Not all equipment is created equal when it comes to production objectives versus safety objectives in an electrical power system. Certain devices should be tested and maintained to ensure uptime; others should be inspected to ensure they perform properly to protect workers in an arc flash event. When considering arc flash safety and selecting the associated PPE, the condition of maintenance for protective devices or circuit breakers is certainly much more critical than for a conductor or load device. Evaluating each piece of power system equipment for its impact on productivity and safety is clearly important.
Whether using an arc flash study, the table method, or another process to determine arc flash values, PPE selection is based on a value for incident energy. Incident energy values are based on a combination of circuit clearing time, distance from the source, and the amount of arcing current in Amperes (among other factors).
Since clearing time is one of the direct critical factors, every single PPE selection is based on the equipment operating speed. In an arc flash study or other method, this speed is based on the equipment’s time current operating characteristics to be in like-new condition. Even equipment that operates a fraction of a second slower can cause arc flash values to be substantially higher than anticipated or indicated on a label.
What does this mean? A worker can believe the incident energy is single-digit calories and dress accordingly. However, the actual clearing time of the unmaintained upstream device can cause this value to be hundreds of calories. Not verifying clearing times essentially makes the arc flash label or table method of PPE selection invalid.
Condition of maintenance is now a key consideration in the overall risk assessment process. But how does this affect worker safety and PPE selection? A 2008 paper on IEEE ESW-2008-21 by Ron Widup and Kerry Heid used survey data from NETA members with results from over 340,000 protection devices. With service-aged equipment, approximately 22% of circuit breakers had an issue with the protective trip unit. Experience tells us that trip units usually operate slower, not faster, so the arc flash energy values downstream of these devices are higher than expected. Even more alarming: 10.5% of circuit breakers would not trip at all, and 42.8% of these problems were mechanical. A circuit breaker that has not operated in years may not operate at all when called upon during an arc flash event. An inoperable protective device will create a high probability of extreme values of incident energy. It’s a scary scenario when selecting PPE.
Table 1 lists six asset classes that must be considered differently based on production versus safety objectives. Items such as cables, rotating equipment, and transformers can have a huge negative effect on production but typically aren’t as likely to create a major safety risk. However, power systems apparatus such as circuit breakers, protective relays, and DC tripping power can directly affect electrical arc flash values and PPE selection. The Canadian standard CSA Z463, Maintenance of Electrical Systems states that these critical safety devices must be included in the electrical maintenance program. The type and interval of maintenance is left up to each individual user based on the installation and operating environment. Production devices can be maintained at a different level based on the business decision by the organization, but devices that negatively affect electrical safety cannot be ignored.
Productivity and uptime can be drastically affected by these items. Failure of these devices can cause power outages or lost production for days and even weeks depending upon the ability to replace or repair them. This will have a major financial impact on the organization, but typically, a limited impact to personnel safety. Note that the devices above in the Safety column can also affect production/uptime.
The items listed under Safety can fail without anyone knowing they failed and thus won’t operate to clear a fault, but production continues on as normal. Failure of these devices to trip and limit the energy during an arc flash event creates a huge increase in the risk to the worker. These failures are normally only found during maintenance or when a fault occurs. For example, a transformer fire could be dangerous to workers. But what is the likelihood that a transformer would fail with someone standing directly beside it? In addition, for a transformer to catch fire or to catastrophically fail would mean the protection failed, so the risk is associated with the protection, not the actual transformer failure.
Electrical Safety Heat Map
CSA Z463, Maintenance of Electrical Systems features a heat map analysis in Annex C (Figure 1). Maintenance for safety objectives can be completely different than those for production objectives. This is exactly what the safety heat map establishes for separate asset classes. This system establishes unmaintained equipment in the green (low-risk), yellow (moderate-risk), and red (high-risk) zones for their effect on the electrical safety program.
