At 04:39 PM 5/29/2003 -0400, you wrote:

A much cheaper and easier to implement external maintenance make-before-break bypass will accomplish the same thing.

I've heard many a story of the paralleling gear causing the problem in the first place, as well...

We have two MGE 150KVA UPSes at our Newton facility. When I designed the electrical system, I originally specified a make-before-break 208V 450A 3PH switch. This would enable us to isolate our internal and customer load panels from the two UPSes and their respective static transfer switches and output transformers - so we would have completely redundant dynamic A/B switching. The ONLY vendor at any price I could find was for a switch made for the FAA for their regional approach control centers critical power systems. It cost $250,000 wholesale with a six month lead time!!! I decided I would take my chances with the integrated MGE static transfer switches and use one output transformer. For that amount of money, I can buy two more UPS units when my load grows to 75% (to minimize the stress on the circuit components and to control the transformer temperature by running it well within limits) and still have $90k in the bank. Since we designed and installed everything ourselves, I know we can bypass one UPS if we ever have a catastrophic failure of the internal static transfer switch. In retrospect, I think it may have been better to have a split output for UPS #2 one feeding the bypass for UPS #1 and one feeding a break-before-make manual switch which normally connects the output of #1 to our circuit panels for quick service restoration in the event that UPS #1 fails in the critical path such as the output transformer or the static transfer switch. The comments about two MGE units failing due to component failure is a bit scary since our serial numbers are actually the same and we just have different suffixes due to being purchased on the same PO! Whatever components are defective on one are probably defective on the other also. So far, 16 months and zero problems... (knock on wood) -Robert Tellurian Networks - The Ultimate Internet Connection http://www.tellurian.com | 888-TELLURIAN | 973-300-9211 "Good will, like a good name, is got by many actions, and lost by one." - Francis Jeffrey