Okay, let's break down your SCCR concerns for this chiller application. This is a common area of confusion, so your questions are very relevant.

Understanding SCCR Basics SCCR (Short-Circuit Current Rating)

The maximum fault current a component or piece of equipment can safely withstand without causing a fire or electrical hazard. This is not the same as AIC (Ampere Interrupting Capacity) of a breaker/fuse. AIC is the maximum fault current a breaker or fuse can safely interrupt.

Available Fault Current (AFC)

The magnitude of current that would flow in a short circuit at a specific point in the electrical system. This is what you calculate using impedance-based methods, point-to-point calculations, or software.

Series Rating

Allows use of downstream circuit breakers with lower interrupting capacity than the available fault current as long as they are protected by upstream breakers with higher interrupting capacity.

Analyzing Your Chiller Situation1. Chiller SCCR and Separate Control Feed



Yes, it is highly likely the 5,000A SCCR applies to the entire chiller unit. This is because it's a complete, listed piece of equipment. The manufacturer is certifying that the entire assembly (compressor motors and controls) can withstand 5,000A of fault current. The separate 120V control circuit does not change the 240V SCCR requirement for the chiller itself. The control circuit is still part of the chiller unit. Therefore, you need to focus on the AFC on the 240V side (19,000A). This is the critical value you need to mitigate.2. SCCR of Only the Control Board

It's extremely unlikely that only the control board has a 5,000A SCCR. Manufacturers usually provide a single SCCR for the complete listed assembly. It is possible, but you would need written clarification from the manufacturer to prove it. If they provide documentation like a nameplate showing an SCCR for just the control panel, that would change things. Without definitive documentation from the manufacturer, you should assume the entire chiller has the 5,000A SCCR rating.3.

Contactor and Fault Location

Your thinking is correct. If the 120V control circuit energizes a contactor that switches the 240V compressor power, that contactor (or any other component in the 240V power path) could be the point of failure during a 240V fault. The fault current path matters. A fault could occur in the wiring to the compressor motor, the motor winding itself, the compressor contactor, wiring to the contactor, etc. All of these are part of the 240V power path within the chiller.4.

Fuses to Reduce AFC

This is a valid approach. However, there are important considerations:

Fuse Type

You

must use a current-limiting fuse. Class J fuses are often used in industrial applications and may be suitable. Fuse Sizing and Coordination

The fuse needs to be sized correctly to protect the chiller's components and allow the chiller to operate normally without nuisance tripping. You can't just slap in any fuse. You'll need to consider the chiller's full load amps (FLA), starting current (LRA) and duty cycle when selecting the fuse. This requires careful coordination with the chiller's manufacturer's specifications and potentially a short-circuit coordination study.

Proper Installation

The fuse (and its fuse holder) must be correctly installed in the circuit.

Important Note

Just adding a fuse does not automatically solve the problem. You need to verify that the fuse is listed and labeled for use in systems with available fault currents up to your calculated value (19,000A in your case). Many fuses are not.

Coordination with Control Components

While the primary concern is the 240V power path, the fuse will affect the overall system. Consider how the fuse will affect the starting characteristics of the compressor.

Your Next Steps1. Contact the Chiller Manufacturer

This is the most important step. Ask them: "Can you provide documentation stating that the SCCR of 5,000A applies to the entire chiller assembly, including the compressor motor(s) and the control panel?" "If we use a Class J current-limiting fuse with a specific amperage rating (you'll need to propose one), will that be acceptable to increase the SCCR of the chiller to match our available fault current?" Ask them for specific fuse recommendations for improving the SCCR.2.

Review the Manufacturer's Bulletin

Since they mention component replacement to increase SCCR, ask them for detailed instructions and a list of approved components. This may involve upgrading the contactor, overloads, or other components within the chiller.3.

Perform a Short-Circuit Study

If you haven't already, perform a comprehensive short-circuit study of your electrical system. This will confirm your AFC calculations at the chiller's location.4.

Consider an Isolation Transformer

If the fuse solution is problematic or not recommended by the manufacturer, an isolation transformer can be a reliable way to reduce the AFC. You'll need to properly size the transformer, consider its impedance, and ensure it's installed correctly.5.

Document Everything

Keep detailed records of all communications with the manufacturer, your calculations, the components you select, and the installation.

In Summary The 5,000A SCCR likely applies to the entire chiller. You need to address the 19,000A available fault current on the 240V side. A current-limiting fuse can be a solution, but it requires careful selection, coordination, and documentation. Contact the chiller manufacturer for specific recommendations and documentation.This is an overview; always consult with a qualified electrical engineer for the final design and to ensure compliance with all applicable codes and standards. Flag for review