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Consideration in selecting fused switches in a MCC


This article provides recommendations on fused switches in a MCC for short circuit protection of motor and feeder branch circuits.

The Electrical Engineer selecting the appropriate device should understand what the potential available fault current is at the MCC incoming and select devices whose rating is greater than the available fault current. It is also important to understand what the ratings mean and under what conditions they are tested so the protection devices used can perform properly and not create additional hazards in themselves.

Fusible disconnects offer a number of important safety features such as the availability of visible blades and the ability to withstand and limit the maximum fault current available downstream if properly rated. Conversely, it is these two requirements that have been the biggest barrier to widespread implementation of molded case circuit breakers (MCCBs). Improvements in MCCBs, however, have made a re-examination and comparison of their features advisable in light of the present business needs. Current limiting MCCBs are approaching the capabilities of fuses in current limitation and interrupting capabilities. In addition to degree of protection, other factors deserving evaluation are operational, maintenance, and safety considerations, and cost, both initial and replacement.

The decision process on whether to specify MCCBs or fusible disconnects in MCCs can be very straight forward based upon technical requirements or very complex due to local requirements based upon operations and safety procedures.

Downstream Equipment Protection

In order to properly apply protective devices, Electrical Engineer should first understand the function of the fuse or circuit breaker in typical MCC usage. The function of these protective devices is somewhat dependent upon whether they are used in a branch feeder or a motor circuit. In a branch feeder circuit, the protective device provides both short circuit (fault) protection and overload protection for the cable and, in some cases, for the load if it is not protected by another device. In a starter or other MCC mounted motor controller the protective device protects the components within the "bucket," the cable and the load for short circuit only since the thermal overload device is the primary protection for motor overload.

The degree of protection offered by fuses or circuit breakers is dependent on their interrupting capabilities and their ability to limit energy (electromagnetic and thermal). A device that is incapable of passing or interrupting the currents and energies available in the circuit can fail in two unsafe modes: 

  • It can blow apart causing molten metal and hot gases to be violently propelled from the device.
  • One or more of the contacts could weld closed making it impossible to open the contact(s) of the disconnect.

In the case of a fused device, the switch is not intended to interrupt the fault, but it must withstand the short circuit let-through of the fuse. The fuse is the interrupting device and, as such, must be capable of interrupting the maximum fault current available. In the case of circuit breaker protected motor and feeder branch circuits, the circuit breaker is the interrupting device and it therefore must be capable of interrupting the maximum fault current available.

There are important differences in how interrupting ratings for fuses and circuit breakers are determined under UL test procedures. In UL198C (fuse Classes G, CC, J and L) and UL198E (Class R) the test for interrupting rating ensures the fuse carries the full current values for the interrupting value it is being tested, i.e., 100,000 Amps, 200,000 Amps, etc. However, a circuit breaker's interrupting rating test parameters are described in UL489.

This interrupting test provides for 10 inches of "rated wire" on the line side and 4 feet of "rated wire" on the load side of the breaker. This means that, even though the test circuit has been calibrated to generate the rated interrupting current, the additional impedance of these lengths of rated wire reduces the actual current interrupted by the circuit breaker under test. To further confuse matters, when used in MCCs, an MCC starter can be rated under UL845 and will be labeled a "Unit short-circuit-current rating." The test for this rating allows for 4 feet of wire on the load side of the unit before the fault initiation; therefore, not only does it introduce the impedance of the wire but also the additional impedance of the internal starter wiring, the contactor and the overloads. This information is significant when considering circuit breakers for MCC applications especially on applications with relatively high available fault. 

Table below shows the calculated currents interrupted by circuit breakers under test conditions required in UL489 and UL845. Note that in the UL845 column that the additional impedance shown is only for the overload heaters that should be the most significant resistance contributor. The 200 amp and 400 amp units typically use CT (current transformer) fed overload heaters and are therefore not calculated in this table (shown with"*").

