Negative 48 Volts DC

Me either ;-) Brain fart

208/120

OK you have a regular 120/240 transformer connected to one phase of the medium voltage distribution (13kv or whatever) Then I add a second transformer from another phase and a 3d transformer from the last phase. I already have my ground in the first one so I will connect the secondaries in Delta. Voltage l/l around the triangle are 240v but I still have the original one that is still

120/240. That gives me the best of both worlds. Plenty of 240 3p and as much 120/240 as I size that transformer for. Now as was mentioned before, you can actually remove one of those transformers and it still works, with about a third less capacity on the 3p. Typically you will see two transformers, one big one and one smaller one. The big one is the 120/240.
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120 * 1.73 = 207.6 call it 208
Reply to
gfretwell
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You are right, brain fart on my part.

Yup except it is really the same basic transformer, they just don't connect the center tap.

Reply to
gfretwell

???? 25/1/2020 7:24 ?.?., ? Grant Taylor ??????:

I know two examples:on a common microwave oven, so as to avoid the microwave gun being at high voltage potential, it is earthed and the magnetron filaments are being at negative high voltage potential. Also the diode is being earthed. If the oven is not properly earthed there is the danger of electrocution. Also a modern IGBT inverter welding power supply, when welding all electrodes but aluminium, the electrode is on the plus pole and the ground is negative. When welding aluminium the electrode is negative and the ground is positive.

Reply to
Dimitris Tzortzakakis

I'd like to take a moment to thank everybody that has helped me along my journey to learn about Negative 48 Volts DC. I believe that I have learned, and unlearned, enough to have a acceptable decent understanding.

I have learned that the terms "hot" / "common" / "return" / "ground" do not correlate with the polarity without indication of "negative ground" (what I'm used to) or "positive ground".

In a negative ground system, the "hot" wire will be positive compared to "ground" / "return".

In a positive ground system, the "hot" wire will be negative compared to "ground" / "return".

Reply to
Grant Taylor

volts earthed wye. Medium voltage was 15 kV, now is 20 kV all over Crete, the island where I live, and high voltage is 150 kV. There's a plan of connecting the island to the greek mainland with 2 150 kV underwater cables and one 750 kV HVDC so completely shut down the on-the-fly electricity generation on the island, with mazut and diesel and some renewable. There's a very old power station near the city I live, in Linoperamata, with some small steam units, 2-stroke diesels and gas turbines, burning in excess 300,000 tons of heavy fuel annually. When the interconnection will be realized the plant will shut down.

Reply to
Dimitris Tzortzakakis

I ran across something while reading ANSI C84.1 Electrical Power Systems and Equipment ? Voltage Ratings (60 Hertz).

Table 1 note d has the following to say about 208Y/120:

(d) A modification of this three-phase, four-wire system is available as a 120/208Y-volt service for single-phase, three-wire, open-wye applications.

I think that exactly describes some of the cabinet distribution units (power strips) at my office; two hots (lines) and ground, for a 208 volt single-phase + ground power feed.

In hindsight, it sort of makes sense that the small number is first, thus indicating single phase, seeing as how it really is feeding a single phase to the equipment.

Reply to
Grant Taylor

That is the convention. It is similar to 240/120 (3 phase center tapped delta) and 120/240 (single phase center tapped 240). In fact it can be the same service with the 3d phase added by installing one more transformer (open V delta or red leg delta). To get full rated power you really need a 3d transformer tho. It is not commonly done but I have seen it.

Reply to
gfretwell

Hello, and at my place of employment we have devices such as UPSs and test equipment that require a 208 VAC 50A single-phase service. These circuits are gotten from a breaker panel that has an incoming 120/208 VAC 3-phase, 4-wire service. The single phase 208V service is most often terminated in a wall-mounted receptacle that is physically a larger version of the standard u-ground 120 VAC 15A receptacle. Sincerely,

Reply to
J.B. Wood

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