Each piece of apparatus has a different risk factor for production and for safety, and it’s typically a business decision for production or uptime versus a safety decision for equipment that poses a risk to personal safety. The values in the tables are subjective and need to be aligned with the installed equipment vintage, loading, fault interruptions, operating environment, and apparatus manufacturer to rate the likelihood and impact of failure.
The safety heat map plots the likelihood of an occurrence versus the impact the device would have on worker safety should it become inoperable (Figure 1).
- Likelihood is the chance the equipment would not work as intended (mis-operate) if it was completely ignored over a lengthy in-service period.
- Impact is the potential exposure to a worker should the equipment not work as intended.
Low-Voltage Air Circuit Breaker: High Risk
The on-board protection system in a low-voltage air circuit breaker (LVACB) includes a series trip unit or an electronic trip unit consisting of current transformers, wiring harness, protective relay, and some type of mechanical actuator. Maintenance staff must operate it on a regular basis to create an electrically safe working condition. When not exercised on a regular basis, these devices can become inoperable without warning. Because they are rackable, frequently operated, and include a complete on-board protection system, the likelihood and impact variables on these devices are some of the highest in the electrical power system. As noted earlier, a 2008 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 at all when service-aged maintenance was undertaken.
Power Fuse: Moderate Risk
Power fuses are protective devices that have no movable parts and are generally very reliable. However, they can create extended clearing time above expectation if not applied properly following a replacement-in-kind process. Electrical workers commonly replace a fuse with one that is adequate for the loading level but is not the correct size or rating as designed in the power system study or arc flash hazard analysis. This can create a much higher arc flash value than anticipated.
Buried Underground Cable: Low Risk
Buried underground cable has limited arc flash risk and associated PPE level because the likelihood of failure and impact to personal safety are both very low. These devices have no moving parts and are highly reliable if installed correctly. The likelihood of a failure is typically very low, and the impact this device could have on safety during a failure is quite low.
As seen is these examples, the attention paid to maintenance depends on the likelihood the equipment will fail and the impact that it will have on worker safety. Power systems equipment that is more likely to fail and have a higher impact must be included in the electrical maintenance program due to the serious safety concerns it can bring.
An arc flash hazard analysis will usually indicate that protective setting changes can reduce the incident energy values and thus lower the level of PPE required. However, if these protective devices do not function as designed due to a lack of maintenance, the new energy levels are invalid and render the PPE assessment inadequate. Arc flash studies should always indicate that any setting changes made to lower incident energy require testing of the devices to ensure they follow their time current characteristics.
Certain apparatus in a power system can have a drastic negative impact on incident energy evaluation and the associated level of required PPE when not maintained properly. Field test data shows that a lack of maintenance can severely affect the operation of protective devices. Any item that can directly extend the tripping time of a protection system needs consistent maintenance and testing to ensure it operates according to its specified time current characteristics. PPE selection is completely dependent on this equipment working properly, and an electrical maintenance program is critical to this operation.
Kerry Heid is an Executive Consultant at Shermco Industries, a leader in electrical power systems reliability, engineering, and field services. After beginning his career with Westinghouse Service, Kerry founded the Magna Electric Corporation (MEC) office in Regina, Saskatchewan, in 1996 and became President of the company in 2001. The company grew to over 1,000 employees and won many awards as one of Canada’s 50 Best Managed Companies and Canada’s Top 100 Employers. MEC was acquired by Shermco Industries in December 2013, and Kerry served as CEO of Shermco Industries Canada until 2019. Kerry is a NETA Certified Level IV Technician and is active in Canadian standards development. He has served as Chair of the CSA Z463, Maintenance of Electrical Systems technical committee since 2010, as a member of the CSA Z462, Workplace Electrical Safety technical committee since its inception in 2006, and received the prestigious Award of Merit from the Canadian Standards Association in 2019. Kerry served on NETA’s Board of Directors from 2003–2014, is a past-President, and received NETA’s Outstanding Achievement Award in 2010.