Breaker Contact Timing Test, Measurement Modes and Interpretation

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. A single measurement therefore shows the contact behaviour, and through the coil current it gives an indirect view of the operating mechanism. The contact travel itself is not measured by a timer without a motion input.

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 mandatory in the routine test. At commissioning and periodic maintenance it is common practice and often a manufacturer requirement.

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 CIBRE timers offer five measurement modes. These are instrument functions. IEC 62271-100 works with rated operating sequences such as O–t–CO–t’–CO. Each measurement mode tells you something different.

O — Open (trip)

The breaker is closed and receives an open (trip) command. The timer measures the time from the initiate signal to the moment the contacts separate (the opening time) on each channel. This is the most basic test and verifies trip speed.

C — Close

The breaker is open and receives a close command. The timer measures the time until the contacts first touch (the closing time). Contact bounce after that is recorded separately. 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. C-O is one of the most demanding measurement modes, 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 measurement mode. It 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.

After each measurement, the circuit breaker timer displays three key parameters per contact channel:

Phase synchronicity is another key measurement to check. For a three-phase breaker, all three phases should switch near-simultaneously. The permissible difference is given in the manufacturer specification, typically a few milliseconds. A larger spread indicates mechanical asymmetry that requires 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.

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 switching overvoltages on closing.

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.

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 every one or two years, 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.

IONIO supplies the HighTest CIBRE series for contact timing measurements:

CIBRE-L3 — 3-contact circuit breaker timer. One channel per pole, so on a three-phase breaker with one interrupter per pole you measure all three phases in one test. Features a built-in printer, optional Li-ion battery and IP67 protection.
View the CIBRE-L3

CIBRE-L6 — 6-contact circuit breaker timer. Intended for breakers with two interrupters per pole, again measuring all three phases in one test. It can also test two breakers at the same time.
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.

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 main 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?