Overview
EN ISO 17657-3:2007 (ISO 17657-3:2005) specifies requirements for toroidal current sensing coils used as sensors in welding current measurement systems for resistance welding. The standard applies to both 50 Hz/60 Hz alternating current and direct current, and defines how coils are classified, specified, tested and marked for use with welding current meters and measuring systems.
Key topics and requirements
- Scope and environment
- Designed for use in typical welding atmospheres (gas, fine dust, oil mist, spatters).
- Operating conditions: ambient +5 °C to +40 °C, relative humidity up to 95 %, altitudes up to 1 000 m.
- Sensor type
- Focus on toroidal (multi-wound) coils that encircle the conductor carrying welding current.
- Conversion coefficient (K)
- Defined as output voltage per welding current (mV/kA). Standard rated values at 50 Hz: 150 mV/kA, 220 mV/kA, 1.5 V/kA.
- Value is proportional to frequency; conversion between 60 Hz and 50 Hz is specified (60 Hz values divided by 1.2 to obtain 50 Hz equivalent).
- Accuracy classes
- Highly accurate: ±0.5 % of full scale (laboratory)
- Accurate: ±1.0 % of full scale (routine high-accuracy systems)
- Ordinary: ±3.0 % of full scale (ordinary systems)
- Electrical and mechanical requirements
- Coil output is proportional to the derivative of the current waveform - integrators/amplifiers are required to recover current magnitude.
- Output load resistor recommended between 200 Ω and 1 kΩ; measurement instrumentation should use >500 kΩ input impedance for conversion checks.
- Design measures to avoid sensitivity to external magnetic flux; low-inductance, protected leads and connectors.
- Tests and verification
- Procedures for measuring the conversion coefficient, accuracy, mechanical strength, setting-position error, thermal behavior and contact integrity are specified.
- Product designation and marking
- Requirements for indicating type (flexible/rigid), size (length or inner diameter), conversion coefficient, frequency and accuracy class.
Applications and users
- Practical uses:
- Monitoring and controlling welding current in resistance spot welding, seam welding and other resistance welding processes.
- Calibration, verification and quality assurance of welding systems.
- Primary users:
- Sensor and welding equipment manufacturers, calibration laboratories, welding engineers, quality and process-control teams, procurement specialists specifying measurement hardware.
Related standards
- ISO 17657 series:
- Part 1: Guidelines for measurement
- Part 2: Welding current meter with current sensing coil
- Part 4: Calibration system
- Part 5: Verification of welding current measuring system
- Normative reference: ISO 669 (resistance welding equipment requirements)
Keywords: EN ISO 17657-3:2007, current sensing coil, toroidal coil, welding current measurement, resistance welding, conversion coefficient, accuracy class, welding current meter.