Okay, let's break down the SCCR evaluation process for your ICP and address your questions. Understanding the concepts and applying the proper methods is crucial for safety.

Key Concepts & Standards SCCR (Short-Circuit Current Rating)

The maximum prospective short-circuit current that a component or assembly can withstand without suffering damage.

ICP (Industrial Control Panel)

An assembly incorporating two or more control circuit and/or power circuit components.

UL 508A

The safety standard for industrial control panels. It outlines the methods for determining the SCCR.

Series Rating

An evaluation of downstream component, such as a controller or a distribution block, used with a larger upstream overcurrent protection.

Understanding Your Circuit

You have the following circuit topology:`Main CB (50kA) -> PDB -> SPD (200kA) -> Combination Controller (50kA) -> TB's`Answer to Question 1: PDB SCCR

You are correct in your general understanding of the protection principle. However, here's a more nuanced approach for determining the PDB's SCCR: If the PDB is UL Listed/Recognized

Look for the PDB's marked SCCR. This is the ideal scenario. If it's marked, use that value.

If the PDB is NOT UL Listed/Recognized and the upstream circuit breaker is NOT specifically series-rated with the PDB

You cannot simply assume the main CB's interrupting rating applies to the PDB.

This is critical. UL 508A requires you to apply default SCCR values if the PDB is not marked and no series rating exists. Table SB4.1 of UL 508A typically dictates the default SCCR based on the type of PDB and its construction. The default SCCR for power distribution block is 10kA If the Main CB is Specifically Series-Rated with the PDB (and Documented)

If the manufacturer of the main CB has tested and published a series rating with that specific PDB model, then you can use the series rating. This must be documented by the manufacturer (e.g., a series rating combination sheet).

This is usually found through the manufacturer's data.In summary, if the PDB is not rated and no series rating is provided, 10kA is likely the correct default value to use for the PDB.Answer to Question 2: Terminal Blocks (TB's) and SCCR Evaluation

Here's the breakdown for the TB's: General Rule

UL 508A requires you to include all components within the control panel in the SCCR evaluation.

BUT... Simplification with Combination Controllers

You're right to think about the combination controller.

The important detail is whether the combination controller has a branch circuit protective device (BCPD) within it. If the combination controller has an integral BCPD (e.g., a circuit breaker or fuses)

And the terminal blocks are only connected to the load side of that integral BCPD, then you generally do not need to independently consider the SCCR of the terminal blocks. The combination controller's SCCR is what matters for that portion of the circuit. The controller is providing branch circuit protection to the downstream components. This is the most common scenario for motor controllers.



If the combination controller does not have an integral BCPD

Then you must consider the SCCR of the terminal blocks separately. In this case, determine the minimum SCCR rating between the branch-circuit protective device (BCPD) and the Combination motor controller (CMC). Then, choose the device with the lowest SCCR rating.

Default TB SCCR

If you do need to consider the TBs, many terminal blocks are marked with their SCCR. If they aren't marked, you'll likely need to use a default value (often 5 kA) based on UL 508A Table SB4.1.

Therefore, evaluate the controller if it has an integral BCPD, so the TB can be excluded from the evaluation.

Practical Steps for Your SCCR Evaluation1.

Gather Component Data

Collect the SCCR, voltage rating, and current rating for every component in your control panel, including the main CB, PDB, SPD, combination controller, and terminal blocks (if required).2.

Combination Controller SCCR

Look carefully at the combination controller's documentation. Does it have an integral BCPD? What is its SCCR?3.

PDB Verification

Check for PDB Listing/Recognition and Marked SCCR. If none, and no series rating exists, apply the appropriate default value.4.

SPD Consideration

SPD are usually a component of a system. Its purpose is for safety, and the SCCR should not be a consideration for the system.5.

Apply UL 508A Methods

Use the methods in UL 508A (primarily Section SB4) to determine the overall ICP SCCR. The simplest method is the component-by-component method, which uses the lowest SCCR of any component in the power circuit. Series ratings, if properly documented, can allow for a higher overall SCCR.6.

Document Everything! Keep a detailed record of your calculations, component data sheets, and any series rating documentation. This is essential for demonstrating compliance.

Important Considerations

Component Selection

Consider selecting components with higher SCCR ratings to simplify your evaluation and potentially achieve a higher overall ICP SCCR.

Manufacturer Support

Contact the component manufacturers. They can often provide guidance on SCCR, series ratings, and other application-specific information.

Qualified Personnel

Ensure that the SCCR evaluation is performed by qualified personnel who are familiar with UL 508A and relevant electrical codes.

Don't Assume; Always verify.

Example ScenarioLet's say: Main CB: 50 kA PDB: Not listed, no series rating (default 10kA) SPD: 200kA Combination Controller: 50 kA with an integral BCPD TB's: (Not relevant because of the integral BCPD in the controller)In this scenario, using the component-by-component method, the overall SCCR of the ICP would be 10 kA (limited by the PDB).By following these steps and understanding the nuances of UL 508A, you can properly evaluate the SCCR of your ICP and ensure its safety and compliance. Consult the latest version of UL 508A for the most up-to-date requirements. Flag for review