480vac

This is what is known as a corner grounded delta system. There is no neutral on this system. To ground the system, one of the three lines is tied to ground.

Thanks sounds sensible.

North of 60,

You are right, the connection is delta, the line-line voltage between any 2 corners (A&B,B&C and C&A) of the delta are 480V. One of the windings (assume AB) is divided into 2 equal parts (AM and MB). Assume C is grounded.

If the system is symmetrical, the voltages should be as follows:

UBC = UAC = UAB= 480V

UMA = UMB = 240V

UMC = 415V

But in this case between which points was the mentioned 440V voltage measured ?

guest; if you measure from center leg to each phase leg & read 255 volts ac you have a star/wye system, usually the center is grounded & should not read any voltage between the two. perry

Thanks I will check that.

I think it depends on what is meant by the term "center leg". I, like North of 60, interpreted that to mean that one of the 3 phases (i.e. "B" phase) was reading 0V with reference to ground, in which case it is a corner grounded delta (3 wire) system.

You have interpreted the term "center leg" to mean a star (Y) point in a 480Y277V 3 phase 4 wire system, in which case the Star point is grounded at some point. I would not have used the term "center leg" to describe a 4th conductor in a 4 wire system.

My experience has it that above 300 volts is not normally grounded as described. Usually at 480v this condition would be considered dangerous and a second leg making it to ground would be a disaster. This is why the neutral in 277/480 4 wire is grounded (it is less than 300v) but a 3 wire 480v system is not grounded. I saw the reference to corner grounds, but that is for 240v delta.

I'm always open to expanding my experience though, such as if you are not in a country where my understanding holds true.

When measuring voltages on a straight 480v system, the 'to ground' voltages will be similar and fluctuate a little bit, but when a ground fault occurs, then the system will measure as if "corner grounded" 480-0-480 etc. If this condition is found it is time to find the cause of the ground as soon as possible and remove it before another 'corner' gets grounded, saving the equipment and manpower from damage.

Above 300v the grounding of a current carrying conductor is accomplished through a resistor or grounding transformer bank to control the KABOOM and still accomplish the circuit protection.

The short answer to your question "what type of circuit is 440vac checked with a meter has center leg to ground 0 volts, between the 3 legs its 480vac" is therefore "a dangerous one." You need to locate and remove the ground on the center leg ASAP.

Guidance on how to locate by request, since that was not the question.

^{"I saw the reference to corner grounds, but that is for 240v delta."}

No, not at all true. Legacy 480V corner grounded delta systems exist all over the US.

As they do here in Canada as well, though mostly with the older facilities of companies who's original infrastructure was engineered by the engineering resources of their US parent organizations.

Current Canadian code has a prohibition on the installation of any new systems of this description, though there is language in the code that makes the issue debatable.

This discussion has prompted me to ask my colleges for an interpretation on the question. I'll get back to the group on the current Canadian perspective on this.

I acknowledge the existence of many dangerous existing legacy systems, however in 25 yrs with major elec Utility (1/3 of California) in metering tech and management my experience was the basis for my post. The OP description of 'center' leg was a physical descriptor of what he saw as opposed to a diagramatical term.

Some industries have in the past used corner grounded 480v systems for certain differences in how they protect equipment as opposed to personnel, however in my 25 yrs I did not experience one of these systems, in fact I answered in reference to our solid policy of stopping our procedures upon finding voltage measurements as described by the OP and instituting an urgent effort to locate the party responsible for the installation. The action was to initiate a swift resolution for the sake of personnel safety and equipment protection.

Quite often, after determining if the grounded corner was on the utility side or the customer side (Utility side required line crew intervention ASAP) it would be determined that some replacement wiring had recently occurred in conjunction with new or replacement equipment, leading to discovery of wiring scraped and grounded in the process of pulling the wire. There were profuse thank you's on several occasions.

Hopefully the OP's system is not 'legacy' enough to be intentionally solid corner grounded, but as stated, I am always ready to expand my experience. I will have to wait for specific detail as requested in other posts.

I am including a .ppt link with specific detail on the entire subject for the OP's benefit.

http://www.i-gard.com/pdf/I-Gard_Presentation.ppt, but you may have to use openoffoce.org or a powerpoint reader.

Also I humbly appreciate and enjoy this forum, and responders with differences to my post were quite polite, thank you.

It happens I was called in for a service call today and had a chance to again look at the system we are talking about. It certainly was a corner grounded delta system and this particular company using this plant had leased it as is with the Bus duct 480vac. It also has a 207vac/120vac. The breaker panels are 3 pole and all disconnects have three fuses. I have learned from the landlord that this delta corner ground was done in the substation transformer and it was intentionally

Thanks LionLFlyer, greater detail always aids in the discussion as opposed to conjecture. I'm curious if there is a solid ground, a low resistance ground or a high resistance ground. Were you able to see into the substation, possibly well enough to identify the grounding method? Also were you able to download the powerpoint discussion about the implications of each?

In short, it is clear that equipment and personnel are at much greater risk with a solidly grounded system above 300v, and there are several reasons that a high resistance ground has several advantages, cost, safety, maintenance, etc.

It may be possible to offer the advantages of upgrading this system to HRG to your customer. If you have any difficulty reviewing the link, I will work out a better way to get this important info., and it is well worth everyones' time to know the differences.

We are now clear however that you have good voltages on the 3phase system (delta) and also have a 4w wye system at 120/208, with a grounded neutral.

While I dis-agree with your comments about 480 volt corner grounded systems, they do exist, you have brought up an interesting point.

Namely... besides a corner grounded system, this could also be a 480 volt three phase system that IS NOT GROUNDED and a phase to ground fault has occured.

The one things that is very clear is that the OP really needs to check into exactly what the system is. Time to go to the source.