Circuit Breaker Timing And Time Travel Analysis Testing: the What, How, When, Why, AND Where

Circuit breakers are a critical component for protection against overloaded electrical and short circuits in our electrical infrastructure. When circuit breakers fail to perform their duties as designed and specified per application, the results can be catastrophic, expensive, and life-threatening. Every technician from a NETA Level 1 Trainee to a Level 4 Senior Technician understands the importance of testing circuit breakers to ensure they perform when called upon. 

In my last article for the Summer 2021 issue of NETA World, we discussed the “why” of circuit breaker testing. In this piece, let’s dig deeper into one specific set of testing: circuit breaker timing and time travel analysis (TTA). 

• I’ll first remind the reader what the tests are and what they measure, then how and when they are performed, including a snapshot of what some circuit breaker manufacturers say about time and TTA testing. 
• Next, we will discuss the “why” again and delve into what the tests can diagnose. 
• We’ll conclude with a look into the future — what we think the trends might be for breaker timing and TTA testing as innovations in switchgear and electrical safety and testing equipment continue. 

Let’s get started.

WHAT IS CIRCUIT BREAKER TIMING AND TIME TRAVEL ANALYSIS?

Circuit breaker time testing, termed by NETA as the contact timing test, is simple to understand. Its most basic definition is how quickly the circuit breaker operates when called upon to do so — that is, how quickly the breaker opens and closes. The purpose of a timing test is to determine the response time during the opening and closing operation of the breaker under test. 

Closing time is the time that passes between sending an electrical close signal to the close coil (when present) and the instant the contacts join — the arcing contacts in air circuit breakers, the primary contacts in vacuum and SF6 breakers. Likewise, the opening time is the time that elapses between initiating a trip signal to the trip coil and the instant those same contacts separate. The time test of a circuit breaker is measured in milliseconds (or sometimes cycles or hertz) and includes the time it takes for the respective coil to respond, the mechanical operating time of the mechanism to open or close, and the time it takes for the contacts to make contact or separate:

Breaker Operating Time = coil response time + mechanical response time + contact travel time

Now is a good time to mention that the breaker operating time and the actual interrupting time differ as the interrupting time also includes the maximum arcing duration. The rated interrupting time is defined in IEEE Std. C37.04,IEEE Standard for Ratings and Requirements for AC High-Voltage Circuit Breakers with Rated Maximum Voltage above 1000 V as the maximum permissible interval between the energizing of the trip circuit at rated control voltage and rated operating pressure for mechanical operation and the interruption of the current in the main circuit in all poles.

Interrupting Time = breaker operating time + worst-case arcing duration

Circuit breaker timing and time travel analysis testing calculates the breaker operating time; it does NOT calculate the interrupting time.

Time travel analysis testing, termed by NETA as mechanism motion analysis testing, is an extension of the contact timing test. A transducer converts linear and/or rotational motion to an electrical signal for measurement. Whereas the timing test can only provide the duration of the test, the transducer input transmits velocity as well. In addition to providing the timing values, TTA tests illustrate the actuation of the coil, velocity of the contacts, stroke of the contacts, penetration (wipe) of the contacts, contact bounce, and contact over/under travel.

Typically, the contact timing test and/or the mechanism motion analysis test are performed simultaneously on all three phases. Let us now take a detailed look at how the test is performed.

HOW IS CIRCUIT BREAKER TIMING AND TIME TRAVEL ANALYSIS PERFORMED?

Circuit breaker timing tests are performed with a high-speed recording device typically reading in millisecond precision and an AC and/or DC power supply depending on the control power ratings of the circuit breaker under test. Time travel analysis testing requires an additional motion transducer to record the travel in inches or feet and the velocity of the contacts in inches or feet per second. 

Time and TTA test sets are available from a host of manufacturers, including Vanguard Instruments, Doble Engineering, Megger, DV Power, and Vacuum Interrupters Inc. Circuit breaker timers are test sets that perform timing tests only. Meanwhile, more advanced test sets, often called circuit breaker analyzers, add TTA testing. The ease with which a test is set up and performed, as well as the nomenclature of the test leads and their connections, varies by manufacturer and model but is essentially the same in practice. 

