Letters: Reader Feedback

Editor’s Note: On Dec. 13, CSE hosted a webcast entitled, “Beyond Generators and UPS,” which addressed issues of emergency backup power in data centers. (Readers are urged to view the webcast; it can be accessed from the CSE homepage at csemag.com.) As is always the case, we were not able to accommodate all participant questions in the Q&A session following the webcast.


Editor’s Note : On Dec. 13, CSE hosted a webcast entitled, “Beyond Generators and UPS,” which addressed issues of emergency backup power in data centers. (Readers are urged to view the webcast; it can be accessed from the CSE homepage at csemag.com.) As is always the case, we were not able to accommodate all participant questions in the Q&A session following the webcast. What follows answers some of those questions.

Are circuit breakers rated for the number of times they can be manually operated without failure?


Joe Guentert replies: There are published ANSI standards for mechanical endurance based on full-load operations with a duty cycle of 30 operations per hour. The standards vary by size and type of breaker, but typically range from 5,000 operations for breakers 2,000 amps and below, down to 2,000 operations for 4,000-amp to 5,000-amp breakers. All manufacturers publish their own endurance ratings.

Are the batteries for the Masterpact Micrologic Trip Module replaceable with the breaker in service? Is there a recommended way to power this externally such that a battery is not required?


Joe Guentert replies: I have successfully replaced batteries with the breakers remaining in service, but I’d recommend against doing so if the load is at all critical. There is at least a small probability (maybe one in 100) that the breaker might nuisance-trip during the procedure.

I no longer use these batteries on any of my systems. Instead, I use “reliable” sources of remote 24 volt DC power to the trip units (to the F1-F2 terminals), derived from multiple sources of power (engine starting batteries with best-battery selectors to an alternate source). The idea, of course, is to make sure that the 24 volt DC is not interrupted during a breaker trip, or else the “target” information storage is lost.

I have had experience where the finger has fallen off and caused a short as the breaker was being racked in. What is your experience with this type of problem?


Joe Guentert replies: I have personally experienced this just once, but I’ve received a dozen or so reports of similar incidents from around the country. Once I was racking a breaker out, when I heard a loud “pop” and saw a big flash at the rear of the breaker. Fortunately, it blew itself clear and the damage was fairly minor (I always use high resistance grounding on all of my systems, and that probably prevented fault escalation).

Interestingly, on the very evening of the webcast, I received a report of such an incident from a small data center owned by a major wireless company. Employees were racking out a 3,000-amp utility main breaker, when a finger cluster let loose on the line side of the breaker. The fault burned for about 10 minutes before it was cleared by the utility company pulling transformer primary fuses. The entire switchboard was destroyed, with major flames and a Halon dump in the room. Three employees sustained luckily minor burns, one was treated in the ER for chest pains afterward.

This breaker cell was equipped with rear shutters, which exacerbates the problem, because the shutters trap the part such that it can’t blow clear.

I’ve just recently begun recommending to my clients that they have their employees “suit up” when racking any breaker in an energized piece of switchgear. I’ve racked literally thousands of breakers myself in live boards, without giving this much thought until recently.

I would be interested to know the details of your experience, if you are able to share them with me (in confidence, of course). What type of equipment, whose breaker, circumstances of racking, extent of damage, rear shutters or not, etc?

Define “high power” in the term “high power distribution centers.”


Kfir Godrich replies: We use such centers we call MLCs (mini load centers) including 1MVA transformers after the UPS main distribution and feeding remote power panels. We have also designed applications with up to

Review the elimination of PDU points.


Kfir Godrich replies: The fact that the maximum PDU you can buy on the market is 300 kVA (UL limitation) is a problem in the high density environment. Also, the role of the PDU initially was to eliminate the common mode problem years ago. Therefore, load centers of 1 to 4 MW are becoming a reality in today’s data centers. Besides this, a lot of people are seeing this as a possibility to eliminate an element in the serial distribution chain, increasing the overall system availability as well as having a higher power equipment. Again, it’s additional availability improvement. So the distribution chain is UPS - UPS distribution center - Trafo/Load Centers - RPPs - load.

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