Overcurrent protective devices preserve system integrity

The selection of overcurrent protective devices can have a tremendous impact on the performance of an electrical system. Too often overcurrent protective devices are selected based only on voltage and current ratings. There are many other characteristics of overcurrent protective devices that must be taken into consideration to ensure proper protection of the electrical system.

The National Electrical Code 110.9 requires that equipment intended to break current at fault levels to have an interrupting rating sufficient for the current that must be interrupted. A device's interrupting rating is the maximum amount of short-circuit current at rated voltage that an overcurrent protective device, such as fuses or circuit breakers, can safely interrupt under specific test conditions. Protective devices must be able to withstand the destructive energy of short-circuit currents.

Fault current that exceeds the capability of the protective device may cause the device to rupture violently, causing additional damage to electrical equipment. However, devices with adequate interrupting capability to handle expected fault current are not likely to rupture (See "Comparison of device interrupt ratings").

Overcurrent protective devices have varying interrupting ratings. ANSI Standards for fuses and circuit breakers require the interrupting rating to be marked on the device in most cases. Molded case circuit breakers (MCCBs) that are not marked with an interrupting rating are assumed to have a 5000-A interrupting rating per UL 489, Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures. Common MCCBs often have a 10,000-A interrupting rating. MCCBs with higher interrupting ratings are available, but they must be specified. The available short circuit current must be known to select an MCCB with the proper interrupting rating.

Branch circuit fuses that are not marked with an interrupting are assumed to have a 10,000-A interrupting rating per UL 248, Low-Voltage Fuses. Typical current-limiting fuses, such as Class R, J or L, have interrupting ratings of 200,000 A or 300,000 A. These types of fuses can be applied in nearly any electrical system without concern of exceeding the interrupting rating.

Choosing overcurrent protective devices strictly on the basis of voltage, current and interrupting rating alone does not ensure that equipment will not be damaged by short-circuit current. Devices that are not current limiting may not be capable of protecting wire, cable or other components from short-circuit current. The interrupting rating of a protective device pertains only to that device and has absolutely no bearing on its ability to protect connected downstream components.

Quite often, an improperly protected component is completely destroyed under short-circuit conditions while the protective device is opening the faulted circuit. Fuses and circuit breakers must not only be able to safely withstand fault currents, they must also protect the other electrical components of the system.

The NEC states in 110.10 Circuit Impedance and Other Characteristics:

"The overcurrent protective devices, the total impedance, the component short-circuit current ratings and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault, to do so without extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the grounding conductor or enclosing metal raceway. Listed products applied in accordance with their listing shall be considered to meet the requirements of this section."

There are different levels of protection for electrical components as well. Even components that are installed in accordance with the NEC and within their listing and labeling requirements may not be suitable for further use after a fault condition. For example, UL 508, Industrial Control Equipment, includes a short-circuit test-procedure to verify that motor controllers will not be a safety hazard and will not cause a fire. UL 508 tests are for safety with the doors closed, but do allow a significant amount of damage as long as it is contained within the enclosure. UL 508 allows deformation of the enclosure, but the door must not be blown open and it must be possible to open the door after the test.

In the standard short-circuit tests, the contacts must not disintegrate, but welding of the contacts is considered acceptable. Tests allow the overload relay to be damaged with burnout of the current element considered acceptable. For short-circuit ratings in excess of the standard levels listed in UL 508, the damage allowed is even more severe. Welding or complete disintegration of contacts is acceptable and complete burnout of the overload relay is allowed. Therefore, it is not certain that the motor starter will not be damaged just because it has been listed for use with a specific branch circuit protective device.

There is a method to select a branch circuit protective device that not only provides motor branch circuit protection, but also protects the circuit components from damage. Two documents that offer guidance in evaluating the level of damage likely to occur during a short circuit with various branch circuit protective devices are: Outline of Investigation (UL508E) and IEC (International Electrotechnical Commission) Standard Publication 60947, Low Voltage Switchgear and Control, Part 4-1: Contactors and Motor Starters.

Both documents define two levels of protection for the motor starter:

• Type 1— Considerable damage to the contactor and overload relay is acceptable. Replacement of components or a completely new starter may be needed. There must be no discharge of parts beyond the enclosure.
• Type 2— No damage is allowed to either the contactor or overload relay. Light contact welding is allowed, but must be easily separable.

Motor starter manufacturers must verify that Type 2 protection can be achieved by using a specified protective device. Most manufacturers have both their NEMA and IEC motor controllers verified to meet the Type 2 requirements outlined in UL 508E and IEC 60947-4. Current-limiting devices usually are necessary to provide verified Type 2 protection. In many cases, Class J, Class RK1 or Class CC fuses are required because Class H, Class RK5 fuses and typical circuit breakers do not operate quickly enough under short-circuit conditions to prevent damage to the equipment and still provide Type 2 protection¹.

