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All electrical panels must be designed to achieve a minimum rated short circuit current rating. This short circuit current rating (SCCR) is a safety critical rating on assemblies and components that represent the maximum short-circuit current that they can withstand. There are a number of acceptable methods to determine the SCCR. One of the most common is the method described in supplement SB of the standard.
This however requires care in selecting devices with adequate SCCR. The procedure is basically a “weakest-link” evaluation, where the installed device with the lowest SCCR will dictate the overall SCCR of the entire system.
The SCCR must always be stated on the Rating Label and when you consider that an installation may have a SCCR of over 100KA, careful consideration of SCCR is critical to enable connection by the installer.
How do I know when to use UL 1077 or UL 489 breakers in panels or systems? The answer is seemingly easy, but like most easy questions, there is confusion. The Physical difference between the two are the required pole spacings, as branch protectors have higher arcing distance.
The general rule is that a UL 489 circuit breaker can protect multiple devices at one time, such as an entire control panel, AND external fed equipment.
UL 1077 defines supplementary protectors as devices intended for use as overcurrent, over-voltage or under-voltage protection within an appliance or other electrical equipment where branch-circuit overvoltage protection is already provided or is not required.
Most people are aware of the fact that UL508A lists specific cable sizes and suggests cable colour requirements, these include colours such as blue with white strip for the 0 volts of a grounded DC control circuit and prescribe the minimum cables size allowable for the that circuit.
The standard also lists the minimum bending space between a field wiring termination point inside the panel and the directly opposite wall of the panel. The distance required depends on the current rating/size of the field wiring cables.
Insufficient wire bending space will lead to a noncompliance. Clearly, the cable size calculation is critical, and it can affect the dimensions of the panel in some cases.
The general assumption that selecting components that are UL listed and integrating them into a control panel that also carried a UL rating is enough, is simply incorrect. The key here is the understanding of the standard and the due diligence required to ensuring compliance.
Outside the common misunderstanding with branch and the Circuit Protection UL489 vs UL1077, power supplies can also cause confusion, dependant of the class of circuit and the power requirement needed, a UL listed, or recognised device may not be required!
Manufacturers can also fall foul of basic construction elements of the control systems when the standard is not understood, examples include isolator handles and their proximity to fan and/or filter outlets in doors.
The UL 508A certification provides the inspection authority and your customer evidence that the control panel complies with nationally recognised safety standards, which ensure public safety and compliance with national and local electrical codes.
Manufacturers that carry the UL 508A certification are subject to periodic unannounced inspections of their facilities by UL personnel. Through periodic audits, UL makes sure the manufacturer continues to meet the UL requirements for 508A certification.
The UL Mark on a component means that UL has evaluated and tested samples of this component and has concluded that they meet the necessary requirements, thus protecting the quality and integrity of the control panel.
Remember – the control panel builder HAS to be approved to be able to certify to UL508a (i.e. the design could be perfect, but if the machine builder is NOT approved – it’s NOT UL compliant.)
Come back next week for a guest blog by Rockwell Automation detailing the important component considerations in relation to UL.
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