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Topic Title: Need A Neutral
Topic Summary: use star point of Transformer to get a neutral?
Created On: 24 May 2010 08:52 PM
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 24 May 2010 08:52 PM
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Peddler

Posts: 36
Joined: 28 May 2008

Hi Guys, I have a (20kv 208/120v input)UPS to install on a platform.The job has not been well thought through. Anyway I have been informed by the Electrician/Supervisor that the supply DB has no neutral conductor.The DB in question is a 208/120 v board fed from a step down transformer which in turn is being fed from a 480 3 phase board. I am heading there to check it out in the morning.
My question is if the 208/120 board has no neutral how do i get a neutral to my UPS , my UPS must have a separate earth conductor and must have a neutral. I was thinking of taking a neutral from the stepdown X-former at the star point? The thing that is puzzling me is how the board can have 120 at it without the neutral? has he got it wrong or is the 120v circuits being tied to earth. ? I'm thinking he has it wrong but time will tell
thanks for any input

Ped

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 25 May 2010 07:55 AM
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ArthurHall

Posts: 737
Joined: 25 July 2008

Peddler
You can find many strange systems offshore. Ask to see a diagram of the power distribution and follow it through. Sometimes you get transformers with delta secondarys, no neutral IT earthing, Also you often get double pole MCB's.
It sounds like your DB is 208V phase to phase and 120v phase to neutral. check your voltages to earth to see if one pole of the supply is connected to earth.
You also find a lot of American systems offshore so look out for black = live and white = neutral and so on.

Arthur
 25 May 2010 08:10 PM
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Peddler

Posts: 36
Joined: 28 May 2008

Hi Arthur and tks for the reply.
I went out this morning but was up to my eyeballs in Management issues and didn't get to see inside the panel. Plus i only had 2 hours on the platform.the electrical guy tells me now he hasn 't actually seen inside but will tomorrow. An on going saga. ( permit issue)
Anyway i looked at the front of the panel and it says 208/120 3phase 4 wire which tells me it 3phase & Neutral , and also your right there are double pole breakers in there and i suspect they 208v phase to phase.
The board is American and the rig is in Africa so if there is no neutral there i'm in the s*!t. If there is no neutral can you think of any easy solution to the problem? I have visions of long cables and finding a board with a neutral conductor.

ped

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 25 May 2010 08:23 PM
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dlane

Posts: 690
Joined: 28 September 2007

Generally unser these circumstance we use earthing transformers to create a star point for the 3 phase system, I don't know if they would be available for the kind of voltage level you have.

A quick google should produce some manufacturers, just a case of weighing up the costs of the options then.

Kind regards

Donald Lane
 26 May 2010 10:46 AM
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Peddler

Posts: 36
Joined: 28 May 2008

There is a neutral bar preset, only problem i have now is the sizw of the holes , they are too small to accomodate a 16mm2 cable, how frustrating im heading out to see what i can come up with , thanks for input to all !!

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 26 May 2010 10:54 AM
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broadgage

Posts: 1357
Joined: 07 August 2007

If the board can supply both 120 volts, AND 208 volts, then there is no simple way to achieve this without a neutral.
120 volts between phase and neutral.
208 volts between phase and phase.

Without any neutral this would be a 3 phase 208 volt supply, without any provision for 120 volts.
 26 May 2010 09:58 PM
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alancapon

Posts: 5834
Joined: 27 December 2005

This sounds like an American three-phase supply arrangement. This system is rather unusual, as it is a delta-connected arrangement, with an earthed neutral connection at the center-point of one of the three windings. This gives a phase-to-phase voltage of 240v, and the phase-to-neutral voltages of 120v, 208v and 120v.

Regards,

Alan.
 27 May 2010 11:44 AM
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ArthurHall

Posts: 737
Joined: 25 July 2008

The system described by Alan is a new one to me. However I looked it up in my old (1999) copy of the NEC (American regs book)
The system seems common enough and the following should be applied.
The phases should be marked A,B,C the B phase should be the Hi-leg ie. 208V
The Hi-leg should be coloured orange.
The neutral should be white.
The A and C conductors should be identifed with a note at the source explaining the identity.

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