By ABB Low Voltage
A short circuit is a low-resistance connection established by accident or intention between two points in an electric circuit. This excessive electric current potentially causes circuit damage, overheating, magnetic stress, arcing, fire or explosion. The amount of current that is available in a short circuit is determined by the capacity of the system voltage sources and the impedance of the system, including the fault. In circuit analysis, the term short circuit is used by analogy to designate a zero-impedance connection between two nodes. This forces the two nodes to be at the same voltage. In an ideal short circuit, this means there is no resistance and no voltage drop across the short. The electrical opposite of a short circuit is an open circuit, which is infinite resistance between two nodes.
Short-circuit current ratings (SCCR) are a critical specification when designing industrial control panels. Determining the appropriate SCCR actually requires no calculations. Instead, there is a simple, four-step process to follow.
The UL Standard for Safety for Industrial Control Panels, UL 508A, includes instructions for calculating a panel’s short circuit current rating (SCCR), but many people have trouble working through that process. Determining an accurate SCCR is essential to ensuring the safety of people who work on or around power equipment. A panel with an incorrectly calculated SCCR could fail or cause an arc flash, with the potential to cause series injury or death, as well as significant damage to the facility.
People refer to “calculating” the SCCR of a panel, but there actually are no calculations required. Rather, identifying an SCCR requires only that you investigate the fault capacity of the components in the panel circuit. With the list of those values in hand, you need to identify the component with the lowest capacity, which is literally the weakest link in the circuit. The SCCR of the entire panel assembly is the capacity of that component.
Panels must be rated for the available fault current at the time they are installed and for future potential needs, if they may be higher at some future date.
What is SCCR?
Rather than “calculating” the SCCR of a panel, it actually only requires that you investigate the fault capacity of the relevant components and then identify the component with the lowest capacity … the weakest link in the circuit. The SCCR of that component is the SCCR of the entire panel assembly.
Before 2005, the NEC required industrial machinery electrical panels to be marked only with the interrupting rating of the main overcurrent-protective device. This, however, did not ensure the electrical panel was adequately protected against short circuits. The new standard includes the entire combined power circuit in determining the SCCR requirements.
Determining a panel’s SCCR
How is the SCCR calculated? There are three steps in the process:
Step 1 – Determine the short circuit current rating (SCCR) of each component or combination in the power circuit. (SB4.2)
Step 2 – Determine whether feeder circuit components limit fault current (SB4.3) circuit-protective devices, like the fuse.
Step 3 – Determine overall short circuit current rating for industrial control panel (SB4.4.).
Step 4 – List the SCCR marking on the control panel nameplate (SB5.1).
Following is more detailed information about each step.
Step 1 – Determine the short circuit current rating of each component in the power circuit
The first step is to determine the SCCR of each component or combination of components, which is usually on the component label or its instruction manual. You don’t need to include the SCCR for power transformers.
Another source of SCCR information is the Assumed Maximum Short Circuit Current Rating For Unmarked Components, Table SB4.1 in the UL 508A Standard. This is also referred to as the standard fault. All components must have a standard fault current rating, and it is typically very low.
There are resources available that provide device ratings for recognized components, which include the component’s UL file and manufacturer’s installation instructions. Also, the UL website includes a table of Short Circuit Current Ratings for Combination Motor Controller Components. These components typically must be used with another component to get the desired rating.
Feeder-circuit components that modify fault current include:
-Current-limiting circuit breakers
You need to locate these parts and include them in your consideration of the SCCR.
On the branch circuit, you need to consider transformer ratings. For transformers rated 10kVA or less, the transformer secondary is assigned an available current of 5kA, and all secondary side components in the power circuit must have a SCCR of 5kA or greater. On the primary side, only the primary overcurrent protection is relative to the overall panel SCCR. An example is Class CC fuses used on the primary side of the transformer, which have a SCCR of 100kA.
The branch circuits must have a SCCR equal to or higher than the let-through current of the feeder circuit. If they don’t, the overall rating for the panel is the lower rating of the panel or the branch circuit.
Step 2 – Determine whether feeder circuit components limit fault current
Once you’ve identified the SCCR for the components, the next step is to determine whether feeder-circuit components, specifically circuit-protective devices like fuses, limit the fault current.
Circuit breakers must be marked “Current Limiting” to use SB4.3.2. The breaker let-through current will not exceed a defined value. One of two conditions apply:
1. If the devices on the load side of this breaker have a higher SCCR than the interrupt capacity of the breaker, then you can use the interrupt capacity of the circuit breaker. These may also be a combination that was tested by the manufacturer or panel shop.
2. If the devices have a lower SCCR than the interrupt capacity of the circuit breaker, the SCCR for this circuit is the lower value.
The maximum let-through for a circuit breaker is determined by the manufacturer. For fuses it is determined by a standard, allowing you to use a Table SB4, “Peak let through currents, IP and clearing I2T for fuses”.
In determining the panel SCCR, the SCCR on the line side of any current-limiting circuit breaker can’t exceed the SCCR of any branch-circuit protection or the interrupt capacity of the circuit breaker. The peak current let-through cannot exceed the SCCR for any branch circuit on the load side. This basically states that the device on the load side of this breaker can withstand the let-through breaker energy and current.
For fuses, use the values in Table SB4.2 “Peak let-through currents, IP and clearing, I2T for fuses” to get the I2T and IP for the fuse used in the combination circuit. Any fuse with a lower value for both the I2T and IP can be used. If your fuse size is not shown, use the next larger value in the table.
Step 3 – Determine overall short circuit current rating
With the component research complete, you have the information needed to determine the panel’s SCCR. You do this by determining three different SCCRs. The lowest of the three is the panel SCCR.
The three values to determine are:
-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. (SB4.4.1)
-Determine the lowest SCCR of all feeder components.
-If current-limiting components are supplied in the feeder circuit, determine the modified SCCR for the feeder component and all branch circuit(s) [from A) above] connected to load side. (SB4.3), see step 2 above.
Compare these values from this panel. The SCCR is the lowest of the three.
Step 4 – List the SCCR marking on the control panel nameplate (SB5.1)
The value from step 3 above must be listed on the panel rating or name plate label. The name plate marking must include the SCCR in kAmps rms symmetrical, at the rated voltage.
Know Your Panel
The people who design and build industrial power-control panels need to understand the required level of short-circuit-current protection for the people who own, operate and maintain those panels. The short circuit current rating provides a key piece of information in ensuring the appropriate level of protection. Panel makers rely on the steps laid out in the UL 508A standard to calculate/determine the SCCR of their products and provide this information.