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The National Electrical Code requires the proper application of overcurrent protective devices with regard to their interrupting rating.

This requirement is addressed in NEC 110.9 and requires overcurrent protective devices to have an interrupting rating equal to or greater than the fault current available at their line terminals. In order to properly comply with NEC 110.9, a short-circuit study must be completed to determine the available fault current and, thus, the minimum required interrupting rating of the overcurrent protective devices.

For three-phase systems, the most common practice is to do a short-circuit study calculating the available fault current based upon a three-phase bolted fault. This would approximate the fault current that could result if all three phases were solidly connected, and this is typically assumed to be the worst case fault condition. Thus, if the short-circuit study determined an available fault current of 50,000 A, an overcurrent protective device with a marked interrupting rating of 50,000 A or higher would be required per NEC 110.9 as shown figure 1.

However, have all the concerns about the interrupting capabilities of the overcurrent protective device been addressed? The answer to that question depends upon the type of grounding scheme, the type of overcurrent protective device, and the available fault current. An additional concern of the overcurrent protective device interrupting capabilities has been identified in the new 2002 version of the NEC by an addition of a fine print note to 240.85 as shown below.

"FPN: Proper application of molded-case circuit breakers on 3-phase systems, other than solidly grounded wye, particularly on corner-grounded delta systems, considers the circuit breakers’ individual pole-interrupting capability."1

As indicated in the fine print note, the consideration of the individual pole (or single-pole) interrupting capability is especially important for molded-case circuit breakers on corner-grounded delta systems. In the previous issue of the IAEI News, Michael Johnston explored "Installations and Inspections of Corner-Grounded Systems." This article serves as a follow-up to his article by indicating additional concerns for installations and inspections of this type of system. In addition, other grounding schemes, such as resistance-grounded and ungrounded, will be explored for similar concerns.

The first step to understanding the rationale for this new fine print note in NEC 240.85 is to understand how molded-case circuit breakers are tested in accordance with UL 489.

Molded-Case Circuit Breaker Testing – UL 489
The standard interrupting tests for molded-case circuit breakers involve single-pole and multiple-pole tests. Table 7.1.7.2 of UL 489 provides the minimum single-pole (individual) and multiple-pole (common) interrupting capabilities for listed molded-case circuit breakers. Table 1, as taken from Table 7.1.7.2 of UL 489, indicates these values for straight-rated 2-pole (including additionally marked 1-phase–3-phase) and 3-pole molded-case circuit beakers since these are the types typically used for applications involving the fine print note.

Figure 2 shows the test set-up for the individual pole.

Figure 3 shows the test set-up for the multiple-pole test.

Molded-case circuit breakers can be tested for a higher multiple-pole short-circuit interrupting capability as indicated by 7.1.11 of UL 489. The test current value can be equal to any value listed in Table 8.1 of UL 489, from 7500 A to 200,000 A. The higher short-circuit interrupting capability test requires a common (multi-pole) operation with a set-up similar to figure 3, but with a higher current than indicated in table 1. If tested to a higher value, the molded case circuit breaker is marked with the higher multiple-pole interrupting rating.

However, an individual-pole interrupting test, with a set-up similar to figure 2 is not required for these optional higher interrupting capability tests. Because of this, the marked multi-pole interrupting rating can be much higher than the tested individual pole interrupting capability. In addition, the single-pole capability is not required to be marked on the molded-case circuit breaker, it can only be determined by reviewing the UL 489 standard.

Based on the testing requirements of UL 489, it can be seen that a three-pole, 100-amp, 480-volt circuit breaker of any marked interrupting rating is tested to interrupt 8660 amps at 480 volts across one pole. For instance, if a three-pole molded-case circuit breaker was marked for an interrupting rating of 65,000 A, the maximum tested individual-pole interrupting capability, required by UL 489, would be 8660 amps at 480 volts. Thus, performance of this molded-case circuit breaker for faults above 8660 amps at 480 volts across only one pole is not known.

The question now remains, what type of grounding schemes should be considered when verifying the single-pole interrupting capability of molded-case circuit breakers. As indicated in the fine print note of NEC 240.85, the first type of system that needs to be considered is the corner-grounded delta system.

Corner-Grounded Delta Systems (Solidly Grounded)
The system in figure 4 has a delta-connected secondary and the B-phase is solidly grounded.

If the B-phase should short to ground, no (or minimal) fault current will flow because it is already solidly grounded.

However, if either Phase A or C is shorted to ground, a phase-to-ground fault condition will exist. The available phase-to-ground fault current, at 480 volts, can be as high as 87 percent of the three-phase short-circuit current. As shown in figure 5, this requires one pole of the three-pole circuit breaker to interrupt the fault current at 480 volts. To assure proper application, the available phase-to-ground fault current must not exceed the tested single-pole interrupting capability of the molded-cased circuit breaker. Thus, if the circuit breaker shown in the branch panel in figure 5 were a three-pole, 100-amp, 480-volt circuit breaker, the line-to-ground fault must not exceed 8660 amperes.