• A START CIRCUIT is set up via the circuit breaker’s secondary disconnect to begin the timing measurement through the CLOSE and the TRIP CIRCUITS as applicable.
• A STOP CIRCUIT is set up through the circuit breaker primary disconnect(s) to indicate when to conclude the timing. 
• The breaker is charged either electrically or mechanically, and the test is initiated and recorded via the test set.

Here are the details of the test setup and what the technician should consider for each step:

1. When testing a metal-clad power circuit breaker in the field, remove the circuit breaker from the cubicle in accordance with the manufacturer’s instructions and all jurisdiction and industry safety regulations.
2. The test setups for the CLOSE and TRIP timing are identical for their respective circuits:
   • a. Power is provided via a reliable AC or DC power source to the CLOSE and/or TRIP CIRCUIT through a START CIRCUIT dry contact on the test set.
     i. Ensure the control power is de-energized prior to connecting the START CIRCUIT connections.
     ii. It is paramount that the tests be performed at the nominal voltage rating. When a power supply is used, it must be capable of producing full coil current without voltage drop. Unreliable and inconsistent power supply voltages will have a significant adverse effect on timing results. The importance of reliable, consistent control power cannot be overstated.
     iii. In the field and whenever possible, the preference is to utilize control power either from the cubicle control power or a test station using local control power.
     iv. The connections to the CLOSE and/or TRIP circuits are primarily made via alligator-style clips (or an adapter) directly to the secondary disconnects of their respective control circuits, requiring knowledge and/or drawings of the circuit breaker control schematic.
     v. Some test sets have onboard control power for the START circuit.
   • b. Connect the STOP CIRCUIT(s) by connecting leads to each phase under test.
     i. Some test sets require each phase to be tested individually, but most modern test sets allow all three phases to be performed simultaneously.
     ii. Simultaneous measurement of all three phases is preferred.
3. When TTA (mechanism motion analysis) testing is performed, install the transducer(s) in accordance with manufacturer instructions or industry best practices.
4. Charge the circuit breaker either electrically or mechanically in accordance with the manufacturer’s instructions and all jurisdiction and industry safety regulations.
5. Energize the control power.
6. Initiate the CLOSE timing test via the test set. Read, record, and evaluate test results.
7. Deenergize the control power.
8. Ensure the control power is de-energized, remove the START CIRCUIT leads from the START CLOSE CIRCUIT, and apply them to the START TRIP CIRCUIT.
   • a. Some test sets allow connecting the START CLOSE CIRCUIT and START TRIP CIRCUIT simultaneously.
9. Energize the control power.
10. Initiate the TRIP timing test via the test set. Read, record, and evaluate test results.
11. Deenergize all control power and secure all test connections. The test has concluded.

The reader may think we glossed over installing the transducer when TTA testing is required. There are many circuit breaker types and mechanisms combined with linear and rotary-style transducers from an abundance of test set manufacturers. Explaining where and how to install a transducer could be an article unto itself. In fact, Robert Foster from Megger authored such an article titled “Circuit Breaker and Transducer: Where Do I Connect?” in the Spring 2019 issue of NETA World. The article, and all past NETA World articles, are available at no charge at www.netaworldjournal.org. I encourage the reader to dive into transducer placement, but not until after we continue our discussion into when the testing is recommended.

WHEN IS TESTING RECOMMENDED?

The requirements for when to perform circuit breaker timing and TTA testing are listed in ANSI/NETA ATS–2021, Standard for Acceptance Testing Specifications for Electrical Power Equipment & Systems, and ANSI/NETA MTS–2023, Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems. Table 1 summarizes their content.

Some highlights to note from the table:

• Timing and TTA testing are not required in insulated- and molded-case or low-voltage power circuit breakers in any case but are required in all OIL-type breakers.
• The acceptance and maintenance testing standards are aligned except that TTA is a required test in SF₆ circuit breakers during acceptance testing, but optional during maintenance testing.

