The 2005 National Electrical Code now requires most control panels to be marked with their short-circuit current rating (SCCR). This change affects all industrial control panels (409.110), industrial machinery panels (670.3(A)), and non-residential HVAC equipment rated greater than 60A (440.4(B)). Furthermore, all industrial control panels listed to UL508A must be marked with their SCCR. Supplement SB in UL508A, which became effective on April 1, 2006, is referenced by the NEC as an approved method for establishing the SCCR of a panel.

As with most every change to the NEC, safety is the primary concern of this new requirement. Under abnormal operating conditions, such as an electrical fault, a control panel may be subjected to thousands of amperes of let-through current before any protective device(s) inside the panel clears the fault. Such an event may cause severe damage to power components and electrical wiring, which can compromise the integrity of the Panel and result in injury to nearby personnel.

Proper matching between the maximum available current from the power supply and the SCCR for the panel is fundamental to ensure safety. From the Panel users point of view, Panels with a higher SCCR allows them the flexibility to connect their Panel to electrical systems with different available currents. From the Panel builder’s point of view, establishing this higher SCCR may represent significant challenges and additional costs.

Even though supplement SB from UL508A defines how the SCCR for an Industrial Panel is established, determining the Panel’s actual SCCR may not be so straightforward. The simplest method to obtain the Panel’s rating is by analyzing the individual power components in each branch of the circuit. The component with the lowest rating determines the SCCR for the entire branch. Subsequently, the branch with the lowest rating determines the SCCR for the entire panel.

Despite the fact that the SCCR rating for SSRs was not available in the past (nor was such a rating required) it is obvious from the above example that this is a critical part of determining the rating for the entire panel. Per UL508A supplement SB, components that do not have such a rating are given one based upon their specific category. SSRs, which do not have a specific category, are considered to fall under “Switches”, “Switch Units”, or “Motor Controller, 0-50HP”. The rating for unmarked components in these categories is 5kA, and the use of such component in a branch circuit effectively reduces the entire panel to the same rating.

Per UL508A, short-circuit current ratings can be conveniently established for individual power components, or for combination of power components, or for the Control Panel as a whole. Crydom has elected to test a combination of SSRs with protective devices (fuses initially) in order to meet the conditions of acceptability necessary to assign the standard 5kA rating to the combination. However, most Crydom customers desire a SCCR rating higher than 5kA for their panel in order to be more competitive in the market. For this reason, Crydom is undertaking a series of tests in order to significantly increase the UL approved SCCR ratings for their relays.

The tricky part is selecting the right fuse for not only the application, but also for the desired SCCR rating for the combination. It is understood that under sizing the fuse may compromise performance, while over sizing the fuse may impact the safety of the Panel. Not to mention add unnecessary cost. To further complicate the issue, fuses of the same physical size may have different current ratings (i.e. a 60A fuse is the same physical size as a 45A fuse). This is a significant UL concern, as field service technicians could easily replace a fuse with one of identical size but having a higher current rating. Such an error could potentially result it damage to the relay, the load, or nearby personnel.

Therefore, UL requires that the relay-fuse combination be tested with an “umbrella fuse”. That is, the fuse used in the combination must have the highest possible current rating available for that particular fuse package. Since UL approval with a rated fuse covers all fuse ratings up to that value, the “umbrella fuse” method adequately addresses this concern. Crydom’s test plan involves multiple relays of different load-current ratings in combination with the highest possible “umbrella fuse” combination. This not only meets UL’s safety requirements, but also provides panel builders with flexibility in selecting the right relay for their application and their specific SCCR requirements.

Similar to fuses, solid-state relay manufacturers tend to follow a typical industry-standard footprint. However, unlike fuses, each manufacturer’s relays may have significant design differences that affect thermal performance, power dissipation, mechanical stress/fatigue, etc. Therefore, the “umbrella” concept does not apply in the same manner as it does for fuses. So simply substituting one relay with an “equivalent” relay from another manufacturer will not likely result in an equivalent short-circuit current rating.

It should be remembered that SCCR is strongly related to safety, and therefore great caution must be used when changing solid-state relays in control panel.

While the new NEC code focuses primarily on the North American market, Panel Builders selling outside of the US may also be affected by the change. Any panel requiring UL Listing, regardless of its final destination, must be marked with a SCCR. In addition, some regions outside of the US may enforce NEC 2005 as an additional safety measure for local installations. Panels in such locations must also have their SCCR clearly marked.

To add to the confusion, some states in the US have not yet adopted NEC 2005. Therefore, they do not yet require the SCCR marking on panels. However, as stated above, a UL Listed panel installed in that particular state must have an approved SCCR, regardless of the state’s regional requirements.