Circuit Breaker Timing Test: What It Measures

Short answer

The operating time test measures how long a circuit breaker takes to open and close from the moment it receives the command, and how closely its three poles operate together. It reveals slow mechanisms, weak coils and misaligned poles. A slow breaker lengthens fault clearing even if the relay tripped on time.

What the operating time test is

The operating time test records, with a circuit breaker analyzer, the moment each contact changes state after a close or open command. The analyzer sends the command to the coil, measures the time of each pole and, on extra-high-voltage breakers with several interrupting units, the time of each unit within the same pole. At the same time it can record the coil current and the auxiliary contacts.

It is one of the central tests among the circuit breaker tests, because the breaker is the last link in the protection chain: the relay decides in milliseconds, but the fault is cleared only when the contacts separate and the arc is extinguished.

What it measures and which defect it detects

IEC 62271-100 defines the breaker timing quantities, taken from the International Electrotechnical Vocabulary (IEV 441-17). The ones measured in the field are:

Opening time: from the moment the trip coil is energized until the arcing contacts separate in all poles.
Closing time: from the start of the closing operation, in practice the energizing of the closing circuit, until the contacts touch in all poles.
Close-open time (CO): in a closing followed by an opening, the interval from the moment the contacts touch in the first pole until the arcing contacts separate in all poles (IEV 441-17-42). It is what the breaker goes through when it closes onto a fault and the protection trips it immediately.
Pole discrepancy and, on multi-unit breakers, between units of the same pole.
Bounce of the contacts on closing.
Auxiliary contacts (52a and 52b): when they change relative to the main contacts. Anti-pumping, pole discrepancy and the logic of the breaker failure protection 50BF depend on this.
Coil current: the shape of the curve shows the movement of the plunger and the instant the mechanism latch is released. We explain it in the guide on trip and closing coils.

This data reveals slow mechanisms from hardened lubrication or friction, weakened springs, worn coils or latches, misaligned poles, miscalibrated auxiliary contacts and pre-insertion resistors that do not engage in time. The exact cause of an abnormal time is clarified with the contact travel and speed test, which shows how the mechanism moves.

In maintenance the first trip is also measured: the first opening after a long period without operations, before any other operation.

Which equipment and levels it applies to

Level	Equipment	Factory	Commissioning	Maintenance
LV	Open-frame breakers with operating mechanism	Per equipment standard	Timing and functional tests, per the manufacturer	Occasional
MV	Vacuum or SF6 breakers in switchgear, reclosers, contactors	Routine mechanical test	Yes	Yes
HV	Live-tank and dead-tank breakers, GIS breakers	Routine mechanical test	Yes	Yes, with first trip
EHV	Multi-unit breakers, single-pole operation, pre-insertion resistors	Routine mechanical test	Yes, per unit	Yes, with first trip

At extra-high voltage, the pole and unit discrepancy and the engagement of pre-insertion resistors carry more weight; see extra-high-voltage circuit breaker.

On GIS circuit breakers

In a GIS the breaker terminals are inside the enclosure and cannot be touched: the timing channels and the current injection reach the contacts through the earthing switches on each side of the breaker. Working with both earthing switches closed is the safest condition, but then the ground loop sits in parallel with the contact and the analyzer may not see the change of state. For that condition there are measurement techniques with both sides grounded, for example with a sensor that detects the loop current, or the removable ground link on one side is taken out if the design and the safety procedure allow it. Contact resistance measured by the same path is explained in the contact resistance test.

When commissioning a GIS breaker, the timing test is done after the gas filling, with the SF6 density at its rated value, and before the on-site voltage test. Three control checks that depend on the same mechanism are added to the timing:

Minimum operating voltage: the closing coil must operate within the range of IEC 62271-1 (from 85% of the rated supply voltage) and the opening coil on DC from 70%. A coil that only operates at full voltage fails exactly when the battery bank is low.
Anti-pumping: with a sustained close command and an open command, the breaker opens and does not close again until the close command is removed and repeated. It is tested in local and remote control.
Interlocks and supervision: alarm and lockout for low SF6 density, trip circuit supervision, pole discrepancy and interlocks with the bay disconnectors.

