480v Delta systems

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If you're looking to learn all about it, that is a LARGE amount of information to take in. Is your system grounded or ungrounded?

I'm going to go with the biggest one first: safety. 480V is no joke. Do not work on it live. Arc flash is a major concern and danger.
here's a few video links of 480V arc flashes:

https://www.youtube.com/watch?v=P35HRYHFz7c
https://www.youtube.com/watch?v=-iClXrd50Z8
https://www.youtube.com/watch?v=DFw8t7tG1Y0

I've seen an industrial electrician cause a fault to ground in a 480V MCC... for a 1/2 HP shaft drive motor. The fault tripped the 2000A main. The second time he went phase to phase across a MCC for a 150HP pump... that arc flash put him out of work for six months.

eta: here's another video. Dunno the voltage, probably higher than 480V, but same results:

https://www.youtube.com/watch?v=g_mzKyze4Vk&t=6s

7:00 in to about 10.
 
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Are you about to go to work in a 480V Delta system? It's becoming more rare every day, because utilities don't really want to supply it any more, they prefer 480Y277V and if you want a Delta, you have to ask for a variance.

The purpose of using a delta system stems back mainly to WWII when defense related industries were asked to run at capacity full tilt with as little shutdown as possible, even at risk of life and limb. I like to describe it as a "Damned the torpedoes, full speed ahead!" attitude. That's because in an ungrounded delta system, a line-to-ground fault does NOT cause the system to shut down, you simply move from an ungrounded delta to a corner grounded delta system, and everything keeps running. So you had some sort of ground indication system that let the electricians know there was a ground fault somewhere, but generally you didn't know exactly where at first, you had to go find it. When I started back in the late 70s at a steel mill, that's what we had. We had three light bulbs on the wall and if one glowed brighter than the other two, that phase had a ground fault somewhere and we would jump in a golf cart to run around looking for smoke, or a production manager running around pulling his hair out because his machine stopped running.

But some time in the 80s, the NEC stopped allowing ungrounded delta systems unless you added ground fault PROTECTION, not just indication, OR you used a corner grounded delta system; one or the other. Both options however removed the "Damned the torpedoes..." benefit of using a delta system anyway, so it fell out of favor. Even later it became evident that delta systems are really unkind to power electronics, such as VFDs, UPS systems, large DC power supplies etc., and that became the final death knell for utilities offering it without a thorough vetting of why you want it and how it will be implemented.

So if you are going to work somewhere that uses a 480V delta system, it's going to be either a corner grounded delta, or there will be a GFP system involved that can shut down your entire system, and if you have any VFDs etc., you will need to pay a lot of special attention to how you connect and protect them. If this is just out of curiosity, you may never see one in your career.
 
Jraef said:
Are you about to go to work in a 480V Delta system? It's becoming more rare every day, because utilities don't really want to supply it any more, they prefer 480Y277V and if you want a Delta, you have to ask for a variance.

The purpose of using a delta system stems back mainly to WWII when defense related industries were asked to run at capacity full tilt with as little shutdown as possible, even at risk of life and limb. I like to describe it as a "Damned the torpedoes, full speed ahead!" attitude. That's because in an ungrounded delta system, a line-to-ground fault does NOT cause the system to shut down, you simply move from an ungrounded delta to a corner grounded delta system, and everything keeps running. So you had some sort of ground indication system that let the electricians know there was a ground fault somewhere, but generally you didn't know exactly where at first, you had to go find it. When I started back in the late 70s at a steel mill, that's what we had. We had three light bulbs on the wall and if one glowed brighter than the other two, that phase had a ground fault somewhere and we would jump in a golf cart to run around looking for smoke, or a production manager running around pulling his hair out because his machine stopped running.

But some time in the 80s, the NEC stopped allowing ungrounded delta systems unless you added ground fault PROTECTION, not just indication, OR you used a corner grounded delta system; one or the other. Both options however removed the "Damned the torpedoes..." benefit of using a delta system anyway, so it fell out of favor. Even later it became evident that delta systems are really unkind to power electronics, such as VFDs, UPS systems, large DC power supplies etc., and that became the final death knell for utilities offering it without a thorough vetting of why you want it and how it will be implemented.

So if you are going to work somewhere that uses a 480V delta system, it's going to be either a corner grounded delta, or there will be a GFP system involved that can shut down your entire system, and if you have any VFDs etc., you will need to pay a lot of special attention to how you connect and protect them. If this is just out of curiosity, you may never see one in your career.
Click to expand...

I once had a facility rewired in 1979 that used un-grounded 480v delta. We installed two 160kva UPS systems in 2010. Another PE argued there was no such thing as an un-grounded delta. I finally put four 100w light bulbs on 2x4 and went from each phase to ground - minor glow from leakage. That shut him up. We had to install a 500kva delta to wye transformer in front of the systems along with all the required fusing. Big bucks!.
 
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