Developing an Effective SCCR Plan

One of the most fundamental calculations made on a power distribution system is that which yields available short-circuit current. Maximum available short-circuit current is an important parameter for every power distribution system as it provides a data point necessary to ensure equipment is being applied within its rating and the system is performing to meet expectations. Available short-circuit current is used in many other applications as well. The National Electrical Code demands this data point for enforcement of such Sections as 110.9 “Interrupting Rating,” 110.10 “Circuit Impedance, Short-Circuit Current Ratings, and other Characteristics,” and 10.24 “Available Fault Current.” Today we will discuss the development of an effective short-circuit current rating (SCCR) plan. Having a good plan in place can help increase electrical safety.

BACKGROUND

SCCR is the amount of current an electrical component or assembly is able to safely withstand in the event of a short-circuit (also commonly referred to as a short-circuit current event) when properly applied. SCCRs apply to a lot of equipment including many industrial control panels used for operating machinery and equipment.

The Occupational Safety and Health Administration (OSHA) and the National Electrical Code® (NEC®) require sufficient short-circuit current protection of industrial control panels to protect equipment and personnel from certain risks in the event of a short-circuit event. Protection from short-circuit current events is often properly specified and applied in electrical switchgear and distribution equipment (such as panel boards and switchboards), but it is often misunderstood or misapplied when it comes to industrial control panels.

The NEC® requires industrial control panels be marked with the assembly SCCR as well [409.110, 670.3(A), 440.4(B)]. The NEC® and OSHA require that electrical equipment provide sufficient protection against short-circuit current events. 1910.303(b)(5) of the OSHA regulation requires all electrical equipment, including equipment that is already installed or new equipment being installed, meet this requirement and does not provide for any exemptions. Section 409.22 of the NEC® prohibits the installation of industrial control panels in locations where available short-circuit currents exceed the equipment’s assembly SCCR. The available short-circuit current is the amount of current that would be available in the event of a short-circuit event and can vary depending on the location in the electrical distribution system, among other factors.

RISK

Panels with insufficient assembly SCCR that are subjected to a short- circuit event can expose personnel and equipment to serious danger. Without sufficient assembly SCCR, it is likely that the devices inside the panel will sustain and cause damage within the panel, and it’s also possible that damage may extend outside the panel.

Insufficient assembly SCCR poses the following hazards:

Electric shock and burns

Burns associated with arc flash and contact with heated surfaces

Injury associated with flying debris

Damage to equipment or the facility

Arc blast (shock waves, shrapnel, etc.)

Vaporized metal

It is very important that all electrical equipment be applied within its rating. SCCR is one such rating that when exceeded can be quite hazardous to those in and around the equipment.

DEVELOPING AN EFFECTIVE EQUIPMENT SCCR PLAN

There are two primary areas that must be addressed in order to achieve proper equipment SCCR levels and regulatory and code compliance:

Existing installed equipment

New equipment

Addressing and resolving these areas can vary in approach and implementation costs. All aspects influencing available short-circuit current levels and equipment SCCR should be carefully considered for each site and situation. It is not safe to assume that one particular method will work equally well in all situations. It is good to leverage concepts that can be applied and result in developing an equipment SCCR plan that will provide protection for employees and equipment while taking into account immediate and future needs.

Included here are some concepts to stir your thinking and help you begin the process of developing an effective SCCR plan for your facility.

Determine present available short-circuit current levels. This includes the available short-circuit current at each location where equipment is installed and at distribution points, such as panelboards and switchboards.

Consider factors that may affect future short-circuit current levels. These include changes to the electrical distribution system, which may increase the available short-circuit current. These include but are not limited to:

Future power needs that result in an increase in size of the upstream transformer or power source

Improved efficiency of the upstream transformer or power source

An improvement in the efficiency of the conductors or equipment in the electrical distribution system

A decrease in a conductor length

Consider factors that may require a higher equipment SCCR in the future. One such factor could be relocating equipment to different locations in the circuit where theshort-circuit current may be higher. There are applications where equipment is relocated for process, manufacturing, re-organization, or other similar reasons. Appropriate SCCR can help ensure moving equipment does not present a hazard due to higher available short-circuit currents.

Consider methods to resolve insufficient existing equipment SCCR and their implementation. Such methods may include introducing a small transformer ahead of the location of equipment with inadequate SCCR. Other forms of impedance may be used, or it may be possible that a qualified resource can alter existing equipment to improve its SCCR.

Define a minimum acceptable equipment SCCR specification for new equipment that takes into account present and future available short-circuitcurrent levels as described above.

Where appropriate and beneficial, consider grouping equipment types by their power demand or by their location. For example, some equipment types may have high power demands with higher available short-circuit current levels and higher equipment SCCR needs. These equipment types are likely to have higher SCCR protection needs than other types of equipment.

Require that equipment suppliers provide documentation detailing the individual component SCCRs and method by which the equipment SCCR was determined. This not only supports the accuracy of the determined equipment SCCR but also identifies the required replacements parts so maintenance personnel can retain the equipment’s SCCR during regular maintenance activities.

Document requirements. Documentation is a critical step in communicating equipment SCCR requirements to both suppliers and personnel and is a key part in sustaining an SCCR plan. Documentation should be considered in the following forms:

Labels indicating available short-circuitcurrent (and date calculated) and minimum equipment SCCR requirements including downstream equipment. Consider placing these labels at each electrical distribution system point where equipment may be installed, such as a panelboard or a transformer.

Equipment operation and maintenance manuals including specific text that precludes component substitution that may invalidate the equipment SCCR and possibly void the warranty

The NEC®, OSHA, and UL® recognize the need to provide adequate protection against short-circuit events. This can only be achieved by understanding available short-circuit current, communicating protection requirements to personnel and equipment suppliers, and proper application of components in the industrial control panel. Execution of a proper equipment SCCR plan will provide adequate protection for employees and equipment.

Remember to integrate safety practices into the home and help them become second nature for the workplace.

As always, keep safety at the top of your list and ensure you and those around you live to see another day.

Thomas Domitrovich, P.E., is a National Application Engineer with IEC Platinum Partner Eaton Corporation in Pittsburgh, Pennsylvania. He has more than 20 years of experience as an Electrical Engineer and is a LEED Accredited Professional. Domitrovich is active in various trade organizations on various levels with IEC, International Association of Electrical Inspectors, Institute of Electrical and Electronic Engineers (IEEE), National Electrical Manufacturer’s Association (NEMA), and the National Fire Protection Association (NFPA). He is involved with and chairs various committees for NEMA and IEEE and is an alternate member on NFPA 73. He is very active in the state-by-state adoption process of NFPA 70, working closely with review committees and other key organizations.