Letters: Reader Feedback
Staff -- Consulting-Specifying Engineer, 9/1/2005
In last month's Codes section ("New NEC Could Change Emergency Power Distribution System Design," p. 21), the discussion of Fig. 1 indicated that the bottom part of the "blob" is the time at which the breaker contacts part. This is a simplification of the facts. The upper portion of the instantaneous "tail" gives the total clearing time of low-voltage breakers, including contact parting time, operating time and arcing time. The contact parting time is not available from looking at the manufacturers' curves and could be anywhere within the shaded area or even below the 0.01-second mark.
In looking at Fig. 3, the discussion indicates the 400-amp fuse will clear before the 100-amp fuse, which is probably true in this case because of the 4:1 difference in fuse sizes. If the size ratio were 2:1 on the time-current curve, daylight would be shown between the two characteristics from 0.01 seconds (0.6 cycles) to 1,000 seconds. Just because there is daylight between the curves does not ensure the fuses will operate in sequence for a fault. The time-current curve does not show what happens in the 0.001- to 0.01-second region. The fuse characteristic could be overlapping in this current-limiting region. Unless the manufacturer has tested the two in series, the user does not know. Often, manufacturers provide tables that list the ratio of fuses where they will be fully selective. Using fuses of different types and from different manufacturers creates another unknown to contend with.
Regarding the comment on Fig. 4, where the instantaneous portion of the main breaker is turned off, the trip unit may have the instantaneous portion turned off, but the majority molded-case circuit breaker (MCCB) has a high-set magnetic instantaneous unit to protect the breaker from delayed short-time tripping at very high currents, since most MCCBs do not have a short-time short-circuit rating. The manufacturer determines where this override is set. Unless the main, in this case, is a low-voltage power-circuit breaker with a short-time rating, I would not say that Fig. 4 is fully selective.
CONRAD ST. PIERRE , ELECTRIC POWER CONSULTANTS, LLC, SCHENECTADY, N.Y.
Into the archivesAugust's Codes story was timely and intriguing. Presently, I am grappling with how to retrofit an overcurrent protection device (OCPD) scheme on a 600-kW emergency generator that will meet the new selective coordination requirements of the 2005 NEC while reducing arc-flash incident energy. Several comments and suggestions in the article forced me to dig a little deeper into my 30-year accumulation of revised NEC books. Here is what I divined:
No past or present NEC article allows the instantaneous trip feature of an OCPD to be bypassed. The article suggests that in order to meet the new NEC selective coordination requirement, one should turn off the instantaneous feature on the main OCPD. This practice would circumvent the problem but would also make wearing an OSHA-approved arc-protection suit necessary for servicing the system. The practice would also create another NEC violation. NEC article 240.4(a) allows the overload feature to be turned off only if doing so will prevent creating another hazardous condition. This article does not permit turning off the OCPD's instantaneous feature.
Second, fuses, when used as an OCPD in an emergency power system, do not make selective coordination any easier as suggested in the article. If an emergency power system's available fault current is limited—and most emergency sources are—then a fuse will clear a faulted circuit no quicker than a circuit breaker. A system short-circuit study is first needed to see which device works best with the actual available fault current. Fuses also add another problem to the system: singles phasing. Blowing a single fuse on a three-phase emergency system may only trade one system abnormality for another.
Finally, what I found most curious was what was not brought to light: "Why past designed protection schemes for emergency power systems failed to work as designed." Similarly curious were the vague comments and proposals made by the board members who worked on adding the selective coordination article to the 2005 NEC. Visit www.NFPA.org/freecodes for a look at their ROCs and ROPs.
JOE C. CRAWFORD , P.E.
Editor's note: Next month, NEC code analysis author Keith Lane will address these questions and others in his continuing look at the 2005 edition of the NEC.
