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Yesterday, I wanted to test some MCBs and power supplies that I had just received. Since we only have one 480V socket in the shop, I asked my coworker if I could use the power source he was using. He went with me and unplugged his (working) PLC system which used a 3 pole MCB powering a 24VDC power supply. His parts are nearly identical to the parts I'm testing.

I took an identical power cable we had and wired in three of the wires to one of my MCBs. I left the remaining green wire that had a ring terminal on it unconnected.

When I plugged in the cable and measured the voltages at the MCB, I was confused at the measurements. If we call the three phase wires A, B, and C, and the green wire D, then these were the measurements I made:

Probe Probe Voltage (VAC)
A B 225V
A C 475V
A D 250V
B C 246V
B D 33V
C D 238V

We're in western Canada, and I've heard that some systems use a "high leg", but I don't think that's what we have here. I also read about "corner grounded" and "open" delta systems, but I don't know if that's what we have.

What kind of distribution system is this?

Given that my coworker's system hasn't malfunctioned, it may be safe to say that I can simply treat it as a 480V three-phase connection and not worry about how it works. Is this stupid?

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  • \$\begingroup\$ Is the unconnected D green wire from the source, or from your MCB. \$\endgroup\$ Apr 2 at 23:49
  • \$\begingroup\$ Hello. The green "D" wire is within the power cable, which has four wires in total. The MCB only has three poles which I've wired A, B, and C to. Out of the four prongs on the plug end of the cable, the "D" wire is tied to a prong which is slightly larger than the others. \$\endgroup\$
    – masuat
    Apr 3 at 0:17
  • \$\begingroup\$ Key line is one 480V socket in the shop. So your shop is not true 3-phase. If you check co-workers wiring, odds are they are only using two wires, not three. 24VDC does not need three phase. \$\endgroup\$ Apr 3 at 0:50
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    \$\begingroup\$ B is neutral, D - ground. Looks like 480/240V two phase system. Some old stuff. \$\endgroup\$
    – user263983
    Apr 3 at 1:20
  • \$\begingroup\$ @StainlessSteelRat, a few questions about your comment. You seem to be implying that having one 480V socket directly implies that the connection is not three-phase. Why? Also, my coworker was definitely using three wires to power his power supply. It's a Delta DRP024V240W3BN; datasheet deltapsu.com/jp/products/download/Datasheet/DRP024V240W3BA. Why does the power supply function normally when supplied with our single/two-phase voltage? \$\endgroup\$
    – masuat
    Apr 3 at 20:42
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It looks to me like a 480 Volt single phase with a center tap - using our 120/240 volt terminology, A and C would be the two "hot" wires, and B would be the Neutral (but not connected to Safety Ground in your case).

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  • \$\begingroup\$ You meant 480/240? \$\endgroup\$
    – user263983
    Apr 3 at 1:22
  • \$\begingroup\$ Yes - I'm suggesting the OP has 240/480 V, similar to the common 120/240 V we have in North America. \$\endgroup\$ Apr 3 at 1:47
  • \$\begingroup\$ Canada is North America too. You may edit the answer. \$\endgroup\$
    – user263983
    Apr 3 at 11:24
  • \$\begingroup\$ @user263983: I'm Canadian - that's why I said "North America", not "USA". \$\endgroup\$ Apr 3 at 15:55
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I do not know what you have, nothing appears correct. 480 has nominally 277V between phase and ground. This is used a lot in the US for lighting. It appears you have a real problem, "D" I believe is your safety ground not a power connections. 3 phase only has three power connections hence its name and a safety earth ground. I highly recommend getting a qualified electrician to check it out for you. I am assuming there is no phantom phase generation in the system.

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It is 480/240V, two phase system. Transformer output is 480V with center tap. You can check the colors of wires, neutral will be white or gray, phase is usually black and red. The system is very old, not used for new installation. But if you have some old type of equipment, required that voltage, keep using it. Voltage between neutral and ground way too much. Neutral has to be grounded at transformer. My suggestion order inspection from ESA, continue to use it may be dangerous.

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