Consulting-Specifying Engineer - November 2007 - (Page 24)

1,500 kW ESB 277/480V, 3Ø, 4W 2,500 1,200 400 ATS-EQ 1,200 ATS-CR 1,200 Figure 3 Panel serving two automatic transfer switches. adjustable. Here the engineer must carefully adjust the settings to achieve satisfactory coordination. The NEC does not clearly define what must be selectively coordinate. Therefore, it is up to the engineer to work with the local AHJ to determine an interpretation of what is selective coordination and at what the requirements will be. In Florida healthcare, the local AHJ has required selective coordination plots, signed and sealed by the engineer of record, for years. The requirements for selective coordination have evolved through the years in this region and currently require selective coordination from 0.10 s on. In order to understand the protection of the generator and the selective coordination of the generator, some basic understanding of time-current curves is required. Time-current curves are log graphs that represent the tripping time versus the current of breakers. They also are used to demonstrate the damage curve for generators, transformers, motors and wires, and to evaluate the fault contribution from a generator, or the inrush seen from a transformer. To do a complete selective coordination study, all of the above items must be considered. For a generator the two important components are the damage curve and the decrement curve. The damage curve (i.e. overload curve or withstand curve) is a measure of current from which the generator will be damaged. The decrement curve, as stated above, is a measure of the fault current produced by the generator. In the event that the generator faults, we want the breaker closest to the fault to trip and protect the system. The fault current must be sufficient to allow the breaker to trip. A standard generator cannot maintain fault current for long, so standard practice is to use a permanent magnet generator. This generator will maintain approximately 300% of the rated current for approximately 10 s, long enough for the downstream breakers to trip and protect the system. At approximately 10 s, the decrement (fault) curve will intersect with the damage curve for the generator. The selective coordination for the emergency power system must protect the generator, but allow the downstream breakers to trip. Figure 3 shows a panel serving two ATS. Panel ESB is the parallel switchboard (only one generator is shown) that serves a transfer switch ATS-EQ via a 1,200-amp breaker. This board is fed by several 1,500-kW generators with 2,500-amp breakers protecting each. Selective coordination requires that the ATS-EQ breaker trip before the GEN breaker. But if the fault is greater than both breakers, what will happen? We want to avoid a situation where a fault will take a generator offline before it takes the faulted branch of line. In Figure 3, we don’t want a fault in ATS-EQ to take the generator offline because it will remove the power for ATS-CR. This is not selective coordination. The goal is simple: design the protection that allows the downstream breakers to trip before the generator breaker trips. The generator breaker in Figure 4 is set to the right of the decrement curve from 0 s through 2 s, and crosses to the left of the decrement curve before damage to the engine occurs. Remember, the decrement curve is the fault output for the generator. It is important to note that many generator controllers have internal overcurrent protection that will take the generator off-line as well. Make sure you work with the manufacturer to confirm you have a complete selective coordination system. Emergency system design is a critical first step toward the proper operation of today’s healthcare facilities. Identifying the issues and addressing the challenges one at a time, we will be able to provide a safe and reliable emergency system, and do our part toward enhancing patient safety. 1,000 Current in Amperes GEN-0001 EDP-CB6 Trip 1200 A Settings Phase LTPU/LTD (A 0.4-1.0 x S) 1 (1200A); 0.5 STPU (1.5-10 x LTPU) 4 (4800A) STD (0-0.4) 0.1(I^2 T Out) INST (2-15 x S) 2 (1600A) Damage Curve 100 Decrement Curve 10 Time in Seconds GEN-MCB Trip 2500 A Settings Phase L LTPU/LTD (A 0.4-1.0 x S) 1 (2500A); 0.5 L STPU (1.5-10 x LTPU) 10 (25000A) STD (INST-0.4) 0.4(I^2 T Out) INST (2-15 x S) 12 (30000A) 1 0.10 0.01 10 100 1K 10K 100K Figure 4 Time current curve showing genset decrement curve. 24 Consulting-Specifying Engineer • NOVEMBER 2007

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - November 2007

Consulting-Specifying Engineer Contents Editor's Viewpoint Letters In The News M/E Roundtable Emergency Power for Healthcare 2007 Products of the Year Industrial-strength Lighting Hazardous HVAC Codes and Standards Management Report Jobs/Classifieds Specifier's Notebook On-Peak Performance Contents Best Battery Selector in Engine Starting Applications NEC 708: Practical Impact on Backup Power Systems Protecting Power: Specifying Outdoor Generator Enclosures Investing in Backup Power Systems Coes, Consultants, Manufacturers, and Standby Power Systems

Consulting-Specifying Engineer - November 2007

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