What about Other Overcurrent Protective Devices?
If the phase-to-ground fault current in figure 5 exceeded the single-pole interrupting capability of the molded-case circuit breaker, what options would be available?

One option would be the airframe/power circuit breaker. Per UL 1066 and ANSI C37.13 and C37.16, a low-voltage power circuit breaker has a single-pole interrupting rating of 87 percent of its three-pole rating. If the marked three-pole rating is equal to or exceeds the available three-phase fault current, it will have a sufficient single-pole interrupting capability.

Another option would be a current-limiting fuse. Current-limiting fuses are tested individually per UL/CSA/ANCE 248 to obtain their marked interrupting rating. Their marked interrupting rating applies for applications in 1-phase and 3-phase systems and further investigation is not required.

What about Other Types of Systems?
Although corner-grounded systems are of prime concern regarding proper application of overcurrent protective devices’ single-pole interrupting capabilities, other systems require a similar analysis. These systems include inpedance-grounded and ungrounded systems.

Impedance-Grounded Systems
The most common type of low-voltage impedance-grounded system is the "high" impedance-grounded system. Often these systems are referred to as high resistance-grounded systems because of the addition of a grounding resistor. The high impedance-grounded system offers the advantage that the first fault to ground will not draw enough current to cause the overcurrent device to open. In addition, this system reduces the stresses, voltage dips, heating effects, and so forth, normally associated with high short-circuit current for the first fault to ground. Because of these advantages, this system is becoming very popular in today’s industrial installations. One drawback with high impedance-grounded systems is that line-to-neutral loads are not permitted per NEC 250.36(4).

When the first fault occurs from phase to ground as shown in figure 6, the current path is through the grounding resistor. Because of this inserted impedance, the fault current (typically about 5 amperes) is not high enough to open protective devices and the plant remains "on-line." Ground detectors, as required by NEC 250.36(3), are then triggered by the ground-fault condition.

Even if equipped with ground detectors, the ground fault may not be investigated until scheduled maintenance is planned, which can be many weeks. During this time, if a different phase in another part of the system goes to ground, a 480-volt fault across only one pole of the affected branch-circuit device may occur. The magnitude of this fault current can approach 87 percent of the three-phase short-circuit current. Because of this possibility, as shown in figure 7, the single-pole interrupting capability must be investigated.

Ungrounded Systems
The ungrounded system offers the same advantage for continuity of service that is characteristic of high impedance-grounded systems. Although not physically connected, the phase conductors are capacitively coupled to ground.

The first fault to ground is greatly limited by the large impedance path as shown in figure 8. Since the fault current is reduced to such a low level, the overcurrent devices do not open and the plant continues to "run." With grounded systems, ground detectors should (but are not required by the NEC) be installed, to alert the maintenance crew of the ground-fault condition.

If a second phase in another part of the system goes to ground, a 480-volt fault across only one pole of the affected branch-circuit device may occur. NEC 250.4(B)(4) requires a permanent, low impedance path back to the source to promote a high fault current and the opening of the overcurrent device when the second fault occurs. Because of this possibility, as shown in figure 9, single-pole interrupting capability must be investigated.

Conclusions
An overcurrent protective device must have an interrupting rating equal to or greater than the current available at its line terminals for both three-phase bolted faults and for one or more phase-to-ground faults. Although most electrical systems are designed with overcurrent devices having adequate three-phase interrupting ratings, the single-pole interrupting capabilities are easily overlooked.

Simple solutions exist to provide adequate interrupting ratings if molded-case circuit breaker single-pole interrupting capabilities as shown in table 1 are not sufficient. First, low-voltage power circuit breakers that have tested single-pole interrupting ratings that are 87 percent of the published three-pole rating can be considered. Alternatively, current-limiting fuses are available that have tested single-pole interrupting ratings of 200,000 and 300,000 amps.

1 NFPA 70, National Electrical Code, Section 240.85, (Quincy, MA: National Fire Protection Association, 2002), p. 70-93, 70-94

Dan Neeser is employed by Cooper Bussmann and holds the title of manager, Technical Sales. He participates in IEEE, NEMA, NFPA, NJATC, IBEW, NECA, and IAEI activities. He specializes in training on the design and application of overcurrent protective devices in electrical distribution systems in accordance with the National Electrical Code. Prior to his position with Cooper Bussmann, he was a sales engineer for Cutler-Hammer focusing on construction project sales. He has a BSME from the University of North Dakota.

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