SHORT CIRCUIT CURRENT RATINGS How to Determine and Increase Short Circuit Current Ratings for Industrial Control Panels WHITE PAPER How to Determine and Increase Short-Circuit Current Ratings for Industrial Control Panels Introduction if a panel had a main feeder circuit protective device with an interrupting rating of 22 kA, then the manufacturer of an A short-circuit current rating (SCCR)1 is the maximum current unapproved panel would also label their panel with an SCCR a device or system can safely withstand for a specified of 22 kA. time (such as 0.05 seconds), or until a specified fuse or circuit breaker opens and clears the circuit. SCCR is usually However, each power component within the panel has a expressed in kiloamperes (kA). different SCCR or IR, many of which are likely to have a rating of 5 kA or less. Therefore, if a 22-kA fault occurs, even if the SCCR labeling requirements create a safe installation and main circuit protective device can safely interrupt the fault, help manufacturers, owners and managers meet OSHA one or more of the power components within the panel can regulations and NFPA codes and standards. still potentially catch on fire or violently explode. Both the NEC and OSHA forbid equipment from being located The NEC specifically requires SCCR markings for: near any point where the available short-circuit current is more than what the equipment can withstand. • Services (Article 230) Available fault currents are increasing in industrial facilities • Industrial Control Panels (Article 409) due to a continuously increasing demand for electric power. When available fault currents at the equipment exceed the • Motors, Motor Circuits, and Controllers (Article 430) panel’s SCCR, there can be catastrophic results. • Air-Conditioning and Refrigerating Equipment (Article Current-limiting fuses can optimize a panel to give it a strong 440) competitive advantage over molded case circuit breakers (MCCB) so long as SCCR is considered in its design. Most • Elevators, Dumbwaiters, Escalators, Moving Walks, MCCBs do not provide adequate SCCRs. Platform Lifts, and Stairway Chairlifts (Article 620) The most traditional way to increase a panel’s low SCCR • Industrial Machinery (Article 670) that is caused from an MCCB is to replace the MCCB with current-limiting fuses. But after making this improvement • Emergency Systems (Article 700) to the panel, how can you increase the panel’s SCCR even further? • Legally Required Standby Systems (Article 701) • Optional Standby Systems (Article 702) Why Must Industrial Control Panels Be • Critical Operations Power Systems (Article 708) Marked with Their SCCR? These markings allow installers and inspectors to compare NEC Article 409.110 requires industrial control panels (ICP) fault current studies at the facility where the panels are to be visibly marked with the panel’s overall SCCR. NEC intended to be installed to the SCCR of the control panel Article 409 requires an analysis of panels and facilities to be to minimize potential hazards in industrial or commercial conducted for potential electrical hazards. The marked SCCR facilities. must be based on either the SCCR of a listed and labeled assembly or on an SCCR that was established by an approved For safety and reliability, it is essential to compare accurate method. Overcurrent protective devices must also be marked SCCR markings on equipment with the available fault currents with an interrupting rating or an ampere interrupting capacity. on a regular basis. A common misconception among equipment manufacturers is that the ampere interrupting capacity or the interrupting rating (IR) of the main circuit protection device is the same as the industrial control panel’s (ICP) SCCR. This can confuse personnel about where the panel may be safely used. For example, before the introduction of NEC Article 409 in 2005, 1 Short-circuit current ratings were known as “withstand ratings” before the NEC first defined it in the 1999 edition. © 2019 Littelfuse, Inc. 2 Littelfuse.com How to Determine and Increase Short-Circuit Current Ratings for Industrial Control Panels What is a Current-Limiting Overcurrent How Does Current Limitation Increase Protective Device? SCCRs? Current-limiting overcurrent protective devices, as defined Current limitation increases SCCRs because it reduces the by Article 240.2 of the NEC, reduce the current flowing destructive thermal energy. Figure 1 shows the maximum in a faulted circuit to a magnitude that is substantially instantaneous peak current (Ipeak) during the first half cycle less than what would otherwise be obtained if the device after the initiation of a fault. The gray area represents what were replaced with a solid conductor with a comparable the Ipeak will be without the use of current-limiting fuses, while impedance. When operating in their current-limiting range, the green area depicts the Ipeak when current-limiting fuses these devices can clear a