While NETA testing standards are an excellent guide, especially in the absence of manufacturer recommendations, to quote the standards: 

It is important to follow the recommendations contained in the manufacturer’s published data. Many of the details of a complete and effective testing procedure can be obtained from this source.

As you can see, manufacturer recommendations for contact timing tests are all over the board. Many do not require testing at all or give a specification for qualifying test results. Only one manufacturer, Westinghouse, gave TTA guidance on their DHP model breaker. However, some manufacturers do provide at minimum an opening/closing time specification with a few at least providing guidelines on how and when to perform the test. 

For more information regarding OEM stances on timing tests, I recommend an EPRI report: Circuit Breaker Timing and Travel Analysis. This report is available for free download on their website (EPRI.com) and includes guidance from several manufacturers. 

But the big question remains: If there is so much inconsistency in timing tests between manufacturers, why perform them in the first place? We’ll tackle that question now.

Manufacturer Instruction and Maintenance Manuals

This section lists contact timing and travel tests from a cross-section of medium-voltage circuit breaker instruction/maintenance manuals. 

Siemens Model GMSG (Vacuum)

• No testing requirement/recommendations given
• Opening/closing time published (≤56 ms (5-cycle) ≤33 ms (3-cycle) / ≤65 ms)
• Siemens Model FSV/MSV (Vacuum)
• No testing requirement/recommendations given
• Opening/closing time published (2.0/4.5 cycles 33/75 ms)  

Siemens Model FC (Air) 

• A comparison of circuit breaker timing at any period of maintenance with that taken when the breaker was new will indicate the operational condition of the breaker mechanism. A time variance of more than ½ cycle on opening and 2 cycles on closing indicates a maladjustment or friction buildup. A hole in the movable contact arm is provided for connecting a speed analyzer.
• No opening/closing time published

Eaton Model VCP-W (Vacuum)

• Timing test recommended
• Opening/closing time published (30–45 ms / 45–60 ms)

Westinghouse Model DHP (Air)  

• The mechanical operating speed of the breaker should be satisfactory as received. Some users include timing as part of inspection and maintenance. If or when a mechanical timing check is made…the following values and limits should be obtained. Contact speed and separation should be measured at or referred to the extreme tip of the moving arcing contact. 
• Closing speeds are provided from 6.5 ft/sec to 10.5 ft/sec depending on momentary current rating

GE Model AM Magne-Blast (Air)

• No testing requirement/recommendations given 
• No opening/closing time published

GE Model VB1 PowerVac (Vacuum) 

• Timing may be checked by monitoring control circuit voltage and using no more than six volts DC and one ampere through the vacuum interrupter contacts to indicate closed or open condition. 
• Typical time ranges vary with coil voltage: Opening/Closing time (35–45 ms / 60–90 ms)

ABB/BBC Model HK (Air) 

• After operation intervals noted previously or a change in bridge pivot adjustment, it is recommended that opening and closing time be checked by use of a cycle counter, time-travel analyzer, oscillograph, etc. to monitor the time from energizing to arcing contact touch or separation.
• Opening/closing time published (50–95 ms / 23–35 ms)

ABB Model AD/AMVAC (Vacuum) 

• No testing requirements/recommendations
• No opening/closing time published in maintenance manual, present in technical guides
• Opening/closing time published ADVAC (22.9–38.6 ms / 30–80 ms) / AMVAC (11.8–33.7 ms/ 30–80 ms) dependent on ratings

WHY CONDUCT CIRCUIT BREAKER CONTACT TIMING TESTING?

Compared to the consistency manufacturers and industry place on acceptance testing of circuit breaker insulation resistance or contact resistance, timing tests can seem almost second tier, especially in medium-voltage applications. So why take the trouble?

The most obvious answer is that it is required by NETA organizations that follow the ANSI/NETA ATS and ANSI/NETA MTS testing standards and those OEMs that recommend it. A better answer, however, is what the test can tell us about the circuit breaker’s condition — especially a breaker that has had a mechanism repaired, components replaced, was re-adjusted to factory settings, or all the above after overhaul or a reconditioning process beyond standard preventative maintenance. 

