between 85% and 110% of nominal and frequency between 99% and 101%, with the circuit loaded by the device’s loading equipment.
• Measurement category: Equipment intended for general distribution system use must be at least Measurement Category III per IEC 61010-2-030; devices intended only for socket outlets may be Category II.
• System phase angle influence: Instruments must account for system phase angle (inductive effects), especially when measurements are made close to the transformer (noting possible phase angles up to ~30°).
• Safety limits and protections:
  • Avoid creating fault voltages > 50 V at the point of measurement (automatic disconnection required if exceeded).
  • Equipment must tolerate 120% nominal voltage without damage and survive accidental exposure to 173% rated voltage to earth for 1 minute as per IEC 61010-2-030 (protective devices may operate).
• Marking & instructions: Clear marking of range, nominal system voltage/frequency, and any calibration offsets (e.g., external resistance zero offset) plus enhanced operating instructions are required.
• Referenced tests: Tests for operating uncertainty, external resistance behavior, fault voltage, and overvoltage are specified.

Applications and users

IEC 61557-3 is used for:

• Commissioning and periodic verification of electrical installations
• Fault loop impedance testing to confirm protective device operation and calculate prospective fault currents
• Equipment manufacturers for design and compliance of loop-tester instruments
• Electricians, electrical inspectors, maintenance engineers, calibration laboratories, and certification bodies concerned with low-voltage safety testing.

Keywords: IEC 61557-3:2019, loop impedance, loop resistance, low voltage distribution systems, measurement category, operating uncertainty, protective measures, loading method.

Related standards

• IEC 61557-1:2019 - General requirements for equipment for testing protective measures
• IEC 61010-1:2010 / AMD1:2016 - Safety requirements for measurement equipment
• IEC 61010-2-030:2017 - Particular requirements for testing/measuring circuits
• Other parts of the IEC 61557 series (for broader protective-measure testing requirements).