Survey results on air-magnetic breakers
Author: Jeff Phelan
06/01/2005 - Volume II - Issue II
Over 100 of the T&D Guardian's end-user circuit breakers 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.
Respondents' most common maintenance interval on air magnetic breakers is three to five years which includes 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 here; 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 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 them with vacuum circuit breakers. 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, circuit 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 participants' 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 percentage 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 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 they perform on one manufacturer's circuit breaker versus another. Nonetheless, there was a significant percentage of users that apply more maintenance hours to certain manufacturers of air magnetic gear.
Roughly one quarter of our respondents have replaced over 25% of their installed base of breakers with roll-in vacuum replacement circuit breakers.
Despite their advanced age, medium-voltage air magnetic breakers have performed remarkably well; many over 50 year old circuit 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 that 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 breaker fails catastrophically, causing forced downtime and costly collateral damage to neighboring cubicles.
While it is difficult to apply hard probability of failure rates to air magnetic breakers, it is safe to say that modern vacuum technology is much less likely to fail than old air magnetic breakers (Siemens' testing indicates that mean time to failure of Siemens' vacuum bottles is over 36,000 years). 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 breakers should also be considered. This is a particular concern for the users that do keep up with OEM recommendations on maintenance. 20% of the T&D 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 utilizes the 3AF/3AH operator. The recommended maintenance interval for this circuit breaker is 10 years or 10,000 operations, whichever occurs 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 circuit breakers should consider the following inputs from installation to installation.