EN
Moving and Fixed Contact Test
Contact test on breakers: touch resistance (micro-ohmmeter) measurement, open-close timing and sync checks by engineers with reports.
Moving and fixed contact test verifies the connection quality between the fixed contacts and the moving contact rods completing the energy path of power circuit breakers (CB) and disconnectors, through micro-ohm (Ductor) contact resistance measurement and mechanical timing trials. Rising contact resistance is an early sign of surface oxidation, spring pressure loss, adjustment drift or overheating damage at the weld zone, and under current it initiates the I²R loss thermal runaway chain.
Contact Resistance Measurement
- Method: A 100-200 DC constant current is injected and four-point (Kelvin) resistance read at microohm level; each phase separately with symmetrical connections (current and potential probes apart).
- Acceptance: Manufacturer value and phase-to-phase consistency are essential; in practice deviation above 50% between phases is suspect, and a several-fold growth in mΩ a fault precursor. Contact surface cleaning and tightness effects before measurement are noted.
- Trend: Not a single reading but installation-commission-control series compared; in oil-air breakers the oxide layer after lid opening affects the resistance history.
Mechanical Timing Tests
Beyond contact soundness the motion chain is verified: energising open-close coils, contact travel time, phase asymmetry (deviation of simultaneous closing, typically within a ~2 ms band), velocity (stroke time) and open-closed position dialogue are measured. Wear-play in spring-cam mechanisms and leakage-delay in hydraulic-pneumatic transmission organs are root causes of timing drift. Timing also concerns opening stability (latching) against magnetic push under short circuit current: an unadjusted lock equals the contact parting under arc. First arcing pre-strike bouncing during closing is also searched in the oscilogram; recurring bounce is mechanical proof of spring pressure loss or contact surface damage and accelerates erosion.
Thermal and Electrical Complements
Under load, thermographic ΔT of contact zones verifies contact resistance with field reality (mΩ good at low current, hot spot under load — a loose external link). Secondary injection through the protection relay chain and total fault clearance time are the breaker's performance contract value. On the LV side motor-circuit breakers can be evaluated with similar measurements.
Legislation and Outcome
Breaker tests sit under the periodic maintenance duty of Law No. 6331 and the High Current Installations Regulation; references IEC 62271 (circuit-breakers), IEC 60694 and the manufacturer service handbook. The result connects to a signed report with the measurement table (phase-to-phase mΩ), timing oscilogram/protocol and proposals; cleaning-lubrication or part replacement decisions are set economically with this data. Controls are in our Transformer Tests and Analyses group; for planning together with MV cell tests, contact us.
FAQ on Moving & Fixed Contact Test
The contact resistance of moving (breaker-disconnector-arcing contact) and fixed (busbar-bushing-joint) contacts is the direct source of I²R heating under load current. Low-resistance measurement with a micro-ohmmeter hunts oxidation-contamination-under-torque-worn-surface or a non-gripping contact and catches loose connections. High contact resistance is a hidden weakness producing a local hot spot, arcing and finally a transfer parting.
Current injection with a calibrated micro-ohmmeter (at least 100 A, four-terminal-Kelvin) and Kelvin connections are used; the thermal effect of the test current is managed, surface-insulator-layer cleaning-dryness-joint oxidation is removed. A serial measurement is taken per pole-position (open-closed-close sequence) and joint. The result is compared with the manufacturer reference and phase inconsistency; temperature normalisation is applied.
A high inter-phase difference (e.g. above 50 %) or a rise against the reference value marks contact surface-oxidation-under-grip-blowout traces or a loose-wet-corroded joint. Thermal scanning (thermal control) verifies hot spots under load; a high-resistance contact is removed by planned contact-closure comparison and, where needed, contact-joint-bushing change. Interpretation separates main from auxiliary (arcing) contact.
On breakers-disconnectors the operating count, eventful trips and protection fault trips trigger contact resistance control; the practical routine is generally inside the yearly periodic control package (with grip-open time-mechanical characterisation). Values are stored as a trend; rising resistance is the precursor of progressing fault. It is matched with thermal scan findings to produce early warning.