The Debate Over Circuit Breakers vs. Fused Solutions Continues

12/01/2005


I have a number of reservations about the technical issues in Mr. Lane's articles, which broke down changes to the NEC and suggested fused distribution may not always be necessary when considering selective coordination requirements.

Both circuit breaker (CB) and fused systems provide safe and reliable overcurrent protection when systems are carefully engineered, installed and maintained. But there are significant differences that must be recognized.

The first problem with all three articles is they do not look at a complete system. The one-line diagram in part two of the series, The Skinny on Switchgear and the New NEC ,reaker would require a short delay trip set at 12 cycles. The vast majority of transformers in commercial and industrial systems are protected on the primary side by current-limiting fuses. What would the coordination be? It's difficult enough to coordinate CB having instantaneous trips with primarycurrent-limiting fuses.

If the primary fuses open—admittedly, this rarely happens—the system is down until the utility replaces the fuses, and only after determining there is no problem in the transformer or main switchgear.

Any facility requiring an 800-amp CB for the emergency system must be fairly large. It is reasonable to assume a 2.5% impedance high-efficiency 1,500- or 2,000-kVA transformer. Available short-circuit current could easily be more than 75,000 amps. What is the cost of CB switchgear-type construction opposed to a bolted pressure switch and fuse switchboard? Considering only space, fused switchboards are readily available. They may actually take less total space than CB switchgear and be less expensive. The use of Class-J time-delay current-limiting fuses may also reduce the size of equipment. What about the short-circuit rating of other components such as busway? How will they be affected by the short-delay trips? It is common in high-rise construction to run busway from the main switchgear to distribution panels or to use plug-in busway to feed main-lug only (MLO) branch panels.

The same diagram shows an 800-amp CB with short delay trips protecting the automatic transfer switch (ATS). Will the ATS have an adequate short-circuit rating? One ATS manufacturer tests and UL lists its units only when protected by current-limiting fuses or CB with clearing times of less than three cycles. Generators commonly have short-circuit X/R ratios of 20:1, and the asymmetrical short-circuit current may exceed that of breaker or ATS.

All of the time-current data sheets, with one exception, only show a maximum short-circuit current of 10,000. This gives a distorted impression of the system coordination. If available short-circuit current is only 10,000 amps, it is easy to assume that a fault could be less than 2,000 amps. With 50,000 amps available, a "minor" fault could easily be 5,000 amps or more. Also, the coordination studies do not accurately indicate the conditions. The time-current curves for the dual-element fuses are inaccurate. They do not indicate the time delay portion of the curve. When attempting to coordinate systems, time-current curves must be accurate. Available short-circuit values should also be indicated on the T-C curves, as should the primary transformer protection.

Also, for higher values of current-opening times of less than 0.01 seconds, the shape of fuse curves changes significantly. Simply said, the instantaneous trip of standard thermal-magnetic breakers does not truly respond to the strength of the magnetic field.

The field for short periods of time—less than 0.01 seconds—depends on the rate of rise as well as the peak current. Published data and tests show that molded-case circuit breakers may unlatch in 0.001%%MDASSML%%0.002 seconds. Published low-voltage fuse curves are average melting time curves. Minimum melting time and maximum clearing time curves are not generally available. At currents high enough to cause the fuse to open in less than 0.03 seconds, arcing time increases in relation to melting time. For example, with a 10-sec. fuse opening time, melting time is about 9.999 seconds, arcing time about 0.001 seconds. As current increases, melting time may be 0.002 seconds and arcing time 0.006 seconds. Tests have shown that fuses on the load side of thermal-magnetic MCCB may not prevent the CB from opening.

Figure 3 in part one (8/05, p.24) does not show dual-element fuses as indicated in the text. It shows very fast-acting non-time delay fuses. This becomes very important when coordinating systems. Mixing various fuse classes with or without CB can destroy coordination.

