Low-voltage circuit breakers are predominant in virtually every electrical power system. Several years ago, Shermco Industries surveyed NETA-accredited companies for the IEEE Electrical Safety Workshop and published the results (IEEE ESW-2008-21). The NETA survey and associated IEEE paper showed that 22% of these breakers did not follow their time-current characteristics, and 10.5% did not operate when initial service-aged maintenance was performed. The majority of failures were mechanical, which indicates regular maintenance must be a priority.
Low-voltage air circuit breakers (LVACBs) come in a wide variety of ratings and styles from numerous manufacturers, and it can be a challenge to ensure these devices are tested to operate within their time-current bandwidth.
The 2021 edition of CSA Z462, Workplace Electrical Safety, offers requirements for these devices to be assessed on a case-by-case basis as outlined in an electrical maintenance program such as the example in CSA Z463, Maintenance of Electrical Systems. It is always advisable to use competent maintenance personnel (such as those employed by NETA-accredited companies) for field testing that follows the latest ANSI/NETA maintenance testing specifications for the assessment of electrical power equipment and systems.
Why LVACBs are a higher-risk item
A recent assessment by the CSA Z463 Technical Committee deemed the rackable LVACB a device that carries a higher risk than most other devices in an electrical power system. This device is quite often operated by maintenance staff to establish an electrically safe work condition. LVACBs have an onboard protection system, including a series trip unit or an electronic trip unit consisting of current transformers, a wiring harness, and a protective relay as well as some type of mechanical actuator to open the device.
When not exercised regularly, an LVACB can become inoperable, without any prior warning or indication to the operator. Combining the features of rackable, frequently operated, and on-board complete protection systems makes the likelihood and impact of failure one of the highest exposure points in the electrical power system (Photos 1a and 1b).
2021 CSA Z462
The chapter on “Safety Related Maintenance Requirements” in the 2021 edition of CSA Z462, The Standard for Electrical Safety, has been revised to emphasize electrical maintenance that directly affects worker safety.
“Condition of maintenance” is to be considered numerous times during the risk assessment process. The biggest issue in the industry is whether the equipment trips to clear a fault at the time specified. To ensure that it does, maintenance personnel must perform inspection, testing, and verification to prove the method chosen to determine the incident energy exposure is valid. When not properly maintained, these devices often operate slower — or, in some cases, not at all — especially when service-aged in an industrial operating environment or due to other factors, such as age, lack of maintenance, cleanliness, etc. When protective devices do not follow their time-current characteristics, workers can receive a false estimate of the incident energy exposures and may choose the required personal protective equipment (PPE) incorrectly from the arc flash and shock label or the table method.
The operating speed of a protective device is a major factor. Maintaining protective devices is critical to a worker’s safety because it affects their ability to determine the true value of incident energy exposures in the field. Tables list typical fault-clearing times of overcurrent protection devices — all dependent on equipment maintenance and proper operation. For this reason, Z462 has upgraded its safety-related maintenance clause to focus on items that affect worker safety, not just general maintenance requirements.
The 2021 edition references the maintenance requirements for low-voltage circuit breakers. A removable LVACB aligns with all these requirements for maintenance:
- Protective devices “shall be maintained to function in accordance with their designed operating times.”
- Overcurrent relays, commonly called trip units on low-voltage breakers, “shall be tested to operate in accordance with their time-current characteristics.”
- Ground fault protection “shall be inspected and tested to ensure they are functioning correctly and within the trip or alarm time specified.”
- Key interlocks “shall be tested for proper operation.”
- Mechanical systems that need regular exercising “shall be inspected, exercised, tested, and maintained to ensure proper electrical and mechanical operation and circuit isolation of the contacts and mechanical operation of the racking mechanisms and, where applicable, shutter mechanisms and safety interlocks.”
- Settings “shall be reviewed to ensure they meet the protection coordination study and incident energy analysis.”
Maintenance survey
At the end of calendar year 2020, Shermco Industries surveyed a group of NETA technicians about their issues and most recent failures with low-voltage power circuit breakers. The breakers were from a wide variety of manufacturers, vintages, and industry types. Some highlights of the survey included:
- Mechanical operation is always an issue, often due to improper lubrication, lack of exercise, wear on the main operating mechanism, or even overlubricating — particularly when applied to the finger clusters instead of the stabs, which may be energized or not accessible (Photo 2). It is extremely important to understand the consequences of lubricating the clusters versus the stabs or using dissimilar lubrications, as this frequently causes contact or racking problems.
- Sluggish initial operation for the first couple of operations can usually be avoided with regular maintenance of the operating mechanism and proper lubrication (Photo 3).
- Trip unit failures are common across the board with all types and manufacturers. These failures are discovered during time current testing through primary or secondary injection.
- Damage to contact surfaces from years of maintenance scrubbing with scratch pads for no reason other than to make it shiny removes the silver plating. Be very careful when performing maintenance on silver-plated surfaces.
- Generator breakers, specifically those mounted on the same chassis as the generator, tend to have a higher failure rate. This is often a mechanical failure that causes the breaker to be inoperable.
- Coil failures can include a failed undervoltage coil or trip coil as part of the trip and control scheme.
- Vintage parts are becoming more difficult to acquire from the original manufacturer; this applies to most makes and models. It is always good to have a local NETA service company or a NETA-affiliated used equipment vendor available.
Ease of maintenance
Not all LVACBs are created equal. Many breakers can be quite easy to maintain, while others can be much more difficult; it depends on the installation. In the author’s opinion, the following circuit breakers are listed from the easiest to hardest to perform electrical testing on. The list is based on ease of access, shutdown required, and testing with secondary injection versus primary injection among other factors.
- Rackable power circuit breakers with solid state or electronic trip units.
- Bolt-in insulated-case power circuit breakers with adjustable solid-state trip units and manufacturer’s test sets.
- Bolt-in molded-case circuit breakers with electronic trip devices, adjustable settings, and secondary injection test ports.
- Rackable power circuit breakers with series trip units (oil or air dash pots or other electromechanical trip units).
- Bolt-in power circuit breakers with oil or air dash pots or other electromechanical trip units.
- Bolt-in molded-case circuit breakers with electromechanical trip units.
Conclusion
Low-voltage circuit breakers, which typically include trip units or integrated tripping devices, should be tested to operate within their time-current bandwidth or per alternative equivalent methods recommended by the manufacturer of the trip unit or circuit breaker. Industry standards are also great tools to use for proper maintenance requirements.
Just assuming electrical power distribution equipment will operate as designed without verifying operating conditions can result in an unexpected increase in incident energy values. In some cases, devices do not operate at all. The lengthening of operating time vastly increases incident energy values, arc flash boundaries, and PPE requirements. Regularly performing basic maintenance tasks, such as proper visual and mechanical inspections and electrical tests, are essential to ensure low-voltage circuit breakers can provide their important safety functions.
This article was provided by the InterNational Electrical Testing Association (NETA), www.NETAworld.org. NETA was formed in 1972 to establish uniform testing procedures for electrical equipment and systems. Today the association accredits electrical testing companies; certifies electrical testing technicians; publishes the ANSI/NETA Standards for Acceptance Testing, Maintenance Testing, Commissioning, and the Certification of Electrical Test Technicians; and provides training through its annual conferences, PowerTest and EPIC – Electrical Power Innovations Conference, and its expansive library of educational resources.
