Electronics > Beginners
How to wire up a 240VAC receptacle
Spork Schivago:
--- Quote from: Gregg on May 17, 2018, 07:38:05 pm ---
--- Quote from: C on May 17, 2018, 06:23:59 pm ---Actually Split-phase can be safer.
First you have GFCI or RCD to think about.
To reduce EMI or RFI X-cap's or Y-cap's are used. With two wire power and one side Neutral you are connecting 1/2 AC voltage to somethiing, the Neutral lead or safety ground. To work around this the GFCI or RCD has to be made less sensitive. With the center Neutral you have 0 volt AC EMI/RFI connected to Neutral so a more sensitive GFCI/RCD is possible..
There is no perfect wire so there is always a voltage drop on the Neutral lead making it NOT ground. A short from Neutral to something is connecting the voltage drop to something and not detected with fuse or circuit breaker in the hot lead. With split-phase this is detected. Shorts like this do happen and can have many watts of power turned in to heat.
Then you have 1/2 voltage difference to actual ground, again safer
The big cost is often just the one added wire and dual breakers.
Equipment designed for split-phase can be designed three ways.
1. You have Equipment that needs some 110-120 for internal use in addition to 220-240. Here three wire or safer 4 wire connection.
2. like above but with no 110-120 internal use so that the Neutral lead supplies no power.
3. A variation of #2 is Two hots and where Neutral lead is just used for a safety ground.
Three phase is just a shift to three hots vs two hots.
C
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Three phase is far from just a shift to three hot leads. There are several methods of connection used in the US, but all of the large data centers I have worked with use 480 volt 3ph UPSs connected to 208/120V transformers and then breaker panels. Several customers had large cabinets that had 40 amp 208V 3ph power supplied. 3ph when rectified has far less ripple than single phase and the 3ph SMPS are more efficient.
Neutral conductors should never be used as ground connectors. True, there are a few cases where the NEC allows a ground conductor to carry a small neutral current; but I would never wire my house in such a manner, even if it is officially OK.
Back to the subject at hand; Richard Crowley is absolutely correct and it is sad that Spork purchased the wrong PDU. It isn’t our fault he didn’t ask first and now there isn’t any way to easily fix the situation short of taking it apart and seeing if it can be modified to install 2 pole breakers and to ensure the switching mechanism switches both hot legs of the 240V.
If I were in Spork’s shoes, I would try to return the PDU on the grounds that they didn’t properly describe it in their literature and instead purchase two of the P9S13A units if he needs the capability to switch each branch circuit remotely.
Let’s consider the possible unsafe modes of failure using the P9S16A PDU:
Suppose there is an internal ground fault in one of the server power supplies that is from the hot without the single pole breaker. The single pole breaker doesn’t respond and the fault remains until the 40 amp two pole breaker trips after things catch fire. Or the fault gets to the point where it blows up the power supply and possibly ruining the whole server. If the 40 amp breaker trips, and nothing much else happens your whole system still goes down until you figure out the problem. Assuming your server racks/cabinets are properly grounded; if they are not, you could be in for the shock of your life.
Suppose you figure out there is something wonky with one of the server power supplies and you turn it off remotely because you are not at home; but the PDU switch only turns off one hot leg. Failure modes are the same as above.
I’m sure others on the forum could add to this list.
--- End quote ---
This is getting old guys. It's not like I just surfed the net and randomly picked a PDU. I'm running a business, so I call one of my VARs, I tell them what our requirements are. They place a call to one of the various companies. I like HPE, so I wanted HPE. At first, we didn't have income to purchase everything we needed. This is how most start-up companies operate. They don't make a return for 2 years or so. We got lucky and made a return early on, which allowed us to purchase the upgrades to do what we wanted. But then we called our VAR and said hey, because we have x, y, and z added to the servers now, will the 120VAC 500 watt PSUs that are currently in the servers handle the new load? They say let me forward that along to our HPE technicians. They come back and say no, you need the 1400 watt PSUs part number <blah>. We purchase the PSU part number <blah> and they say you also are going to need the PDU part number <blah>. Our only other option was 3-phase, and if anything, we should be saying it's sad that Spork couldn't afford to get 3-phase ran to his house so he could purchase the 3-phase equipment that would be available to him. We also need a UPS, but eventually a whole home generator with an automatic transfer station.
It'd be like me saying it's sad that you are running 3-phase to your rack. You made a mistake in purchasing your equipment and should be running split-phase to your rack. Your rack cannot handle split-phase, it needs 3-phase. Why? Because the equipment was designed for 3-phase, just like our equipment was designed for 240VAC single phase, not 120VAC - 0 - 120VAC split-phase.
I thought this was a simple question, I didn't expect 6 pages of people trying to get me to switch to something that just won't work, and granted, I guess it's more my fault for not giving out enough information (such as the power supplies we use, the exact equipment we use, etc), but my fear is violating the NDAs that we signed. There's some claus that talks about us paying a lot of money if we violate it. We have people working for us that have to sign an NDA, but we as well had to sign NDAs. We have on-site and off-site record storage where there's different levels that different employees have access to. There's the TRADE SECRET level, the CONFIDENTIAL LEVEL, and then the GENERAL level. Our employees get access to the GENERAL level documents, which include stuff such as how to connect to the VPN tunnel, how to use the two factor authentication properly, what to do if there is a suspected compromise, SOPs, etc.