Mr. Lane makes some good observations about coordinating small CB-to-CB systems. As opposed to the 225-amp panelboards used in the examples, lighting panels in typical emergency systems are 100 amps. Thus, any fault on the load side of a branch breaker exceeding 900 amps (90% of 1,000 amps) may open the CB feeding the panelboard.

The time required to restore power to a fused system after a short-circuit outage may actually be less than that for a CB system, especially if it is not coordinated. First, consider a coordinated fused system. It is easy to determine the approximate location of the fault or overload. Once located and corrected, the switches need to be tested and serviced as necessary, then replaced with new ones.

In a non-coordinated electrical system two or more circuit breakers may open. The location of the fault is in question. Consider an emergency system with all thermal-magnetic CBs. A 400-amp CB feeds a subpanel of four 100-amp CBs that feed four 100-amp MLO panelboards. A 5,000-amp fault on the load side of a branch breaker may cause all three CBs to open. If power to the subpanel is lost, where would an electrician look? After the fault is located and removed, the switches and CBs need to be tested and serviced as necessary. According to all manufacturer's instructions and the International Electrical Testing Assn.'s Maintenance Testing Specifications, similar steps are required to examine, service and restore power to both CBs and fused systems, with the exception of fuses that do not require testing because they are new.

There is no "convenience" in simply closing a CB after it has opened. To reclose a breaker into a fault can be dangerous and increase the damage to equipment and property. However, even though manufacturers, NETA and others say it should not be done, the temptation is there. It is almost a knee-jerk response. While some industries have planned shutdowns for service and maintenance purposes, commercial and other industries seldom can do so except at late hours. This increases the cost of scheduled maintenance, so it seldom gets done. Or even worse, equipment is worked on while energized. Project specifications should always include a spare fuse cabinet and an adequate stock of spare fuses. No fused system has ever become obsolete because fuses were not available. But CBs do become obsolete. Replacement CBs for many older panels are not available. A search must be done for used CBs.

The concern for single phasing, especially in the example shown, is non-existent. These are lighting panels. By far, the most common types of short-circuits in lighting circuits are phase-to-neutral or ground faults. If they were being served by a three-phase fused switch and there was a fault, only one fuse would open, leaving two-thirds of the lighting unaffected. Even in complete systems no single device can prevent single phasing.

This is a complex subject beyond the scope of these comments. Suffice it to say there are some 60 causes of single phasing, and what is important is what happens to the system afterward. These low-current arcing faults may be the most hazardous. When no instantaneous trips are provided or when instantaneous trips are set at high levels to improve coordination, arc-flash hazard is increased. Delayed opening on arcing faults may greatly increase damage to equipment. In fact, when coordination is not a problem, CB manufacturers often recommend lowering the instantaneous trip pick-up point as low as possible without causing nuisance tripping.

Mr. Lane in essence is suggesting that the 2005 NEC not be adopted. Therefore, he is advocating the elimination of other improvement in the NEC—not to make systems safer, but to negate the work of the entire code-making committee.

I strongly feel that even if a jurisdiction has not formally adopted the latest NEC, it should be included in every project specification. To do less is to increase hazard to personnel. No engineer can do so and meet the ethics of his profession.

Author Keith Lane responds: My intent was to solicit good conversation in the engineering community, which I think my article has succeeded in doing. I am certainly not against AHJs adopting the 2005 NEC. The heart of the piece was the issue of selective coordination. As noted in my conclusion, I am in favor of good engineering analysis, which would include a comprehensive fault current calculation, coordination study and appropriate settings of the breakers for every emergency or legally required standby system. I suggest that based on the result of that study and an analysis of the type of installation, good engineering judgment can be utilized to determine if fused distribution is required. In either case, with the use of a comprehensive fault current analysis, coordination study, comparison of the fault current with respect to the threshold of non-coordination and appropriate setting of the breakers or with the use of fused distribution, the reliability of electrical distribution systems would be increased over what is required by the NEC prior to 2005.





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