short circuit current in less than 8.3 are used. milliseconds, which is less than one half of an electrical ac cycle (see Figure 1). Depending on the power factor and when the fault occurs, the maximum possible Ipeak without current limitation can Current-limiting overcurrent protective devices must interrupt be as high as 2.3 times the available rms fault current when currents in their current-limiting range to be effective. the fault occurs. However, when current-limiting fuses are used, the maximum Ipeak is only a small fraction of what the potential flow would otherwise be. The area under the curve represents the I²t let-through energy of the fuse. The lower the I²t, the lower the potentially destructive thermal energy will be that passes through the circuit when it is interrupted. Maximum peak-let through current with noncurrent-limiting fuses. (Peak value can be as high as 2.3 times the available rms fault current.) CURRENT NOTE: Total clearing I2t = Melting I2t + Arcing I2t Point at which Maximum peak-let through current a fault occurs with current-limiting fuses Fuse elements melt Arcing energy (l2t) Melting energy (l2t) TIME Melting Arcing time time Arc is extinguished Fuse total clearing time (less than ½ a cycle) Figure 1. Maximum instantaneous peak current during first 1/2 cycle of a fault with current-limiting fuses versus noncurrent-limiting fuses. © 2019 Littelfuse, Inc. 3 Littelfuse.com How to Determine and Increase Short-Circuit Current Ratings for Industrial Control Panels 1000000 1000000 Interrupting Ratings for Overcurrent 800000 800000 600000 B 600000 B Protective Devices 400000 400000 300000 300000 233000 200000 Although fuses and circuit breakers are tested using different 200000 Fuse Fuse methods, a fuse’s interrupting rating (as well as the AIC of a 100000 Ampere Ampere Rating 100000 Rating circuit breaker) is considered to be the same as its SCCR. 80000 80000 60000 600 A 60000 c b 600 A 40000 400 A 40000 400 A 30000 30000 NEC Article 110.9 says that “equipment intended to interrupt 200 A 200 A 20000 20000 current at fault levels shall have an interrupting rating at 100 A 100 A 60 A 60 A nominal circuit voltage at least equal to the current that is 10000 10000 8000 30 A 8000 30 A available at the line terminals of the equipment.” 6000 6000 4000 4000 NEC Article 110.10 specifically requires the coordination of 3000 3000 overcurrent protective devices and the components that they 2000 2000 are designed to protect. This prevents extensive damage to 1000 1000 any component or circuit in the event of a short circuit fault. 800 800 600 A 600 400 400 Fuses and circuit breakers that are Listed or Recognized by an 300 300 Instantaneous peak let-through current in amperes current Instantaneous peak let-through 200 NRTL are tested in a laboratory to verify that they can safely in amperes current Instantaneous peak let-through 200 A interrupt certain fault currents. To obtain certifications, a fuse 100 100 d a 100 200 300 400 600 800 must open safely and not damage itself or anything around 100 200 300 400 600 800 1000 2000 3000 4000 6000 8000 1000 2000 3000 4000 6000 8000 10000 20000 30000 40000 60000 80000 10000 18000 20000 30000 40000 60000 80000 100000 200000 it. It also may not vent hot gases, which can ignite flammable 100000 200000 surroundings. Available fault current symmetrical rms amperes Available fault current symmetrical rms amperes Figure 2. Relationship between available rms fault current Circuit breakers must also open safely under controlled and maximum instantaneous peak during a short circuit. conditions and not ignite nearby objects. Circuit breakers, Figure 14 Figure 15 however, can vent high-temperature gases and can be a source of ignition in hazardous areas. Current Limitation and Peak Let-through Some circuit breakers and supplementary protectors are Charts tested and approved to be able to safely interrupt a high-level short circuit current only one time. After the first interruption, Current-limiting fuses operating in their current-limiting range these circuit breakers may not be operable and cannot be will limit the maximum instantaneous peak current (Ipeak) to approved to be used again. a value that is substantially less than the peak current that could flow if the fuse were not in the circuit. Figure 2 shows the relationship between the available Rms versus Ipeak symmetrical rms fault current and the maximum possible I if a short circuit occurs. Figure 2(A–B) is the maximum The SCCR of components in panels is expressed as rms kA. peak possible I under worst-case conditions (15 % power Root mean square (rms) current is an expression of effective peak factor, asymmetrical), or 2.3 times the available symmetrical current over a time period. rms fault current. The other lines in Figure 2 illustrate how ampere ratings of standard current-limiting fuses affect the Conversely, if the Ipeak of a current-limiting fuse is 23 kA, then peak let-through.