What is UL 508A?
UL 508A is the UL standard for the construction of Industrial Control Panels. This blog gives guidelines to panel builders on various issues including proper component selection, wiring methods and calculation of short circuit current ratings. If a customer follows UL’s recommendations, they can apply to have their panel listed by UL. Upon successful inspection, the panel would bear the UL label. It is important to ask your customer if they are building to this standard, as many panel builders choose to skip this step, and many only list their panel if specifically required by the end customer.
How to calculate the Short Circuit Current Rating (SCCR) of a Control Panel
1. Calculate the SCCR of individual components (identify lowest rating in power circuit)
2. Identify where the rating of these components are increased because they are used in combination with current limiting devices (circuit breakers, fuses, etc.)
3. Compare the two SCCR calculations and see what is the lowest rated (SCCR) devices including in combination (Step 2) – this yields the panel SCCR for UL508A labeling
There are three simple steps to calculating the SCCR
Step 1: Determine the individual short circuit current rating of each individual component in the system. You can exclude transformers, dry-type capacitors, resistors, varistors and voltmeters. If the device is not marked UL provides a table (SB4.1) of standard SCC ratings in the UL 508A guideline.
Example: Look at the diagram below. Determining the SCCR rating of a component, is easy. Simply look at the label on the device and it should be listed. If the component is unmarked, UL provides a table (SB4.1) of standard ratings in the UL 508 guideline. If there is no marking, and the device is not listed in the table, you must look at the documentation to determine if a combination rating is available.
In the breaker example below we know –
Main Breaker is rated 65kA
Starter is rated 10kA
Contactor is rated 5kA
QC breaker is rated 10kA
We need to determine the SCCR of the HMCP and the PLC.
The PLC can be ignored because it is part of the control circuit.
The starter information states that when used in conjunction with a HMCP it carries a rating is 65kA.
Therefore the panel rating is 5kA based on lowest rated component.
Step 2: Calculate the available short circuit current when taking into account current limiting components like control power transformers, fuses and circuit breakers.
Example: Since the available fault current of the system is 50kA, we need to determine if there are any current limiting components in the system that would limit the available short circuit current, and allow us to achieve a higher rating. The UL 508 guideline lists components that can limit the short circuit current. UL 508 states that a power transformer rated 5kVA or less will limit the available short circuit current to 2kA in section SB4.3.Therefore, because the rating of all of the devices on the secondary of the transformer are greater than 2kA, the SCCR of the panel in this example would be 65kA when using Circuit Breakers.
Calclulate the available SCC when taking into account current limiting components like CPT‘s, fuses and circuit breakers. (Left Side). The available fault current is 50kA. Per the UL 508A guideline a power transformer rated 5kVA or lower will limit the fault to 2kA on the secondary side of the transformer (Section SB4.3). Therefore, because the rating of all of the devices on the secondary of the transformer are greater than 2kA, the SCCR of the panel in this example would be 65kA.
Let‘s follow the same procedure and determine the SCCR using a fuse.(Right side)
As with the breaker example, we can eliminate the the branch circuit feeding the transformer, since it limits the current to 2kA. We need to determine if the 100A fuse will limit the current enough to protect the starter rated at 10kA. From Table SB4.2 we can determine that at 50kA, Ip for a Class R 100A fuse is 14kA. Therefore the panel would need to be rated at 10kA.
Step 3: Compare the values obtained in steps one and two to determine the overall short circuit current rating of the panel.
Example: Simply compare the two values obtained. If for instance, the lowest rated
component in Step 1 is 5kA, but in Step 2 you are able to limit the peak let through current use the value obtained in Step 2.
65KA SCCR rating – Circuit Breaker Example
10KA SCCR rating – Fuse Example
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