Short-circuit current rating is not the same as interrupting rating and the two must not be confused. Short-circuit current rating is the maximum short-circuit current a component or equipment can safely withstand when protected by a specific overcurrent protective device or for a specified time. Adequate short-circuit current rating is required per NEC 110.10 (See "What the Code says about short-circuit current rating").

Equipment and controllers with higher short-circuit current ratings make it easier to specify, install and meet compliance. Also, equipment such as industrial machinery is often moved to other locations. High short-circuit current ratings make it easier to relocate equipment without exceeding the short-circuit current rating requirements at new locations and help to ensure safer installations. Protection with current limiting devices is the easiest and most effective way to achieve higher short-circuit current ratings.

Selective coordination can be defined as isolating an overloaded or faulted circuit from the remainder of the electrical system by having only the nearest upstream overcurrent protective device open (Figs. 1 and 2). This term was actually added to 2005 NEC Article 100 Definitions:

"Coordination (Selective)— Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings."

The 2005 NEC has added requirements for some systems to be selectively coordinated such as Emergency Systems (Article 700), and Legally Required Standby Systems (Article 701). The NEC also requires that elevator circuits be selectively coordinated. The objective of these requirements is to ensure system uptime with the goal of safety of human life during emergencies. Although not required by the NEC, selective coordination is the best practice for all electrical systems, and can be critical to reduce costly downtime. Without selective coordination, a single faulted circuit can shut down an entire facility.

The design engineer must verify that overcurrent devices are selectively coordinated for the full range of overcurrent that can occur in the system. And the site inspection should verify the overcurrent protective devices are installed as specified to achieve selective coordination. It is possible for both fusible and circuit breaker systems to be selectively coordinated with proper analysis and selection.

Selective coordination of fusible systems can often be achieved and verified by using selective coordination ratios published by fuse manufacturers. A full short-circuit and coordination study is not necessary to verify selective coordination.

Selective coordination with circuit breakers depends on their characteristics and settings as well as the circuit parameters for the specific application. Generally, it is difficult to achieve selective coordination with common circuit breakers that incorporate instantaneous trip settings. Circuit breakers with short time-delay settings may be necessary. In some cases, MCCBs or insulated case circuit breakers with a zone interlocking feature are specified with the objective of achieving selective coordination.

However, these circuit breakers still have an instantaneous trip that overrides the zone selective tripping feature. Therefore, selective coordination may not be achievable for some fault current levels. If circuit breakers are to be considered, a full short-circuit current and coordination study must be done with proper analysis and interpretation.

Perhaps the two most important considerations for achieving successful overcurrent protection are reliability and maintenance. Overcurrent protective devices must operate as specified to ensure the integrity of the electrical system. Mechanical overcurrent protective devices, such as circuit breakers, must be maintained in accordance with manufacturers' instructions and industry standards.

NEMA AB4, Guidelines for Inspection and Preventive Maintenance of Molded Case Circuit Breakers Used in Commercial and Industrial Applications can provide guidance on proper maintenance and testing requirements. It may be necessary to remove the device from service and test the electrical characteristics in addition to visual inspections.

If mechanical devices are not maintained, they may not operate as specified, which can compromise the integrity of the electrical system. Lack of maintenance could result in longer clearing times under fault conditions. This can allow extensive damage to electrical equipment as well as creating a greater arc flash hazard (See "Arc flash test sequence").

Fuses do not have operating mechanisms that would require periodic maintenance. The only maintenance requirement is to ensure the fuse body is not broken or cracked.

There are many characteristics to consider when selecting overcurrent protective devices. Selecting devices based solely on voltage and ampere ratings may not provide adequate protection, and may result in serious misapplications. Overcurrent protective devices must be specified with adequate interrupting ratings in order to safely interrupt short-circuit currents. In addition to being able to safely interrupt fault currents, overcurrent protective devices must be able to protect other electrical components from damage caused by fault currents.

Often current-limiting overcurrent protective devices are preferred to protect electrical components, allow equipment to attain higher short-circuit current ratings and help to reduce the likelihood or the intensity of an arc flash hazard. Selective coordination is also important to consider in preventing system blackouts. The integrity of the electrical system depends on the overcurrent protective devices operating as specified. Mechanical devices must be properly maintained and operated as intended.

¹ For a listing of Type 2 protection table by motor starter manufacturer visit www.bussmann.com/apen/pubs/

The author is available to answer questions about this article. Mr. Schomaker can be reached at JSchomaker@CooperBussmann.com. Article edited by Jack Smith, Senior Editor, PLANT ENGINEERING magazine, (630) 288-8783, jsmith@reedbusiness.com

The Cooper Bussmann SPD Selecting Protective Devices publication can be downloaded from www.bussmann.com. It has an indepth discussion on selective coordination analysis with published fuse selectivity ratios, simple evaluation rules for coordination of instantaneous trip circuit breakers and illustration of short time-delay circuit breakers.