The most prevalent and important issues timing tests can diagnose are those caused by the mechanism. A study by a CIGRE working group concluded that more than 50% of major circuit breaker failures were due to the operating mechanism (30% were attributed to control/auxiliary circuits). Working for an organization that has specialized in power circuit breakers for over 40 years, I can attest to the accuracy of those statistics.

Circuit breaker timing and travel analysis tests can highlight circuit breaker mechanism issues in breakers that otherwise appear to be operating normally. Examples of issues timing tests can diagnose include:

• Mechanical binding
• Improper trip and close latch adjustment
• Improper wipe adjustment
• Improper mechanism adjustments
• Bent, broken, or worn parts
• Incorrect installation or misapplied parts
• Friction or lubrication issues
• Faulty or failing trip and close coils
• Arcing and primary contact over- or under-travel
• Excessive contact rebound
• Improper contact pressure
• Improper contact penetration
• Misalignment between phases

If this list is not persuasive enough of the value of circuit breaker timing tests, let’s not forget the most important justification: ensuring that the breaker is operating quickly enough to perform as rated.

The validity of circuit breaker short-circuits and related ratings depend on the breaker operating speeds at which it was designed and qualification tested. Most manufacturers attest that when a circuit breaker is properly maintained in acceptable physical condition with all critical adjustments set within specification, the breaker will perform reliably over the course of its lifetime. However, if any of the mechanism conditions or adjustments are unsatisfactory, the breaker may still operate but not necessarily within its design. 

Measuring opening and closing contact timing and velocity via primary contact timing and time travel analysis testing offers an excellent field test that assesses the overall status of the circuit breaker. If the breaker passes all other testing and is mechanically sound and factory-adjusted with a contact opening time that meets or exceeds manufacturer requirements, we can be confident that the circuit breaker’s short-circuit capability — its primary function — will perform as expected.

In addition to the validity of short-circuit ratings, there is also a time effect on arc flash hazards. The validity of arc flash studies and labels is based on circuit breaker interrupting times. Arc energy is directly proportional to clearing time. This means the longer it takes to clear an arc flash, the more intense the energy can become and the more devastating the incident energy can be. We are all familiar with the term “Distance Is Safety” regarding arc flash hazards but ensuring that breakers are operating at the speeds on which arc flash studies are based is equally true.

We have covered the what, the how, the when, and the why. Let’s complete our discussion with the where — not where to perform the test, but where timing tests are going today.

WHERE ARE CIRCUIT BREAKER CONTACT TIMING TESTS GOING?

Throughout the existence of circuit breakers, an overwhelming number of innovations have been to their interrupting method. Breakers started as simple knife switches and a fuse link and progressed to the first resettable fuses before interruption capacity necessitated it be done in oil. After surpassing oil interruption technology, air-magnetic interruption technology ruled. Finally, we reached our current vacuum and SF₆ interruption technology. Test equipment and practices followed along with it, and timing tests became an important means to ensure the decreasing interrupting times resulting in faster circuit breaker operating speeds were maintained. 

Circuit breaker testing — the actual hands-on practical testing — has also progressed. Safety practices have certainly improved, and test sets have advanced dramatically. Test sets have gone from heavy, cumbersome, analog, manually operated devices to digitally controlled (but still heavy), slightly less cumbersome, and at least partially automated processes. Despite these advances, the testing process is still as it was 50 to 100 years ago — a manual hands-on process that requires taking the equipment out of service, testing it, and returning it to service.

One of the largest innovations looks to change testing from off-line current time to on-line real-time via circuit breaker status monitoring systems. On-line circuit breaker monitoring systems seek to detect failures before they occur by monitoring breaker operating characteristics such as SF₆ gas, trip and close coil current, operating temperature and humidity, operations count, the presence of partial discharge, and more, but they are especially adept at capturing circuit breaker timing characteristics. 

Transferring timing tests from off-line to on-line makes sense as the start and stop circuits we delved into earlier are already in place. Circuit breaker status monitors are an established technology for SF₆ GIS switchgear, and that technology is trickling down into the medium- and low-voltage product segments. Medium- and low-voltage relays installed on new equipment or retrofitted into legacy equipment allow the user to monitor circuit breaker operating and interrupting times, compare them to expected, and inform the operator when poor conditions exist or are trending that way. 

