From: Price, Ed Sent: Monday, June 02, 2003 8:19 AM To: 'EMC-PSTC List' Subject: Surge Suppressors on a UPS
Hi Group! Last Friday, I got ambushed in a meeting. I hate it when that happens! A question was asked about whether it's OK to put a surge suppressor on the output of a UPS that is supplying power to some expensive equipment. I opined that I didn't think it should be necessary, but that it also shouldn't hurt anything either. So then somebody asks me why all the UPS manufacturer's sites say not to use a surge suppressor. I expertly reply that gosh, I don't know, but I'll take a look. The next question nails me again. "Are there any standards for UPS output power quality?" Uh, well, I'll look into that too. Now, the market is light industrial, USA, but are there any applicable EN standards also? Just for some background, here's a typical entry from Tripp-Lite's FAQ list for UPS's (not to pick on Tripp-Lite; they just said it most succinctly of several sites I looked at): http://www.tripplite.com/support/faq/tech_ups.cfm Can I plug a surge suppressor or extension cord into my UPS? No. Using an extension cord will void your equipment coverage warranty, as all equipment must be plugged directly into the UPS. Tripp Lite does not recommend plugging a surge protector into a battery backup outlet of a UPS either as this can overload it. Also, when some UPS systems switch to battery power they will output a waveform that a surge suppressor may see as a surge and short-circuit the UPS. Again, this setup will void the equipment coverage warranty. Now this is getting to be a big can of worms! What do they mean by "some UPS"? Is there one kind that does, and another kind that doesn't; and how do you know which is which? And if some UPS will create a voltage transient (is that what they mean?) sufficient to trigger a surge suppressor, then why is it OK to let the UPS apply that transient to my "protected" equipment? All this talk about uninterrupted power isn't worth anything if the UPS kills my equipment when it switches to battery power mode. And who's fault is this? I mean, a surge suppressor is pretty dumb; it just sits there waiting for the voltage to go over a certain level and then it conducts. What's this about the surge suppressor "may see" something as a surge? That's saying the surge suppressor could mis-interpret the waveform it sees. If the surge suppressor is conducting, then I think the UPS has just done something very naughty. I also don't understand the prohibition of an extension cord. Maybe this is a legal issue, as I can't see any valid safety or regulation issues here. We regularly put a UPS in the bottom of a rack system, and then wire a stripline outlet set for the height of the rack. Isn't that the electrical equivalent of an extension cord? What am I missing? Thanks in advance! Ed I'm replying to my own post because, so far, my question has not really been answered. It HAS generated a really large volume of private emails expressing concern about things like what happens when you use a UPS to power equipment with built-in surge suppression. Or questions about just what are these "switching event waveforms" that a surge suppressor might want to suppress, and why should they be allowed to be applied to equipment without surge suppressors (equipment designed with the assumption that the UPS is protecting it from the public mains). It looks like racking and stacking a UPS, with a power distribution strip (the moral equivalent of an extension cord?) feeding a group of discrete electronics boxes (most of which have built-in filters and suppressors) is quite common. Yet UPS manufacturers seem to be saying that this bad practice and will even void the UPS warranty. So far, all comments have been from USERS of a UPS. I would like to hear from the other side, the UPS designers. (Privately if you must, but I would like to be able to later post unattributed answers.) Thanks again! Ed Ed Price ed.pr...@cubic.com WB6WSN NARTE Certified EMC Engineer & Technician Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (Fax) Military & Avionics EMC Is Our Specialty
