“Standard for Electrical Safety in the Workplace” (NFPA70E) was recently updated by the National Fire Protection Association (NPFA). It primarily addresses fire and explosion hazards caused by arc flash in electrical equipment. The ultimate goal of this initiative is to eliminate these events, thereby mitigating risk to facilities and—most importantly—injury to humans.

The U.S. Occupational Safety & Health Administration (OSHA) 29-CFR, part 1910.333, sub-part S (electrical) specifically addresses Standards for Work Practices and also references NFPA70E. The National Electric Code NEC 2005, article 409 (NEC409) was also amended to align with NFPA70E. Subsequently, in April of 2006, UL 508A SB was released as the guideline for panel fabricators to meet the requirements of NEC409. At the municipal, county and state levels, electrical codes are being amended at an increasing rate to include the requirement for short circuit current rating (SCCR) labeling of industrial control panels per NEC409.

Often, industrial control panels used in electrical heating applications house power controllers. For purposes of this discussion, SCCR can be defined as the maximum short circuit current a component, assembly, or equipment can safely withstand when protected by a specific overcurrent protective device, or for a specified time interval. Likewise, the highest current (at rated voltage) that a device is intended to interrupt under standard test conditions is known as the interrupt rating (IR).

The maximum SCCR of an industrial control panel must always be equal to or greater than the fault current (FC) available from the electrical feeders from which it draws power. Otherwise, a catastrophic and violent equipment component and enclosure failure can occur during a short circuit event. (Note that arc flash hazards can still exist in electrical control panels regardless of the assigned SCCR. Safe electrical work practices should always be heeded.)

NEC409 covers industrial control panels intended for general use and operating at 600 V or less. Typically, these house power circuit components that supply main line power to loads such as motors, heaters, lighting, appliances or receptacles. Compliance requires that the exterior of all industrial control panels be marked with the SCCR, as calculated or tested per the procedures outlined by UL 508A SB and stipulated by NEC409.22 as “An Industrial Control Panel shall not be installed where the available Fault Current (FC) exceeds its SCCR as marked in accordance with NEC409.110, paragraph 4.” Therefore, selecting overcurrent protection devices (circuit breakers or fuses) with the appropriate IR specification is a safety essential and a required step in meeting NEC409.

Determining SCCR

Three basic methods exist for determining the SCCR for industrial control panels:

Test Each Unique Panel Construction

This is usually financially impractical, unless numerous panels of identical design are involved. Testing must be witnessed by a UL project engineer (a paid service). The construction must be recorded in a follow-up procedure, which also needs to be reviewed by UL (a paid service). Given the numerous possible component combinations within a panel, this requires a large investment in testing and record keeping.

Purchase Previously Tested Combinations

This method is also known as coordinated selection. Major component manufacturers (typically of breakers, fuses, and VF drive/starters) provide software tools for panel designs using common combinations, to which a documented SCCR is assigned for the grouping. Deviations from these standard groupings will invalidate this approach.

Utilize UL508A, Supplement SB Method

In most cases, this is the best approach, but it requires care in selecting panel devices with adequate SCCR. The procedure is basically a “weakest-link-in-the-chain” evaluation, where the installed device with the lowest SCCR can dictate the overall SCCR of the entire panel.

Many manufacturers do not invest in actual SCCR testing and instead rely on the default SCCR values assigned by UL based on device category. For SCR power controllers, this value is 5,000 amps (5 KA), if UL-certified testing is not performed. Many installations require industrial control panel SCCR to be 35, 65 or 100 KA, so designing an industrial control panel that includes devices with low SCCR can be a major challenge.

Example Scenario

In accordance with NEC409, in order to safely connect directly to the feeder circuit, the panel must be designed for a SCCR of 100 KA. So far, the design shown in Figure 1 appears sound. The feeder circuit FC is 100 KA and the fuse IR of 200 KA exceeds that value, as required. At this point, an erroneous assumption is often made that the panel SCCR = fuse IR = 200 KA. However, this is not always the case.

UL508A, Supplement SB, Section 4.3.3 outlines the method for determining how other branch devices impact overall panel SCCR (even if protected by current-limiting fuses). It is a “weakest-link-in-the-chain” evaluation, meaning that the lowest SCCR circuit within the panel will ultimately determine the overall SCCR of the entire control panel. It specifically addresses the above erroneous assumption that the panel SCCR equals the fuse IR, which is only valid if the remaining devices in the branch circuit have an SCCR not less than the IP (peak let-through current) of the fuse. Referring to the manufacturer’s data sheet or UL508A Supplement SB, Table SB 4.2 for the fuse selected, IP = 42 KA.

Consider the branch circuit, namely the SCR power controller. If the manufacturer has not tested and certified the device with UL to achieve an elevated SCCR for the device, then an SCCR default value of 5 KA is assigned, per UL508A Supplement SB, Table SB 4.1. Since the SCR power controller SCCR of 5 KA is less than the fuse IP of 42 KA, the overall control panel SCCR will be only 5 KA. The panel will not meet NEC409, and if local electrical code requires this compliance it cannot be directly connected to the feeder circuit. Now what?

Possible Solutions

One possible workaround is to install a current limiting reactor (transformer) between the feeder circuit and the control panel. This incurs significant additional cost and requires additional real estate, which may not be available for a given installation.

Alternatively, leading electrical component manufacturers have made the investment to test their devices at UL, in order to achieve SCCR certification much higher than the UL default values. Specifying these superior devices often eliminates the need for additional branch circuit breakers/fuses and intermediate feeder reactors. The overall benefit realized by selecting an SCR power controller certified to high SCCR is evident in Figure 2.

The cost of these best-in-class devices is about the same as other untested devices in their category. Any cost differential is typically more than offset by the savings realized through the elimination of unneeded circuit breakers/fuses, reactors, excess panel wiring and engineering manpower.  

For additional information, contact the lead author at (952) 345-6205 or dbender@ccipower.com, or visit www.ccipower.com.