Schneider Electric NEC 409.2 January 15, 2008 ASSESSING THE HIDDEN DANGERS IN YOUR INDUSTRIAL CONTROL PANELS by Grant Van Hemert, P.E., application engineer, Water and Wastewater Competency Center, Schneider Electric Industrial control panels can be found in lift stations, OEM-supplied equipment, or as main control system cabinets. Traditionally, industrial control panel design depended on several different articles in the National Electrical Code (NEC ). This resulted in confusion and disagreements. In 2005, NEC 409 was introduced to address this confusion. To understand the technical and liability ramifications, an overall introduction to the issue is required. What is an Industrial Control Panel? According to the NEC 409.2, an industrial control panel is an assembly of a systematic and standard arrangement of two or more components such as motor controllers; overload relays; fused disconnect switches; and circuit breakers and related control devices including pushbutton stations, selector switches, timers, switches, and control relays, with associated wiring, terminal blocks, pilot lights and similar components. Industrial control panels are designed to handle a custom application with an enclosure that has one or more doors. When the enclosure is opened, an unobstructed area is available to house controls for multiple motors, valves, etc. Power and control signals are routed via dedicated wires. Another assembly, the motor control center (MCC), could meet this definition. However, MCCs have distinct features, such as a hidden three-phase horizontal bus, that distinguishes them from an Industrial Control Panel. MCCs must comply with separate codes and standard. Furthermore, MCCs are made by mainline electrical equipment suppliers, such as Square D/Schneider Electric. Motor control center Control panel with door open and closed Figure 1 A system integrator, electrical contractor or consultant can identify these different types of equipment more clearly. 1
What Does NEC 409 Say? NEC 409 is mostly calculations and references to other sections within the NEC. However, three sentences in section 409.110(3) require an industrial control panel to have a marked short circuit current rating (SCCR). Satisfying these sentences requires a major shift in industrial control panel design. What is a Short Circuit Current Rating? The SCCR is the amount of momentary energy that a device must withstand without compromising the physical integrity of the control panel. This is to protect workers near the closed panel from arc flash, or shrapnel. If the fault current is less than the device s SCCR, then the device should contain the energy. However, the device could sacrifice its functionality and may need replacing. The SCCR is different than the current rating of the device. The current rating is the continuous current that the device can handle without degradation of the anticipated life (or tripping, in the case of a circuit breaker). Every device has both ratings. Circuit breakers call this the Amperage Interrupting Current (AIC) rating. The final SCCR of the control panel can be influenced by other factors outside of the actual components being examined. Before NEC 409, it was unclear how to address SCCR in a control panel. Control panel designers might have been unaware of the topic, or used a variety of rule of thumb techniques. Often the result was a control panel with potential vulnerabilities. Obviously, vulnerable control panels are not what any operator wants. NEC 409.110(3) increases reliability by ensuring that a fault in the panel can only traverse through devices that can handle the current until a breaker clears. Therefore, if a 17,000 amp fault occurred downstream of the starter, then the starter must hold together until the circuit breaker can open and clear the fault. The intent of the code is clear; but, how can control panel builders ensure they are in incompliance? NEC 409.110(3) says that a panel builder is responsible for marking the SCCR of his panel. While several methods exist to determine the rating, the code suggests UL-508A-2001 Supplement SB. What is UL-508A? Underwriters Laboratories (UL) standard 508A is one of the most popular methods for ensuring quality industrial control panel construction, and satisfying the requirement of NEC 90.7. NEC 90.7 indicates that an inspector, at their discretion, is not obligated to examine third-party labeled panels. A full discussion of UL-508A is well beyond the scope of this article. However, Supplement SB is the short circuit guideline for that standard. Supplement SB provides two basic methods for satisfying NEC 409.110(3). For the purposes of this article they will be referred to as Straight-Line, and Combination. These are illustrated in Figure 2 2
23KA Available 5KA Rated Group tested to 25KVA 15KA Rated Straight Line Method: Each device must be individually rated for available fault current Combination Method: Different rated devices tested together for a rated combination Figure 2 In the Straight-Line method, every component within a fault path has to be rated for the maximum available fault current. The Combination method uses groups of devices that have been tested together to achieve a rating. It is possible that individual device SCCR may be less then the tested rating. A full discussion of this reason is beyond the The Combination method allows for more economical choices than the Straight-Line method. Combinations cannot be created without testing and approval by a third party testing agency. Most panel shops are ill-equipped to determine a combination. Electrical equipment suppliers, like