- Safety Corner | June 4, 2015
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.
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.
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