cascaded surge protectors - good or bad practice? - Page 1
Forum section: electronics / projects

author: gojimmypi
author_rank: Regular Contributor
timestamp: November 15, 2015, 06:18:34 pm
content: I know that it can be a Very Bad Thing to cascade UPS's. I have first hand experience of a downstream UPS causing massive current spikes, which in turned caused all sorts of unpredictable equipment issues. But what about passive surge protectors? There seems to be a variety of differing opinions out there - perhaps mostly urban legend. Does anyone have the real facts on whether having a surge protector downstream from either a UPS (with built in surge protection), or simply a surge protector power strip plugged into another one? I know there's the whole issue of safety and maximum current: I'm not looking to plug in 12 space heaters, rather just all the little wall warts at my workbench for USB hub, router, RPi, etc, etc.Thanks in advance

author: johansen
author_rank: Super Contributor
timestamp: November 15, 2015, 06:29:14 pm
content: surge protectors usually use 330 v MOV's behind a fuse.. the fuse may be circuit board traces, to achieve their rated protection.the idea being the lightning strike doesn't blow up the MOV (shrapnel hazard i suppose), but rather blows the fuse element.however there are two types.. the fuse inline with the MOV, and the fuse inline with the MOV and the outlet strip,usually if the led doesn't light up, its because the fuse has opened.i think most "surge suppressors" put the fuse inline with the mov, not the outlet strip. so there really is no hazard with cascading a crap ton of them.... unless you have enough of them cascaded such that the first outlet strip fails due to overcurrent (I^2T) from say, 1554 downstream surge suppressors and their 666,000 joule surge capacity absorbing a worst case senario..anyhow, how do you cascade UPS's in a way that causes current spikes?and you probably know the UL doesn't approve of cascading outlet strips..

author: gojimmypi
author_rank: Regular Contributor
timestamp: November 15, 2015, 07:17:07 pm
content: Quote from: johansen on November 15, 2015, 06:29:14 pmIn my case, we had a rather large UPS that was probably intended to protect the entire equipment room. It was so large (on the scale of a refrigerator) that the techs either (1) didn't know it was a UPS or (2) didn't know of the issues of having a "cascaded" or "downstream" UPS. So another, more common UPS found in most homes and small shops was simply plugged into the larger one.[Mondo Big UPS] ---- [Smaller UPS] --- [problem equipment]Everythingappearedto work fine. With the exception that the equipment plugged into the smaller one occasionally had unexplained crashes. After eliminating all the usual suspects the only thing left was power. So we hired a power engineer to come in and monitor. His equipment showed these huge (60 amp) current spikes that (at least he claimed) were being caused by each of the active power conditioners "fighting" to stabilize, and "circulating" between the mondo UPS and the smaller one.. I still don't fully understand how that could be, as the smaller UPS should only be regulating thevoltageon the equipment side. Perhaps when it does this, there's an additional load of current on the smaller UPS's supply side, causing a voltage fluctuation for the mondo UPS to see (?) but one would think that would stop there? Well, add to that the dirty nature of switching power supplies at just the right resonant frequency, and maybe it could happen?I don't really know - but removing the downstream UPS solved the problem and the engineer told us not to do that again. I've read elsewhere it is not a good idea for active power conditioners to be cascaded. Fact or myth?And thanks for your response on passive surge protectors. So another vote that one is simply false urban legend.

author: gojimmypi
author_rank: Regular Contributor
timestamp: November 15, 2015, 08:37:10 pm
content: Quote from: johansen on November 15, 2015, 06:29:14 pmhmm.. good point.I found this 13 year old OSHA response:https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=24631copied here:QuoteSo I decided to first buy this PDU (Power Distribution Unit) with a built-in surge suppressor (APC P6W):http://www.amazon.com/dp/B0046BGUWG/Note from APC (now Schneider):http://www.schneider-electric.us/sites/us/en/support/faq/faq_main.page?page=content&country=US&lang=en&locale=en_US&id=FA158852&redirect=trueAlso copied here:Quoteso the plan is: plugging my surge protected PDU (APC P6W) into the wall, and then plugging several surge-protected outlet strips into that. Should be safe & proper now.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 15, 2015, 08:54:17 pm
content: Having surge protectors in series is not principle problem - but having power distributors and outlet strips in series usually is not by the code. The problem is that there may be to many outlets on one wire and wire/contact resistance can get to high for fast tripping of a fuse in case of a short.Ideally there should be several stages of surge protection: one at the utility transformer, one at the meter / breaker board, maybe one at the outlet and one (possibly even 2) in the final equipment. The earlier ones can reduce the strength of overvoltage to a level that the smaller ones do not explode. Just the small protectors in the outlet / distributors usally can't stop a lightning strike, but only inderect effects or what passed the protector in the fixed instllation.