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UL 508 Listed control components used as a main feeder: We will review four of the most common misapplications of components within the panel and the circumstances that may occur in each.ġ. The application requirements for each component are often quite complex and unique to the component, resulting in erroneous interpretation, leading to improper use of the component. Common misapplications in industrial control panelsĮach component in an industrial control panel must be properly applied for safe and reliable operation. This is especially true with regards to the use of current-limiting devices. If this is lower than any power circuit component after the steps above, the assembly SCCR is lowered to this value.Īlthough the overall process may seem simple and straightforward, understanding the rules of UL 508A Supplement SB and the proper application of devices is a complex and challenging concept. The final step is verifying the lowest interrupting rating of the overcurrent devices in the industrial control panel. It is not permitted to increase the branch-circuit overcurrent device interrupting rating. However, when current-limiting overcurrent devices are used, only the component SCCR can be increased. When transformers are used, both the branch-circuit overcurrent device interrupting rating and the component SCCR can be increased. When a current-limiting device (fuse or circuit breaker marked current-limiting or a transformer) is located in the feeder circuit, it can be investigated to determine if it can increase branch circuit component SCCR ratings. Any component or overcurrent device on the lineside of the branch-circuit overcurrent device is in the feeder circuit. It’s possible to raise the ratings of branch power circuit components – those on the load side of the branch-circuit overcurrent device (the overcurrent device closest to the load). In most applications, a default SCCR of 5 kA is not adequate.įor these installations, UL 508A Supplement SB allows the manufacturer of the industrial control panel to achieve the required SCCR by using power circuit components tested by the component manufacturer for a high fault rating and incorporating them into the industrial control panel manufacturer’s UL report (procedure). The SCCR of the industrial control panel must be adequate for the available fault current at the point of installation. If there are no product markings, a default value can be used as indicated in UL 508A, Supplement SB, Table SB4.1. The lowest component SCCR (steps 1 and 2) and the lowest interrupting rating of the overcurrent protective device (step 3) results in the overall assembly SCCR.įor step 1, power circuit components (components that supply power to external loads such as motors, lighting, heating, appliances, or convenience receptacles): either the product markings or the instruction sheets determine the SCCR.
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The panels may include disconnecting means, power distribution blocks, motor controllers, and branch-circuit protective devices.