Fused Distribution May Not Always Be Necessary

Editor's note: This month, we wrap up author Keith Lane's three-part series on the 2005 NEC and emergency power distribution system design, which has included discussions of how the updated code will affect switchgear and the potential repercussions of selective coordination. Visit the Electrical community of csemag.

10/01/2005


According to research conducted by a major manufacturer of circuit breakers, the current in most faults resulting from bolted shorts is nowhere near the calculated worst-case scenario. If this is true, it's possible to increase the instantaneous setting of the upstream breaker, possibly saving some equipment costs. This could be done by changing the breakers in the main emergency or main standby distribution panels from standard thermal magnetic breakers—with or without the ability to adjust the instantaneous trip—to breakers with amp plugs and long-time trip settings.

But let's test this hypothesis. Figure 1 (below) shows a 225-amp and a 20-amp breaker with no adjustment for the instantaneous portion of either time current curve. Neither breaker has an instantaneous trip setting and both breakers have a default of about 10 times their continuous current ratings for the instantaneous trip—e.g., the 225-amp breaker will trip at about 2,250 amps.

Figure 2 represents a 225-amp breaker with an adjustment for the instantaneous portion of the time current curve and a 20-amp breaker with no adjustment for the latter. The 225-amp breaker is set for 10 times the continuous current rating. The 20-amp breaker has default instantaneous settings for about 10 times the continuous current rating.

In Figure 3, a 600-amp breaker is utilized with a 600-amp sensor and a 450-amp plug with a 50% long-time pick-up (LTPU) setting. This breaker could be located in the main emergency or standby distribution panel feeding a main lug only (MLO) 480/277-volt panel with multiple 20-amp breakers feeding emergency lighting circuits. The instantaneous setting has been set at 10 times the constant current rating of the plug (450 amps). The LTPU is set for 50% and reduces the plug setting to 225 amps. As long as the breaker settings are locked off so they cannot be easily changed, this configuration can protect a 225-amp feeder and feed a 225-amp MLO panel per NEC Section 240.6 (C), Restricted Access Adjustable Trip Circuit Breakers, which states:

A circuit breaker(s) that has restricted access to the adjusting means shall be permitted to have an amp rating(s) that is equal to the adjusted current setting (long-time pick-up setting). Restricted access shall be defined as located behind one of the following:

  • Removable and sealable covers over the adjusting means.

  • Bolted equipment enclosure doors.

  • Locked doors accessible only to qualified personnel.

The 10-times setting for the instantaneous trip will be based on the 450-amp plug setting and would be increased to 4,500 amps. An example from last month's installment (Figure 4) illustrated an 800-amp ANSI power breaker with no instantaneous setting and a 225-amp and a 20-amp breaker with instantaneous settings at 10 times the constant current, making the 225-amp breaker's instantaneous setting 2,250 amps. This instantaneous trip setting has been increased by 100% to 4,500 amps in Figure 3. The larger instantaneous setting would certainly reduce the potential of a fault current taking down the upstream breaker. This solution is not perfect, but would probably represent significantly less cost and physical space than the ANSI power breakers and switchgear construction required when utilizing breakers with the instantaneous portion of the time current curve removed. In this situation, fault currents above 4,500 amps could still trip both breakers. Furthermore, there would be additional expense involved in breakers with sensors and plugs.






Look before leaping

In conclusion, I believe a broad change to the NEC as indicated in this series should be evaluated and reconsidered before local jurisdictions adopt the 2005 version. The effect of this change could be quite dramatic. The engineering and overcurrent protection manufacturing community should find some middle ground that would best serve building owners and tenants. Coordination of the overcurrent protective devices within the electrical distribution systems is important in order to isolate faults to the smallest portion of the electrical distribution system as possible. But requiring fused distribution, ANSI-rated electronic power breakers—complete with the instantaneous portion of the time current curve turned off—and switchgear construction is expensive. So is the use of zone interlocking for all systems with emergency or legally required standby distribution, especially for smaller, less critical electrical distribution systems. Fused distribution also enlarges electrical rooms and reduce leasable space. Fused distribution could also require stricter maintenance protocols and necessitate the availability of spares, and could actually lengthen outages once they occur.

Perhaps the answer is to require coordination studies and analysis from a qualified licensed electrical engineer. In other words, improve coordination between overcurrent protection devices, but don't mandate complete coordination because of the potential worst-case scenario of a bolted short. Perhaps this type of analysis should be required to manipulate instantaneous settings of breakers in reference to the available fault current in the electrical distribution system. If done correctly, a coordination study and analysis could go a long way toward enhancing the electrical distribution system's reliability and reducing the extent of possible outages without causing undue expense to the client.





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