Overview
ISO 17657-3:2005 specifies toroidal current sensing coils used as sensors in welding current meters and welding current measuring systems for resistance welding. The standard covers design, classification, performance requirements and test procedures for coils used to monitor welding current for both alternating current (50 Hz / 60 Hz) and direct current (DC).
Key topics and technical requirements
- Sensor type: Multi‑wound toroidal (tore) current sensing coil mounted around the conductor to detect magnetic flux.
- Conversion coefficient (K): Output voltage per welding current (mV/kA). Standard rated values at full‑wave 50 Hz are 150 mV/kA, 220 mV/kA and 1.5 V/kA. The coefficient is proportional to frequency and can be converted between 50/60 Hz.
- Accuracy classes:
- Highly accurate: ±0.5% of full scale (laboratory use)
- Accurate: ±1.0% of full scale (routine use for accurate systems)
- Ordinary: ±3.0% of full scale (routine use)
- Electrical and interfacing:
- Output load (RL) recommended constant between 200 Ω and 1 kΩ.
- Measurement of output should use a high input impedance device (> 500 kΩ) when applicable.
- Operating environment: Ambient temperature +5 °C to +40 °C, relative humidity up to 95%, altitudes up to 1000 m; tolerant of typical welding atmospheres (gas, fine dust, oil mist, spatters).
- Mechanical and thermal requirements:
- Mechanical strength and repeat‑attachment tests (for flexible coils).
- Cover materials must resist contact with wire heated to 60 °C; transformer‑mounted coils follow transformer insulation class.
- Testing & calibration:
- Conversion coefficient and measuring accuracy tests using full‑wave currents (typically 5–10 kA at positions defined in the standard).
- Reference meters/coils and data acquisition systems should be calibrated at least annually per ISO 17657‑4.
- Product designation: Type (rigid/flexible), conversion coefficient, frequency, accuracy class, and coil size (length or inner diameter).
Applications and who uses it
- Ensures reliable welding current measurement in spot, projection and seam resistance welding systems.
- Used by:
- Welding equipment manufacturers (integrating sensors into meters and controllers)
- Quality and process engineers (welding process monitoring and control)
- Test laboratories and calibration labs validating welding current measurement systems
- Procurement/specification teams requiring compliant current sensors
Related standards
- ISO 17657 (general series) - Parts:
- Part 1: Guidelines for measurement
- Part 2: Welding current meter with current sensing coil
- Part 4: Calibration system (ISO 17657‑4)
- Part 5: Verification of welding current measuring system
- ISO 669 - Resistance welding equipment requirements
ISO 17657-3:2005 is essential when selecting, testing and specifying toroidal current sensing coils for accurate, repeatable welding current monitoring in resistance welding processes.