Short Circuit Current Ratings in Industrial Control Panels

Workplace safety ensures employees and equipment are not in danger and systems operate efficiently. Short Circuit Current Rating (SCCR) is crucial for equipment when promoting workplace safety in industrial spaces. An accurately calculated SCCR used correctly ensures that industrial control panels (ICPs) operate without causing catastrophic damage. 

Industrial control panels utilize SCCR for maintaining application needs

What is SCCR?

According to article 100 of the National Electrical Code (NEC), SCCR is the maximum short-circuit current an electrical component may safely handle without causing shock or fire hazard. SCCR is measured in kiloamperes (kA) and each component in a panel is labeled from the moment of installation. To comply with NEC safety regulations the short-circuit current should not exceed the SCCR marked on the panel.   

Protecting personnel and equipment is the main priority of keeping SCCR ratings up to standard. Without this, multiple systems and panels may either shut off or cause catastrophic damage.

 In additional to ICPs, as of 2017, the NEC requires the following equipment to label their SCCR:

• Machinery
• HVAC
• Elevators
• Generators & Transfer Equipment
• Energy Storage and Battery Systems 
• Motor Control Centers

Understanding the Elements Of SCCR

A fault may occur in any flow of electricity. Faults are an abnormal condition in an electrical system that damages equipment and disturbs the normal flow of the current. A short-circuit current is the excess current caused by the voltage traveling an unintended path.

Equipment’s SCCR stays the same based on the specifications of the components and how they are connected. If the available fault current is lower than the SCCR, the equipment is generally considered safe. This should be verified when systems are changed or moved.

Hazardous fires are the result of an internal electrical fault

What are Industrial Control Panels?

Industrial Control Panels are an assembly of two or more power circuit components, control circuit components, or any combination of power and control circuit components, according to the NEC. These panels are factory-wired assemblies consisting of control equipment (such as switches, terminal blocks, contactors, motor drives, and transformers) and overcurrent protective devices like circuit breakers and fuses.

Industrial Control Panels enable the industrial operator to control mechanical machinery and equipment to handle a particular arrangement of devices. A panel with an incorrectly calculated SCCR could fail or cause an arc flash, with the potential to cause severe injury or death and significant damage to the facility. An arc flash is a type of electrical explosion that occurs when a connection is made through the air to the ground or another voltage phase in an electrical system.

Calculating Industrial Control Panel’s SCCR

Calculating SCCR is a challenging process for first-time specialists. One way to get the SCCR for ICPs is by submitting the panel to UL or another nationally recognized testing laboratory for listed and labeling assemblies. The downside to using one of these labs is it’s expensive and impractical for custom panel designs.

Calculating SCCR in Industrial Control Panels

Key Terms to Understand Before Calculating

The three types of circuits inside Industrial Control Panels:

• Feeder Circuit consists of conductors and components on the supply side of the branch circuits.

• Branch Circuits are the circuit conductors between the final overcurrent device protecting the circuits and connectors. This circuit is primarily found in houses or feeder circuits.

• Control Circuits are particularly used to control the operation of completely separate feeder and branch circuits.

• Power Circuits supply main line power to loads such as motors, heating, lighting, appliances, or receptacles

How to calculate SCCR in a 4-step process:

1. Identify the lowest component SCCR and or component combination SCCR rating with an overcurrent protective device (OCPD). The SCCR is located either on the component label or in the instruction manual. For products that do not have an SCCR marked, refer to Table SB4.1 of the UL 508A Standard. All devices are required to have a standard fault and Table SB4.1 lists these ratings.

• For power circuit components, the product information or instructions determine the SCCR using a power circuit. 

• For transformers rated 10kVA or less, the transformer’s downstream side is assigned an available current of 5kA. All secondary-side components in the power circuit must have an SCCR of 5kA or greater.

• Primary overcurrent protection is relative to the total panel’s SCCR, such as Class CC fuses used on this primary side having an SCCR of 100kA.

2. The branch component SCCR or combination of components may be increased with a feeder current-limiting OCPD.

• Only consider circuit breakers that are current limiting.

• Refer to each component’s recommended SCCR in the UL Standard documentation, this points out the minimum and maximum of each component’s current.

3. Determine the overall SCCR after the research is complete by identifying the lowest interrupting rating of overcurrent protective devices in the ICP. This occurs by figuring out the different circuit's current ratings in the ICP and the lowest is the panel’s overall SCCR.

• For each protected branch circuit in the panel, determine the smallest SCCR for the power circuit components on the load side of a branch circuit protective device.

• Then find the lowest SCCR of all feeder components.

• If current-limiting components are supplied in the feeder circuit, find the modified SCCR for the feeder component and all branch circuits connected to the load side.

4. The calculation of SCCR is an evaluation of the individual component’s SCCR(s) with respect to the protection devices interrupting the rating in each circuit. List the SCCR value from step three on the control panel nameplate. 

SCCR ensures industrial control panels operate efficiently

Understanding the required SCCR of industrial control panels is crucial for operating and maintaining the systems that help the workplace function. These ratings save lives and reduce the impact of malfunctions from occurring. Learn more about which products from TTI may help with creating efficient and safe industrial control systems.