Change can be hard. And in the case of industrial control panels, change also tends to come slowly. This is not a bad thing, mind you, because we must take care to ensure safety with control panels so it’s only fitting that our practices be tried and true.
Against this backdrop, it’s not all that surprising that industrial control panel builders often still struggle to build panels that comply with UL 508A, the standard for industrial control panels. UL 508A saw a significant change back in 2005 that still reverberates today. In this post, I’ll give a little background on that change and some advice on how control panel builders can best deal with it in their day-to-day work.
In 2005 UL 508A was changed to require control panel builders to include a short circuit current rating on the control panel assembly. While that seems a reasonable request, and is certainly in the best interest of safety, in practice it can be difficult to comply with.
UL has a number of methods for how to establish a short circuit rating, but most are not practical for control panel builders. A straight short circuit test, for example, can destroy equipment and is expensive; nobody wants that. There’s also a calculation method but it requires special engineering expertise and techniques that are not generally available to control panel builders.
So panel builders came to the component manufacturers and asked for tested combinations of devices, maybe a fuse or circuit breaker and a contactor or overload relay. They were essentially looking for help in determining, if I use circuit breaker A with motor starter B, what will the short circuit rating be?
Manufacturers, including Schneider Electric, complied with the request and soon we all had spreadsheets on our web sites that listed all kinds of component combinations and their accompany short circuit rating. Those spreadsheets soon became unwieldy and nearly impossible to keep up with, given the thousands of different possible combinations of products to contend with and test.
UL sympathized and enabled manufacturers to instead come up with a new category – component short circuit current ratings – for anything used in the power circuit in an industrial control panel. This made short circuit current ratings more generic. For example, a motor starter may have a label that says it’s suitable for a circuit not capable of delivering more than X amps when used with a circuit breaker not exceeding Y amps. The change had the effect of giving more flexibility in terms of combinations of products that could be used and made it far simpler to come up with short circuit ratings.
But it’s not perfect. Control panel builders still have lots of questions as they go about their work. One common one is, what’s considered the power circuit and what isn’t? That may seem like a simple question, but when you get into the weeds it’s not always so easy.
In many cases, panel builders will simply put whatever the UL says is the minimum allowable value for a given component on the control panel and be done with it. And in many cases, that’s just fine. In an automotive assembly plant, for example, the available short circuit currents will likely be quite large, so there’s not a lot to worry about. In an automatic car wash, on the other hand, the story will be quite different.
So there are still a lot of questions around the short circuit topic. To help out, Schneider Electric came up with a catalog, “Motor Control Solutions for the North American Market,” that includes short circuit ratings on our products. You can download the catalog for free here; it should make it easier for control panel builders to come up with a short circuit value for their panels.
Otherwise, my best advice for control panel builders who have questions is to get in touch with us. We have lots of information available online; just search on a part number. And you can always call our technical support group toll free to get information, at 1-888-SQUARED. We have real, live people standing by to answer your questions.
8 years ago
That Tom Fowler is a true technical expert on both IEC products and UL508 applications. So glad that Schneider Electric has him on “our side”!