What is a contact timing test?
During a contact timing test, a DC voltage is applied across the breaker’s contacts. When the breaker opens or closes, the voltage drop across the main contacts changes. From this change, the timer accurately determines the opening or closing time per contact channel, in milliseconds, including any contact bounce.
The instrument also sends the initiate signal — the current to the trip or close coils — and records the coil current waveform. This provides a complete picture of both the electrical contact behaviour and the mechanical drive performance in a single measurement.
Why is contact timing important?
Circuit breakers are components that must operate correctly every time there is a fault. Every breaker is factory tested before delivery (routine testing), tested again after installation on site (commissioning) and periodically tested throughout its operational life. Contact timing is a mandatory element at each of these milestones.
Deviations in contact timing can have different causes: mechanical wear in the operating mechanism, delayed coil response due to ageing or contamination, phase asymmetry where one contact opens earlier than the others, excessive contact bounce reducing service life, or issues with the damping mechanisms.
Without timely detection, a breaker may fail to operate correctly when it matters most.
The five breaker operations explained
O — Open (trip)
The breaker is closed and receives an open (trip) command. The timer measures the time from the initiate signal to full contact separation across all channels. This is the most basic test and verifies trip speed.
C — Close
The breaker is open and receives a close command. The timer records the closing time including contact bounce. Excessive closing time or bounce may indicate a worn spring charge or hydraulic operating mechanism.
O-C — Open-Close
Two sequential operations with a configurable delay between the open and close actions. This cycle tests the breaker’s ability to reclose quickly after a trip — essential for auto-reclosing schemes.
C-O — Close-Open
The breaker closes and immediately opens. This simulates the scenario where a breaker energises onto an existing fault and must trip immediately. The C-O cycle is one of the most demanding tests as the mechanism must execute two opposing movements in rapid succession.
O-C-O — Open-Close-Open
The complete cycle: open, close (with configurable delay), and open again. This is the most demanding operation and simulates a full auto-reclosing scenario. Both the Open-Close delay and Close-Open delay are set independently. The O-C-O cycle provides the most comprehensive view of the breaker’s mechanical condition.
Interpreting test results
After each measurement, the circuit breaker timer displays three key parameters per contact channel:
| Parameter | What it measures | What to look for |
|---|---|---|
| Contact time (ms) | Opening or closing duration per channel | Compare with manufacturer specification. Deviation >10% signals maintenance need. |
| Cycle time | Time expressed in cycles (50/60 Hz) | Useful for comparison with IEC 62271-100 and IEEE C37.09 limit values. |
| Contact bounce | Bounce duration after closing | Longer bounce than normal indicates mechanical wear or insufficient contact pressure. |
Phase synchronicity is another key measurement to check. For a three-phase breaker, all three phases should switch near-simultaneously. A difference exceeding 3–5 ms (depending on breaker type) indicates mechanical asymmetry requiring maintenance.
The coil current measurement provides additional insight into the condition of the trip and close coils and the operating mechanism. An abnormal current profile may indicate increased friction, a worn coil or a spring charge issue.
Contact detection: how the instrument measures
A circuit breaker timer detects contact status using resistance thresholds. For the HighTest CIBRE series: a contact is considered closed at 20 Ω or below, and open at 5,000 Ω or above. The range between 20 and 5,000 Ω is used for resistor contact detection — relevant for breakers with pre-insertion resistors that limit inrush current.
The timer offers three selectable timing windows: 1 second (resolution ±50 μs), 10 seconds (resolution ±500 μs) and 20 seconds (resolution ±1 ms). For standard circuit breakers, the 1-second window is sufficient. The longer windows are intended for slow-operating devices such as motor-driven disconnectors.
When to perform a contact timing test
There are four standard occasions for contact timing tests:
- Factory Acceptance Test (FAT) — document baseline performance as a reference for all future measurements.
- Commissioning — verify after installation that transport and assembly have not affected performance.
- Periodic testing — compare current values against the baseline for trend analysis (typically annual or biannual depending on load and environment).
- Fault investigation — after an incident or unexpected trip, verify the breaker still operates within specification.
Combining contact timing tests with contact resistance measurements (micro-ohm) and where applicable dynamic resistance measurements provides the most comprehensive diagnostic picture of the breaker.
Recommended test equipment
IONIO supplies the HighTest CIBRE series for contact timing measurements:
CIBRE-L3 — 3-contact circuit breaker timer. Suitable for single-phase breakers and phase-by-phase testing. Features a built-in printer, optional Li-ion battery and IP67 protection.
View the CIBRE-L3
CIBRE-L6 — 6-contact circuit breaker timer. Measures all three phases of a three-phase breaker simultaneously in a single test. No reconnection between phases required, saving test time and improving repeatability. The L6 is the standard choice for substation and high-voltage work.
View the CIBRE-L6
Both models work with HighTest’s DMP software (Data Management Platform) for data export, analysis and reporting via PC.
Prefer to rent? The SUWI-120 is a rental multifunctional substation tester that combines contact timing, contact resistance, relay timing and turns ratio measurement in a single instrument — ideal for project-based or occasional testing.
Complementary tests for a complete assessment
Contact timing is one part of a comprehensive breaker assessment. For a complete diagnostic programme, combine timing with:
Static contact resistance measurement — measures the resistance of closed contacts at high current (micro-ohm). Detects oxidation, wear and poor contact pressure.
Read more about static vs. dynamic resistance measurements
Dynamic resistance measurement (DRM) — measures contact resistance during switching. The only method to detect arcing contact wear without disassembly.
View the ARES 200D micro-ohmmeter
Read the full overview: What tests are performed on a circuit breaker?