For example, SEL-700 series relays available from Schweitzer Engineering Laboratories are available for monitoring on-line breaker trip timing in medium-voltage applications, and the AC-PRO-II low-voltage trip unit available from Utility Relay Company includes Sluggish Breaker® detection that warns the user when tripping times fall below expectations. This is especially valuable in monitoring first-trip timing, which is the initial circuit breaker opening time after a breaker has been sitting in the closed position for an extended period, as many breakers may sit for months (or even years) without operation. It is only a matter of time before continuous monitoring of circuit breaker timing is an expected feature from all breaker and relay manufacturers.

CONCLUSION

We have taken an in-depth look at circuit breaker timing tests as space will allow. I have tried to touch on as much of the topic as possible, but it would take the entirety of this publication and more to cover every circuit breaker, every test set, and every aspect. If any of the points have piqued your interest, I encourage you to take the time to research the subject further. There are countless resources online from reliable sources.

Circuit breaker timing tests are an invaluable tool in the technician’s battery of tests for troubleshooting and ensuring the breaker will safely do its job when called upon. For good reason, it is the last step in ANSI/NETA ATS and ANSI/NETA MTS visual and mechanical inspection procedures. The tests are that final checkbox to ensure that the breaker is operating as it should after all maintenance, repair, or reconditioning is performed. 

REFERENCES

[1] Grein, Paul. “When to Consider a Retrofit Replacement Circuit Breaker,” NETA World, Fall 2017. Accessed at https://issuu.com/netaworldjournal/docs/17-fall_nwj, pp 52–60.

[2] IEEE Std. C37.04,IEEE Standard for Ratings and Requirements for AC High-Voltage Circuit Breakers with Rated Maximum Voltage above 1000 V.

[3] Khan, Hammad, Tjandra, Anastasia, Iu, Herbert, and Sreeram, Victor. “A Novel Island Detection Methodology for the Realization of Smart Grid,” Smart Grid and Renewable Energy, 02. 10.4236/sgre.2011.24038.

[4] Megger Sweden AB. EGIL Circuit Breaker Analyzer User’s Manual, p 35. Accessed at https://www.megger.com/sites/g/files/utfabz201/files/acquiadam_assets/2018-04/EGIL_UG_en.pdf?changed=1669710315.  

[5] Foster, Robert. “Circuit Breaker and Transducer: Where Do I Connect?” NETA World, Spring 2019, pp 42–55.

[6] ANSI/NETA ATS–2021, Standard for Acceptance Testing Specifications for Electrical Power Equipment & Systems. Accessed at https://www.netaworld.org/standards/ansi-neta-ats. 

[7] ANSI/NETA MTS–2023, Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems. Accessedat https://www.netaworld.org/standards/ansi-neta-mts. 

[8] EPRI. Circuit Breaker Timing and Travel Analysis, TR-1127, Final Report, May 1999, p 3-2. Accessed at https://www.epri.com/research/products/TR-112783.

[9] CIGRE Working Group A3 06. Technical Brochure 83: Intermediate Results from On-Going CIGRÉ Enquiry on Reliability of High Voltage Equipment. Accessed at https://mtec2000.com/cigre_a3_06/Rio/Rio2.pdf. 

Paul Grein is Vice President of Group CBS and has been with the company since 2008, working primarily in the Dallas area. His primary responsibilities include business development, engineering design and management, technical expertise, standards, and project management. He has worked with industrial electrical equipment for over 20 years, beginning in the Navy as a nuclear-qualified Electrician on the submarine USS Topeka SSN 754 from 1996 through 2002, followed by positions in the steel industry through 2005. Grein has a BSEE from the University of Texas at Dallas (2007) and an MBA from the University of North Texas (2014). He participates in the IEEE/ANSI PES C37 Standards Committee, which publishes and maintains the design and testing standards that govern the industrial power equipment industry.