Circuit Breaker UL Test Comparisons

Amp

Rating

25,000 IAC

42,000 IAC

65,000 IAC

CB(UL489)

Unit (UL845)

CB(UL489)

Unit (UL845)

CB(UL489)

Unit (UL845)

30

20,248

9,960

28,352

10,860

34,722

11,290

60

23,104

18,900

36,288

25,245

50,824

29,670

100

23,956

23,100

38,955

36,235

57,488

50,635

200

24,429

24,160

40,395

39,580

61,179

59,060

400

24,714

*

41,193

*

63,073

*

600

24,763

*

41,334

*

63,145

*


Another significant testing issue for circuit breakers is when they are to be used on a distribution system that is "Corner Delta" or "B-Phase" grounded. Some breaker manufacturer testing does not allow for additional voltage that can be impressed under certain conditions with this type of distribution system. It is important that the manufacturer be made aware of the type of distribution configuration in which the equipment will be used and held responsible for ensuring that proper testing and certification have been accomplished. It is also a NEC requirement, Article 240.83(e) that the circuit breakers have voltage markings to reflect that they are properly applied for a "Corner Delta" or "B-Phase" grounded system.

Operation

When comparing fused switches with MCCBs there are some operational concerns that should be considered. Circuit breakers are re-settable after a fault or overload if the prospective fault current is considerably less than the interrupting rating. From an operating point of view this feature can be highly desirable. Giving the operator the capability to reset the breaker after the overload or fault has been cleared can save operating time and money. This operation can also be done externally without having to expose personnel to the inside of the MCC unit. However it may not be the best policy to have an operator reset the breaker after a fault particularly since the operator runs the risk of closing back in on a fault. This can further damage the MCC and/or the equipment being "protected" and expose the operator to additional hazards.

Fuses must be replaced when a short circuit, ground fault or overload occurs. As this replacement is typically performed by an electrician, it could require additional personnel availability and/or time delays. This must be counterbalanced however, with the likelihood that a short circuit or ground fault will probably require an electrician to resolve.

A circuit breaker can offer operating convenience and potential savings if the operator can accurately determine the nature of the trip (overload, low level fault or high level fault) or if the available fault current is significantly below the actual interrupting rating of the breaker. A good rule of thumb is a minimum of the fault available not to exceed 50% of the breaker UL rating.

Maintenance

MCCBs require maintenance and testing similar to low voltage power circuit breakers. Routine field testing enables personnel to determine, without laboratory conditions or complex equipment, that a MCCB will operate when called upon. Most manufacturers recommend performing insulation resistance tests, connections inspections, contact resistance tests, overload tripping tests, and verification of instantaneous magnetic tripping and mechanical operation. Although fused disconnects do not require specific electrical tests similar to the MCCB, they do require thorough function checks, cleaning, and inspection on a periodic basis.

Safety

One significant safety aspect is the issue around "high energy". This has implications not only in personnel safety but also in limiting damage to protected equipment which could have economic as well as secondary safety concerns.

Shock or electrocution is another highly important safety consideration. Modern molded case circuit breakers are inherently better protected than open blade disconnects required when specifying visible break or visible blade features. Almost all approved circuit breakers are designed to be "finger proof'' (IP20) which means exposed energized parts are protected to prevent a finger or tool greater than 12mm in diameter. The fused disconnect, even with protected blades, have the fuse clips and fuse ferules exposed and energized when the switch is closed.

Cost

The cost issue is very dependent upon the size makeup of the MCCs to be purchased, and the manufacturer being considered. Looking at an "average market basket" and an average of the guidelined MCC products the cost of standard current limiting circuit breakers and fusible disconnects (without the fuses) is almost identical. To this, one must factor in the cost of initial and replacement fuses which could average roughly 3% of the order. Spare parts, depending upon how many types and sizes of circuit breakers you need, would probably weigh in favor of fuses. In summary, although it can vary significantly depending upon the makeup of the order, initial investment of standard current limiting circuit breakers and fused disconnects will be essentially a tradeoff.

Cost of circuit breakers with limiters is essentially the same for either CL circuit breakers or fused disconnects.



Tags: mcc fused switches