Schneider Electric, assist control panel builders by testing various combinations for use. The result is a simple tool to aid the designer. Schneider Electric s tool can easily be accessed via the Web by searching Schneider Electric UL 508A Frameset Commitment. Who has ultimate liability? This discussion is well beyond the scope of this article. In summery, NEC 110.9 says that equipment installed must be able to withstand the available fault current. However, 409.110 only require panel builders to mark the panel. It does not say that they have to provide a panel sufficient for the fault current. According to NEC 90.4 the Authority Having Jurisdiction (AHJ) is responsible for compliance with the code. NEC 100 says that the AHJ can be a governmental entity, or person responsible for the facility. Thus, the facility must insure that consultanting engineers require each panel builder to adhere to NEC 110.9 when complying with NEC 409. To accomplish this, the contract documents need to address this, and the plans should have SCCR ratings for each location where an Industrial Control panel can connect to the facilities power distribution system. 3
How does a facility determine the Available Fault Current? During design, the consultant s power distribution engineer (PDE) will contact the utility to determine the available fault current at the point of tie-in. It might seem obvious that this rating would determine the available current at all points in the facility, but this is not the case. Ohms Law indicates, transformers, reactors, filters, VFDs, and conductor impedance will decrease available fault current. Thus, the available fault current at a given point in a facility is dependent on the upstream equipment, wire diameter, wire composition, and wired distance to the facilities utility tie in. To insure proper control panel protection, the PDE should show the available fault at each level on their power diagrams drawings or one-line (See Figure 3). This will insure that an industrial control panel engineer can properly comply with NEC 110.9 during design. 480VAC at 65KA 480VAC at 65KA 480VAC at 65KA 240VAC at 32.5KA Figure 3 However, calculating the available fault current for every location where an Industrial Control Panel connects to the facilities power distribution system can be laborious. This brings us to the next point. Why not use the Available Fault Current at the point of utility tie-in for all panel designs? There are two reasons for this the first being cost. Devices that have a lower SCCR/AIC rating tend to be made up of materials that are less expensive. For example, in the above scenario, designing for 50,000 Amps when only 6,250 is available would needlessly increase cost. The second reason involves how circuit breakers and fuses respond to fault currents. Under normal operating conditions, a circuit breaker or fuse has an intentional time delay when an overload is encountered to prevent nuisance tripping. However, during a fault condition, this delay is bypassed. The closer the fault current is to the rated interrupting capacity the faster the opening. Let s assume the facility has a 100KA rating at the point of Utility tie-in. If a breaker has a 100 KA 4
rating, and a 93 KA fault is encountered, then the breaker s magnetic mechanism would open almost instantaneously. However, if the available fault current at the connection point of the Industrial Control Panel is actually 4.5 KA, then the 100KA breaker will delay a few microseconds before opening. This delay will allow damaging energy to pass through the equipment. Thus, a 5 KAIC rated breaker should be used with an available fault current of 4.5 KA to insure a instantaneous opening time. When does NEC 409 Go Into Effect? While section 409 was added to the 2005 NEC, states and municipality adoption has occurred at different rates. Figure 5 shows adoption as of January 2007. Color Purple Green Blue Orange Code Enacted 2005 NEC Code 2002 NEC Code 1999 NEC Code State s Discretion Figure 5 What About Existing Industrial Control Panels? Generally, when equipment is installed, it only has to comply with the codes at the time of installation. This means that a control panel installed in 1984 only has to comply with the 1984 code. However, successful lawsuits have been brought against building owners for not complying with current sprinkler codes in older buildings. Thus, a legal precedence has been set. Therefore, a facility might want to create a proactive plan to address older control panels. A consulting engineer 5
or well qualified system integrator may have guidance on how to best proceed. More information also can be found at www.squared-water.com. Compliance with NEC 409.110(3) will better enable industrial control panels to limit the damage from a fault, allowing municipalities to recover faster at a minimized cost, and better protect the health and safety of on-site personnel. About the Author Grant Van Hemert is an applications engineer for the Schneider Electric Water Wastewater Competency Center. Mr. Van Hemert has over 12 years of water and wastewater experience. Previously he was a design and implementation engineer where he designed and commissioned automation and instrumentation systems dealing with aeration, screening, and clarification. Van Hemert, a licensed professional engineer in the state of North Carolina, can be reached at Grant.VanHemert@us.schneider-electric.com. Square D is a registered trademark of Schneider Electric and/or its affiliates in the United States and/or other countries. Other marks used herein are the property of their respective owners. 6