Survey results on air magnetic circuit breakers

Author: Jeff Phelan

06/01/2005 - Volume II - Issue II

Over 100 of T&D Guardian's end-user readers participated in our recent medium-voltage (5-15kV) air magnetic breaker maintenance survey. Despite an increased trend towards outsourcing of services, over 90% of our survey's respondents do all or at least some of their medium-voltage circuit breaker maintenance in-house. The typical survey participant is responsible for maintaining  a variety of models of air magnetic breakers made by several different vendors.

Maintenance practices

 

Respondents' most common maintenance interval on air magnetic breakers is 3-5 years which the following activities: lubricating the mechanism, measuring contact gap, adjusting contact travel, measuring insulation, and contact resistance. As one might expect, the OEM recommendations are that these activities be done on a more regular interval (typically annually).

 

Maintenance issues on air magnetic breakers run on opposite ends of extremes. The one extreme involves applications where the number of operations contributes to the circuit breaker reaching its end of useful life. About 12% of our readers operate their air magnetic breakers more than 50 times per year. Given that these circuit breakers are maintained in less than ideal conditions for over 25 years, many of these end users determined that these circuit breakers reached their end-of-life and replaced with vacuum. The other extreme is a more common yet less obvious maintenance issue; lack of activity. Almost half of the Guardian's survey respondents operate their circuit breakers less than once every three years.

Ideally, cicuit breakers should be cycled twice per year to keep lubricants from hardening and prevent seizing of linkages. This prevalent lack of activity might explain survey participant's needs for replacement parts.

Parts such as trip coils commonly gum-up with contaminants and can fail to operate if not cycled for several years. Perhaps not surprisingly, the percentages of respondents not regularly cycling their breakers (47%) is the same as the percentage that need to replace their trip coils.

 

OEM recommended intervals are often extended by end-users due to cost constraints and difficulty in scheduling downtime. Even within these constraints; however, low level maintenance activities can still be done to limit risk of failure. The common sense approach of keeping circuit breakers clean, dry, and lubricated can avoid costly down time for a minor investment. Environmental intrusions such as rodents, coal dust, and humidity frequently contribute to catastrophic failures.

 

Most of our respondents do not vary the amount of maintenance that they perform on one manufacturer's circuit breaker versus another. Nonetheless, there were significant percentage of users that apply more maintenance hours to certain manufacturers of air magnetic gear.

Giving up the ghost

 

Roughly one-quarter of our respondents have replaced over 25% of their installed base of circuit breakers with roll-in vacuum replacement circuit breakers.

Conclusions

 

Despite their advanced age, medium-voltage air magnetic breakers have performed remarkably well; many 50+ year old breakers remain in service today. Although their performance has been impressive, the increasing trend towards reducing (or even eliminating) maintenance justifies an analytical comparison of the benefits of replacing these circuit breakers with vacuum technology. Note, 5-10% of our readers "never"  perform some of the most basic tasks like lubrication or measuring contact resistance. The goal of reduced maintenance is best achieved after investing in modern hardware designed around this new paradigm. The folksy adage, "If it ain't broke, don't fix it," does not ring true to a plant manager after a 30 year old circuit breaker falls catastrophically causing forced downtime and costly collateral damage to neighboring cubicles.

 

While it is difficult to apply hard probability to failure rates to air magnetic breakers, it is safe to say that modern vacuum technology is much less likely to fail (Siemens' testing indicates that mean time to failure of Siemens' vacuum bottles is over 36,000 years) than old air magnetic breakers. Furthermore, the fact that the arc is more contained in a vacuum breaker reduces the consequential damages that might occur should a failure take place.

 

After reliability concerns, the cost of maintaining air magnetic breakers verse vacuum should also be considered. This is a particular concern for the user's that do keep up with OEM recommendations on maintenance. 20% of the Guardian's readers spend 28 man hours of labor every other year, not to mention parts costs, in maintaining their air magnetic breakers. Compare this with the Siemens family of roll-in replacement circuit breakers which utilities the 3AF/3AH operator. The recommended maintenance interval for this breaker is 10 years or 10,000 operations, whichever occur first; while the circuit breaker's mechanical endurance is rated for 30,000 operations.

 

Repair / replace / maintain decisions are driven by variables such as criticality of application, age, model, number of operations, safety, and environment. A thorough evaluation of air magnetic verse vacuum should consider the following inputs from installation to installation.

 
Capital / O&M investment
Maintenance costs
Risk = [(Probability of failure) x (Estimated cost of downtime + repair)]
Over duty
Replace with Siemens 3AF/3AH operator
Typically $10-20K per circuit breaker
Typically 4-8 man hours every 10 years
Lower probability of failure, lower cost/time to repair, less damage to cubicle(s)
MVA upgrades available
Repair / maintain existing air magnetic circuit breaker
None
Typically 16 man hours every 3-5 years plus unpredictable cost of parts
Higher probability of failure, more consequential damage, longer repair time
Limited options