"They all suck!" Re: UPS failure modes (was: fire at NAC)

Robert Boyle robert at tellurian.com
Thu May 29 21:55:26 UTC 2003

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)


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