Tech Spec 5: Volume 2 - Issue 2
May 1998
Tech Spec
A Newsletter for the Design Professional
Introduction:
The IEEE Industrial and Commercial Power Systems Department Conference was held May 3-8, 1998 in
Edmonton, Alberta, Canada. The Power Systems Protection Committee sponsored two technical sessions and
numerous sub-committee and working group meetings at that conference. The major topic of discussion at the
Molded Case Circuit Breaker Working Group meeting was Single-Pole interrupting ratings.
Single Pole Interrupting Ratings:
What’s all the fuss about single-pole interrupting ratings? We’ve all been taught that overcurrent protective
devices need to have interrupting ratings that are adequate for the available three-phase bolted fault current. What
we typically did not learn was that overcurrent devices may have one interrupting rating for three phase faults and
another interrupting rating for phase to ground faults.
For example, according to Table 7.1.7.2 of UL 489, a three-pole, 600 ampere, 480 volt molded case circuit
breaker with a three-pole interrupting rating of 65,000 amperes has a single-pole interrupting rating of only 8,660
amperes. That’s right, the breaker is tested to interrupt a three phase bolted fault of 65,000 amperes, but it is only
tested for EIGHT THOUSAND SIX HUNDRED AND SIXTY AMPERES FOR FAULTS THAT ARE
SEEN BY ONLY ONE POLE.
Calculating Ground Faults:
Due to the drastically lower single-pole rating of molded case circuit breakers, one obvious question is raised.
How much short-circuit current will flow in a phase-to-ground fault condition?
The magnitude of the phase-to-ground fault depends upon the ground return impedance and fault location. If the
ground return path is relatively short, the ground fault current will approach the three-phase short-circuit current.
In fact, the phase-to-ground fault may be higher than the three-phase bolted fault current.
Application Considerations:
It is where full phase-to-phase voltage can be impressed across one pole that causes the most worry. This is
possible for corner grounded delta systems, resistance grounded systems, and ungrounded systems. Say, for
instance, that you want to protect a 600 ampere feeder on a 480 volt high resistance grounded system, with an
available three phase short-circuit current of 40,000 amperes. The "white paper" described below will take you
through the details of how you can get 480 volts across one pole.
It is quite obvious that you would need an interrupting rating of at least 40,000 amperes. But, we normally just
think about the three phase interrupting rating. For the three phase interrupting rating we might pick three LOW
PEAK® Class J fuses with interrupting ratings of 300,000 amperes, or a power/air frame circuit breaker with a
three phase interrupting rating of 65,000 amperes, or a molded case circuit breaker with a 65,000 ampere
interrupting rating.
However, looking at the single-pole interrupting ratings of these devices is another story. The single-pole
interrupting rating for the Class J LOW PEAK® Fuses is the same as their three phase rating of 300,000 amperes.
The single-pole interrupting rating of the power/air frame circuit breaker is 56,550 amperes. Both of these devices
would work fine. The real problem comes with the molded case circuit breaker which has a 8,660 ampere singlepole interrupting rating.
Conclusion:
When applying molded case circuit breakers, the manufacturer must be consulted to verify single pole
interrupting ratings to assure compliance with NEC Section 110-9. To insure a safe system, the prudent design
engineer should assume that the ground fault current equals at least the three-phase current. If the electrical
system has the potential of applying phase-to-phase voltage across one pole, the greatest hazard exists.
For a "white paper" on single-pole interrupting ratings LINK TO PAPER
If you have any questions, contact Bussmann Application Engineering at (636) 527-1270, (636) 527-1607
(fax), or e-mail (fusetech@buss.com).
© 1998, 2000 Cooper Bussmann, Inc.