Don't Be Felled By Higher Fault Currents

When you look at your system upgrades and additions, be sure to consider increases in available fault current. Here's when and how you might engineer a solution. Suppose you're estimating the electrical cost of a building addition. The new added load requires a transformer size increase from 1000kVA to 2500kVA. You plan on feeding a 15-year-old, 1200A, 480Y/277V switchboard from a new switchboard.All

When you look at your system upgrades and additions, be sure to consider increases in available fault current. Here's when and how you might engineer a solution.

Suppose you're estimating the electrical cost of a building addition. The new added load requires a transformer size increase from 1000kVA to 2500kVA. You plan on feeding a 15-year-old, 1200A, 480Y/277V switchboard from a new switchboard.

All is going great, and you think everything's under control; until you realize the interrupting rating on those old air frame circuit breakers might not be adequate. Sure enough, a check of the nameplate shows they're rated for 30,000A rms symmetrical. That was sufficient for the old switchboard because there was only 19,028A of available fault current. Now, however, the utility engineer tells you there'll be 49,287A available at the transformer terminals. You do a quick short-circuit calculation and find you'll have 45,119A available at the old switchboard. What do you do? To answer this question, we'll discuss some of the issues (including the UL 489 procedures) as well as basic engineering principles.

Option 1: Rip out the existing switchboard and replace it with one using either fused switches or circuit breakers having interrupting ratings of at least 45,119A. Perhaps this is a waste of money, since you maintain those old air frame breakers every year and they don't cause you a bit of trouble. You dismiss this option because you can't afford it.

Option 2: Go back to the manufacturer. No luck here; the manufacturer is out of business.

Option 3: Add a reactor ahead of the old switchboard. Sure, you could use a reactor to reduce the available short-circuit current to less than 30,000A. But, the peak current let through by the reactor still exceeds the peak rating on the old air frame breakers. Besides that, another calculation shows the extra energy loss, due to the reactor, makes it unattractive from a long-term financial point of view. In addition, voltage transients caused by the added reactance will stress system insulation levels, with motor windings as one of your biggest worries.

Option 4: Use a current limiting overcurrent protective device ahead of the old switchboard. This is another way of saying use a series-rated system. You check the equivalent rms let-through current of a 1200A, Class L fuse and find it reduces the 45,119A available down to 21,000A. This is well within the 30,000A interrupting rating of the old air frame circuit breakers, so you think you've solved your problem. Guess again.

Enter the 1999 NEC. Two sections will affect your decision-making.

• NEC Sec. 110-22. The second paragraph of this section states:

Where circuit breakers or fuses are applied in compliance with the series combination ratings marked on the equipment by the manufacturer, the equipment enclosure(s) shall be legibly marked in the field to indicate the equipment has been applied with a series combination rating. The marking shall be readily visible and state the following: CAUTION: SERIES COMBINATION SYSTEM RATED_____ AMPERES. IDENTIFIED REPLACEMENT COMPONENTS REQUIRED.

This seems to indicate you must mark the series rated combination on the old switchboard, and mark the 1200A fused switch in the field with the series combination rating. You could easily mark this switch with the new rating resulting from your engineering calculations, but things aren't that simple. Read on.

* NEC Sec. 240-86. This section reads as follows:

Series Ratings. Where a circuit breaker is used on a circuit having an available fault current higher than its marked interrupting rating by being connected on the load side of an acceptable overcurrent device having the higher rating, the following shall apply.

(a) Marking. The additional series combination interrupting rating shall be marked on the end use equipment, such as switchboards and panelboards.

(b) Motor Contribution. Series ratings shall not be used where:

(1)Motors are connected on the load side of the higher-rated overcurrent device and on the line side of the lower-rated overcurrent device.

(2)The sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker.

The marking requirement in this section, in its reference to "end-use equipment," applies to manufacturers. Since there were no series-rated combinations marked on your old switchgear, it doesn't look like you'd be in compliance with this section of the Code.

The second part of this section requires there be no more than 300A (1% of 30,000A) of motor load fed by the old switchboard. For example, assume you only have 250A of motor load. This means your design complies with the motor contribution requirements of Sec. 240-86.

Now let's go back to those factory marking requirements on the old switchboard. It just doesn't look like you can meet them. You could go to the Authority Having Jurisdiction (AHJ) and explain your situation, but he or she may not agree with your idea for protecting the old board. In fact, he or she may not have the leeway under applicable administrative law to grant such a variance.