I don't know why everyone assumes I purchased the wrong PDU. I don't know why almost everyone is suggesting I switch to a PDU that supports split-phase. I said I need 240VAC single-phase and that's what I need. I just thought in a professional environment, there'd be a lot less this is what I think you need stuff going on or if it was me, I would have done it this way or that way, or for my situation, split-phase works best, therefore, for your situation, it does as well kinda stuff. I just thought I'd get a nice answer saying this is how you can achieve single phase 240VAC, like Mr. Crowley suggested, and a few others.
Spork Schivago:
--- Quote from: Monkeh on May 18, 2018, 01:52:23 am ---
--- Quote from: Spork Schivago on May 18, 2018, 01:46:26 am ---Then the servers themselves, as we verified once again, are designed for single phase 240VAC but learned they won't even power on if we attempt to feed them split-phase. This is probably a safety feature.
--- End quote ---
This seems unlikely. There's no safety benefit.
--- End quote ---
You are saying there's no safety to the equipment that was designed to run off 240VAC single phase, 120VAC - 0 - 120VAC split phase? Didn't we just go over all the dangers to doing that with the PDU? Surely there are dangers to the server as well. I cannot think of any other reasons as to why HPE would design the PSUs not to turn on at all if we tried feeding them 120VAC down the L/+ line, and 120VAC down the neutral line....
Spork Schivago:
--- Quote from: Monkeh on May 18, 2018, 01:19:08 am ---
--- Quote from: Spork Schivago on May 18, 2018, 01:18:04 am ---the PSUs cannot run off split-phase. They won't power up, they won't provide power to the servers.
--- End quote ---
And you know this how?
--- End quote ---
By talking to HPE directly, instead of going through my VAR? I have a direct phone number I can give you, if you'd like to verify this information. I can PM you the part number to the PSUs and you can talk to them directly and ask them for yourself if you don't believe me.
IanB:
--- Quote from: Spork Schivago on May 18, 2018, 01:46:26 am ---Then the servers themselves, as we verified once again, are designed for single phase 240VAC but learned they won't even power on if we attempt to feed them split-phase. This is probably a safety feature.
--- End quote ---
A couple of puzzles with this are:
1. If you take the two line conductors from a split phase supply then that is, electronically, a single phase 240 V supply. The equipment being supplied cannot tell the difference between a 120-0-120 supply and a 240-0 supply unless it specifically measures the line-ground potential difference and acts on it. That seems unlikely as there would be no point.
2. A 240-0 single phase supply is nonstandard in the USA. So any equipment purchased in the USA for use in the USA market is unlikely to have this requirement--for the simple reason that nobody would have a suitable power supply for it. It makes no sense that the vendors would require customers to arrange a special installation with custom wiring, transformers and so forth just to use their equipment. Nobody would be prepared to stump up the cost.
The short answer is that equipment appears to come in two flavors: North America and "Worldwide", where "Worldwide" is everywhere except North America. It is logical that if servers, PSUs and PDUs are purchased with a North American specification for use within North America then they will be compatible with standard North American power distribution arrangements.
In North America the standard arrangements are 120-0 single phase, 120-0-120 split phase, or 208-0 single phase (derived from a three phase transformer). There could be three phase arrangements, but those are not under consideration here.
Monkeh:
--- Quote from: Spork Schivago on May 18, 2018, 02:10:19 am ---
--- Quote from: Monkeh on May 18, 2018, 01:52:23 am ---
--- Quote from: Spork Schivago on May 18, 2018, 01:46:26 am ---Then the servers themselves, as we verified once again, are designed for single phase 240VAC but learned they won't even power on if we attempt to feed them split-phase. This is probably a safety feature.
--- End quote ---
This seems unlikely. There's no safety benefit.
--- End quote ---
You are saying there's no safety to the equipment that was designed to run off 240VAC single phase, 120VAC - 0 - 120VAC split phase? Didn't we just go over all the dangers to doing that with the PDU? Surely there are dangers to the server as well. I cannot think of any other reasons as to why HPE would design the PSUs not to turn on at all if we tried feeding them 120VAC down the L/+ line, and 120VAC down the neutral line....
--- End quote ---
There are no dangers with the individual power supplies. I think it extremely unlikely they designed them not to operate off split phase.
--- Quote from: Spork Schivago on May 18, 2018, 02:11:29 am ---
--- Quote from: Monkeh on May 18, 2018, 01:19:08 am ---
--- Quote from: Spork Schivago on May 18, 2018, 01:18:04 am ---the PSUs cannot run off split-phase. They won't power up, they won't provide power to the servers.
--- End quote ---
And you know this how?
--- End quote ---
By talking to HPE directly, instead of going through my VAR? I have a direct phone number I can give you, if you'd like to verify this information. I can PM you the part number to the PSUs and you can talk to them directly and ask them for yourself if you don't believe me.
--- End quote ---
The answer will be given to me the same way it was given to you: A person on the other end of the phone with no knowledge whatsoever of power supplies reads '240V single phase' off the datasheet and says 'it will only work with this'.
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