UL approval using isolation transformer and circuit breaker


Thread Starter


I have an application where UL approval of the entire panel is required. Some of the components inside the panel are not UL approved. Isn't there a way to use an isolation transformer and circuit breaker with ground fault protection to make it UL OK. Does anyone have any info on this.


Eric M. Klintworth

Assuming you mean an "industrial control panel" covered by UL "Subject 508A":
I attended the UL seminar on becoming a UL listed panel shop. The whole subject is relatively complicated, with a lot of red tape. For
instance, you can't just automatically use any UL recognized component in a panel--the panel shop building the panel has to have the component (or group of components) listed in the UL records for that panel shop. (Although you can use any UL _listed_ component.)

If you are doing just one such panel, you really need to work with a local UL listed panel shop that routinely builds the sort of panels you
are designing. I believe there is a list on UL's web site ( "www.ul.com":http://www.ul.com ).

To answer your specific question: the answer is in 508A paragraph 3.7, and I have a decision tree chart (from the seminar) that I could fax to you. Basically, a non-UL electrical part that is NOT power switching equipment (that controls external loads) or power consuming equipment
(like heaters or motors) can be included in a UL listed panel IF it is powered from a transformer with isolated primary and secondary windings
and a grounded secondary (i.e. an ordinary control power transformer) and IF you have a GFCI between the transformer and the non-UL load.
There are about three more pages of "shalls, ifs, and buts", but that gives you some idea.

Good luck, and give me call if you need the decision tree or more info.

Eric M. Klintworth, PE
Sharp Technologies, Inc.
Columbus, Ohio

Bob Peterson

If you are a UL registered panel shop you can indeed do this. There are some rules about it but they are not onerous.

One rule frequently ignored is that even with this arrangement there are limitations to the size holes you are allowed to cut in the panel door to mount non-listed equipment in. And the allowed sizes are different for metal cased devices versus plastic cased devices.

You can also get a special ruling on a specific part. This requires an investigation and a change to your UL registration, and takes a while and costs a bit of money. This approach is best used if you want to do this on a regular basis.

Bob Peterson