A prospective alternative. Unfortunately, this scenario is often all too real. This was the reason for a 1999 NEC proposal to allow for "engineering" of a system when the load side circuit breakers are passive during the first one-half cycle. The proposal was very controversial, but it passed by a vote of 9 to 3. After public comments (roughly dividing equally pro and con), Code Making Panel 10 reaffirmed support by a vote of 10 to 2. The only negative votes came from UL and NEMA.

However, the Technical Correlating Committee (TCC) decided to hold the proposal for the 2002 Code cycle because of an editorial mistake that resulted in a minor conflict between the new provision and the new Sec. 240-86. Nevertheless, the NFPA Electrical Section and NFPA membership at the Annual Meeting voted to overrule the action of the TCC, which CMP 10 reconfirmed on another ballot. The TCC, refused to budge, and the Standards Council rejected an appeal to override that position. This means you cannot use the 1999 NEC to solve your problem.

Here's what the proposal would add:

A circuit breaker shall be permitted to be used on a circuit having an available fault current greater than its marked interrupting rating if protected on the supply side by a suitable current-limiting device selected under engineering supervision. This additional series combination rating, including identification of the upstream current-limiting device, shall be field marked on the end use equipment.

(FPN): Suitable systems can be engineered when the circuit breaker is passive during the first half cycle of a fault.

This proposal would have allowed you to use the so-called "up-over-and-down" method, under engineering supervision, to determine if current limiting overcurrent protective devices can protect circuit breakers that are "passive" during the first half cycle.

Be careful! Although engineers have used this method for many applications for decades, it does not carry Code recognition. In addition, both fuse manufacturers and circuit breaker manufacturers no longer recognize it for protection of some of the more modern circuit breakers. This is because some of them may exhibit dynamic impedance within the first half cycle after a fault occurs. (See sidebar "What exactly is dynamic impedance in a circuit breaker?")

The Fine Print Note in the proposal tries to address this. Breakers that exhibit dynamic impedance aren't supposed to be eligible for this calculation method. Nevertheless, it's possible to conduct empirical tests to determine the passivity of air frame breakers, molded case circuit breakers (MCCBs), and insulated case circuit breakers during the first half cycle after a fault occurs.

But what about small MCCBs, say 15A or 20A, made in the past 15 years or so? Make no assumptions! High current short-circuit testing has shown opening times of half cycle or less for some circuit breakers having published clearing times of one cycle.

What about larger MCCBs or insulated case circuit breakers, in the 600A to 1200A frame sizes, with published clearing times of three to five cycles? Again, make no assumptions. Contact the circuit breaker manufacturer or, if economically justified, have one of the subject breakers short-circuit tested for passivity during the first half cycle.

Consider UL standards. The engineering also involves how well the circuit breaker can interrupt currents in intermediate range as well. UL 489 imposes an "intermediate interrupting test" on MCCBs. While UL 489 does not apply to air frame circuit breakers, you should still consider similar situations.

The intermediate tests assure the user that a circuit breaker will clear faults of lesser magnitude successfully. In other words, the test assures that short-circuit currents below the current-limiting threshold of the fuse won't damage the downstream circuit breaker.

A review of Table of UL 489 shows an intermediate test value of 27,000A for a 1200A line-side fuse. Since 27,000A is less than the 30,000A interrupting rating of the old air frame circuit breakers, Paragraph would not require the intermediate test to be run.

So where does this leave you? In our example, the air frame circuit breakers take at least three cycles to open. Therefore, you may be able to "engineer" protection for them by choosing a current-limiting device that limits the available short-circuit current to less than 30,000A in the first half cycle.

If the AHJ will allow it (under either a local amendment or under Sec. 90-4), you may be able to use this approach. Just be sure you only apply it to suitable equipment. For example, don't apply it to modern MCCBs or you may create a significant hazard. Properly applied, however, this method may be what allows an overall improvement in a plant electrical system to be economically viable.

Sidebar: What Exactly is Dynamic Impedance in a Circuit Breaker?

Dynamic impedance is another way of saying the circuit breaker contacts begin to part and insert a changing (dynamic) impedance into the faulted circuit. If a modern circuit breaker begins to open a faulted circuit within half cycle, the breaker may try to handle more energy than it's designed to handle. This has to do with the rate of rise of the short-circuit current, magnetic forces, and other issues beyond the scope of this article.

If the downstream circuit breaker starts opening on a high-current fault within the first half cycle, it may "fool" the upstream, fully rated device into not opening, because the fault current steadily decreases as the downstream contacts open. The result is a downstream device clearing the entire fault, even one well beyond the device's rated interrupting current. The outcome: self-destruction.

TAGS: Design
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