Builders and buyers of motor control centers (MCCs) rely on standards, notably UL 845, to ensure panel safety and functionality. This article provides insights and tips to help you better understand key elements of those standards.
Panel builders and OEMs that buy their panels rely on UL standards to ensure panel safety and functionality. These standards are typically quite detailed and can be difficult to navigate, understand, and therefore comply with. Generally, panel builders strive to ensure their designs adhere to the letter of the standard, but that is not always an easy task.
A key problem is something that comes down to a bit of classic, logical reasoning called a syllogism, which involves a (A) major premise, (B) minor premise, and (C) conclusion. For example:
(A) All fish have gills.
(B) All tuna are fish.
(C) All tuna have gills.
Syllogisms are often a solid form of logic, as in the above example, but not always. There are also “faulty” syllogisms:
(A) Most control panels are grey.
(B) Most fish are grey.
(C) Most control panels are fish.
Here’s a more useful example of a faulty syllogism regarding UL 508 panel shops offering products to the UL 845 market:
(A) I select breakers based on UL 489.
(B) UL 489 extensively references UL 845.
(C) I can use a breaker referenced in UL 845 in a UL 489 panel.
Standards for technical panels sometimes establish standards within their groups without completely considering conflicts with other standards. So just because a component is described or specified in one standard, doesn’t mean the same or equivalent component is described the same way in other standards.
One example of some standard-related confusion involves breaker testing based on UL 845 for a motor control center (MCC). This standard requires a magnetic trip test on the breaker following a short circuit to ensure it’s still operational. One might assume, since a breaker is UL 489 rated that it has been tested and the trip test doesn’t need to be conducted. But breaker testing is no longer part of the UL 489 standard which means the magnetic trip test DOES need to be conducted.
This example points out the need to review proposed test plans with a primary UL contact. Reviewing prepared engineering arguments on worst-case tests that will envelope other variations avoids unexpected requirements later in the project
Look beyond the ratings
Selecting a component with the required rating isn’t all you need to know about using it in an application. There are other variables you must consider to ensure a component is an appropriate choice.
An important variable is the volume of the panel. Without sufficient volume, violent outgassing from certain breakers could create enough pressure to blow the doors off the panel, spewing molten copper. Even if a component is fully tested to UL 489 specs, you must also ensure that the panel volume during the breaker testing is equal to or less than the volume in the actual application.
There may also be bracing or bending requirements for the cable entry into the breaker. Component installation and application guides often provide information related to mounting, bracing, and volume requirements.
With UL 845 MCC panels, builders may say, “I have 508 – so I can put anything I want into an MCC.” But if you talk to any UL project engineer involved in MCCs, they will tell you they have to be tested. Some of the confusion is due to the nature of MCCs: which is, that they have a common bus running through the line of connected MCC sections, creating the need for additional safety testing. Panels using enclosed or sealed components like compact starters, and no shared main bus, don’t have this additional feature – and, therefore, require additional testing
The bottom line is that you can’t go by the rating alone. Unless other critical variables are considered, the panel may be non-UL compliant. In addition to enclosure volume, these variables include bracing requirements, spacings around devices, etc. These variables should be stated in the component installation instructions, but it’s unclear how many panel builders consider them or are compelled to violate them because of design limitations.
“One example I’ve seen in my career of some standard-related confusion involves breaker testing based on UL 845 for a motor control center (MCC). This standard requires a magnetic trip test on the breaker following a short circuit to ensure it’s still operational. It was initially assumed that since the breaker was UL 489 rated, the test had been done and didn’t have to be repeated. But it was discovered this test is no longer part of the UL 489 standard, so the trip test was actually required.”Mike Bryant, R&D Manager, ANSI LV Switchgear Systems
Be aware of standard interpretations
Despite how detailed and specific standards typically are, they sometimes leave room for subjective evaluation and opinions. This can cause deployment or commissioning delays. For example, during installation, one field rep for the authority having jurisdiction may find no problems, while another could identify defects that must be corrected.
When submitting a panel for testing, the design team should present a test plan based on their knowledge of the standards with an argument regarding whether or not testing should be required. While it is tempting to avoid this approach due to the time and costs involved, the issues it can avoid may outweigh the time and cost factors. Of course, this could create problems during the installation and increases the risks involved in using those panels.
Differentiate between Recognized and Listed components
The difference between UL-Recognized and UL-Listed is important but often misunderstood. The simple explanation is that a Listed device is tested and can stand on its own – provided that you install it per the manufacturer’s instructions – including mounting, space around the component, volume of the enclosure, ambient temperature restrictions, etc. Listed breakers/fuses are typically used as branch circuit-protection devices like in mains or feeders.
Thermal magnetic breakers, for example, are typically Listed. Mag-only breakers, however, are usually tested and qualified in a combination with another device like a contactor or starter. These additional devices increase the impedance or reduce the short-circuit energy that the breaker may see.
Terminal blocks (a.k.a. distribution or power blocks) used to be UL-Listed. Today, they are Recognized and must be tested with specific cables to help ensure the cables won’t pull out.
More than 10 years ago, VFDs were Listed (UL 508C) with their own short circuit rating all the way up to 100kaic. Now, VFDs have to be qualified “suitable for use” with fuses or a particular manufacturer’s circuit breaker. This is another example of the need to stay abreast of changes in standards.
Panel builders and buyers rely on guidance from standards organizations like UL for the assurance of the safety and uniformity of power-related components and systems. The depth and breadth of UL standards, though, can make it challenging to understand and comply with them. For panel shops interested or engaged in production of MCCs, the insights above will help ensure that your panels are fully compliant.
R&D Manager, ANSI LV Switchgear Systems
ABB Electrification Business