In maintenance, the test is repeated by number of operations, by time in service or after interrupting major faults, and compared with the commissioning baseline.

Where it does not apply, or applies differently: on disconnectors and earthing switches interrupting times do not apply; their operation takes seconds and what is verified is that they complete the travel and that the auxiliary contacts change at the end. On low-voltage molded-case breakers, what is tested is the trip unit with current injection, not a mechanism time with an analyzer. And on breakers with controlled switching, the poles operate out of phase on purpose: the reference is the controller setting, not pole simultaneity.

Standards and criteria

IEC 62271-100 defines the timing quantities and has a pole discrepancy requirement during closing and opening, in addition to the rated operating sequence, for example O–0.3 s–CO–3 min–CO for breakers with rapid auto-reclosing. IEEE C37.09 establishes the breaker test procedures in North American practice. IEC 62271-1 sets the operating ranges of the releases: the closing release must operate between 85% and 110% of the rated supply voltage, and the opening release on DC between 70% and 110%.

The field criteria are:

Manufacturer values: each model has its ranges for opening time, closing time and difference between poles.
Baseline: commissioning times are the reference for maintenance. A sustained increase indicates a mechanism that is degrading.
Comparison between poles and with identical breakers.
Coordination with the protection: the actual opening time must be compatible with the times assumed in the study and with the 50BF setting.

What can distort the result

Control voltage different from rated: the time changes with the coil voltage. Record the voltage on every operation.
Command sent from a relay or a panel instead of directly to the coil: the times of auxiliary relays add up.
Ambient and mechanism temperature.
Operating the breaker before measuring the first trip.
Both sides grounded: with the earthing switches closed, the ground loop masks the contact change of state unless a method prepared for that condition is used.
Grading capacitors and pre-insertion resistors in parallel with the interrupting units, which the analyzer may read as a closed contact.
Induced voltages in air-insulated substations.

What to do if the result is poor

Repeat the operation at rated voltage and directly on the coils to rule out the measurement, and check the contact resistance if the poles were worked on. If it persists, analyze the coil current curve and the travel curve to locate the cause: coil, latch, springs, damper or friction. Mechanism maintenance follows the manufacturer’s manual, and then the times are repeated to set a new baseline. A breaker with out-of-range times should not be left in service without reviewing the effect on protection coordination.

Instruments for this test

This test is performed with instruments from recognized manufacturers; these guides explain what each one measures. In the quotation we recommend the test and the instrument your case needs.

Breaker timing with Tiza Group

Our team measures times, pole discrepancy and coil currents on medium-, high- and extra-high-voltage circuit breakers, in substations and industry, and delivers the result compared with the manufacturer’s values and with the unit’s history. Field testing is done in Colombia, or elsewhere on request. See the other tests in the primary equipment testing guide, learn about our circuit breaker testing service or write to us.

References

IEC 62271-100:2021, High-voltage switchgear and controlgear – Part 100: Alternating-current circuit-breakers.
IEC 62271-1:2017, High-voltage switchgear and controlgear – Part 1: Common specifications for alternating current switchgear and controlgear.
IEC 60050-441, International Electrotechnical Vocabulary – Part 441: Switchgear, controlgear and fuses (section 441-17, timing quantities).
IEEE Std C37.09, IEEE Standard Test Procedure for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis.
ANSI/NETA MTS, Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems.

Frequently asked questions

What is the opening time of a circuit breaker?

It is the interval between the instant the trip coil is energized and the instant the arcing contacts separate in all poles. The break time adds to that value the arcing time until extinction.

Why is the first trip measured before any other operation?

Because a breaker that has gone a long time without operating may have hardened lubrication. The first opening comes out slow and the following ones are already normal, so operating the breaker before measuring loses exactly the data that matters.

How much time difference is acceptable between the poles of a circuit breaker?

The one the manufacturer declares. IEC 62271-100 has a pole discrepancy requirement for closing and opening that serves as a reference when there is no other value. On single-pole-operated breakers used for controlled switching, the difference between poles is intentional.