Confused about a Server PDU - Page 1
Forum section: electronics / projects

author: justinjja
author_rank: Regular Contributor
timestamp: June 02, 2017, 01:31:01 am
content: I'm taking apart a server PDU...It is UL Listed for 208V 24A (30A 80% derated) - APC AP9571it has 2 15 amp breakers in parallel on one leg, the other leg is connected directly to power bars, then the case is groundedShouldn't there be a breaker attached to both legs?Because my understanding of 208v is that it is made from 2 120v legs of 3 phase power. (2 hot legs)What if a device failed in a way that the 2nd leg shorted to ground?Also interesting, the breakers are only rated for 120v, is that ok?If there was a short, they would be exposed to 208v

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 02, 2017, 04:02:58 pm
content: Any evidence it has been "repaired" in the past? It sounds like the original breaker failed and an inexperienced tech tried to replace it with a pair of 15A breakers that were probably common spares in electrical work. As described, it clearly would not meet code.

author: Jono427
author_rank: Contributor
timestamp: June 02, 2017, 06:04:56 pm
content: Based on the PDF I found, it come with 2x15 breakers.Hey justinjja, got any pictured of the inside you could post?

author: justinjja
author_rank: Regular Contributor
timestamp: June 03, 2017, 02:14:30 am
content: I Bought 2 of them, I just took the second apart too, it's the same.Neither of them look like they have been opened before.Here are a few pics

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 03, 2017, 02:52:03 am
content: The breakers are not actually in parallel but each one supplies half of the outlets as might be expected. What's unusual is that not only is the other side unfused, it is white without a marker as if it's a neutral. 208V hot to ground is sometimes found in commercial environments but 2x 120V at 120 degrees is much more common.

author: ConKbot
author_rank: Super Contributor
timestamp: June 03, 2017, 10:31:34 am
content: Since that has a L6-30P plug on it (phase, phase, gnd), but only 1 leg had breakers, I'd say that was expecting a single phase from a 415/240 or 380/208 3 phase circuit, but nothing about the L6-30P guarantees that. A fault to ground from the 2nd unbreakerd leg could pass 30A, set output cables on fire and you'd be relying on the panel breaker to trip. I don't like it, something fucky going on.

author: rstofer
author_rank: Super Contributor
timestamp: June 03, 2017, 02:47:49 pm
content: I think that L6-30P could also be used as phase-neutral-ground. I went looking at the receptacles and one of the terminals is silver which would indicate a connection to neutral.According to the wiring diagram, the phase pins should have as much as 250V between them but it doesn't say how that comes to be. Just that it is!https://www.grainger.com/ec/pdf/Leviton-L6-30R-Wiring-Diagram.pdfSo, if the device really is phase-neutral-ground (and EVERYTHING seems to indicate this is the case), it is all wired correctly. It just isn't for use in the US where our 208V is phase-to-phase.We already know that every PC can take either 120V or 220V input voltage and this seems to be a device used where the higher input voltage is common.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 03, 2017, 03:09:12 pm
content: In commercial environments, 208V to neutral can be found in a "wild leg delta" configuration. Because 208V loads are less common than 120V loads, that "wild leg" tends to be underutilized.

author: justinjja
author_rank: Regular Contributor
timestamp: June 03, 2017, 04:21:45 pm
content: So there is one line on the label in French, translated:Caution: Use only in dry locations.Double pole / fuse on neutralWhat does that mean?

author: rstofer
author_rank: Super Contributor
timestamp: June 03, 2017, 05:35:52 pm
content: Quote from: NiHaoMike on June 03, 2017, 03:09:12 pmIn fact, it is highly unlikely that it can be used because most plug-in breakers are rated either 120 or, if they have handle ties, 120/240. Finding a single pole breaker rated at 250V is probably not going to happen. It has been more than 50 years since I worked on an open delta panel but I seem to recall that it wasn't even possible for a single pole breaker to connect with the center bus bar. I could be wrong about that... My memory fades!UL talks about it in Section 18, page 23, here:http://www.ul.com/wp-content/uploads/2014/04/Panelboards_MG.pdfI have NEVER seen a single phase circuit connected to the high leg. It exists solely as part of the 3 phase system for motor loads.MOST of what I have read concerning open-delta via Google is nonsense. It is simply a way to get 3 phase for motors and 120V for lighting and convenience outlets using only 2 transformers rather than the 3 transformers required for 120/208 wye. The larger transformer is center tapped and provides the two 120V phases and the smaller transformer provides for the 208V leg. Across the phases the voltages are 240V.