480 volt 3-phase

[e27978]

This may be a off topic of the site but I thought since I have gotten help here before that I might ask.
I was changing a 480v 3phase motor about a week ago and all Power comes from allen bradley bucket while troubleshooting motor I read all three leads Top of Fuse to ground L1 to ground Read roughly 480v L2 read 480v And L3 Read 0v.The entire motor control center all buckets read the same way.Even on other working and running equiptment.An amp reading showed that all legs on running equiptment were drawing ballanced load.
I have taken Voltage readings in many places and diffrent jobes but I have never seen this before expecting 277 to ground on each leg.lead to lead they all read 480v am I missing something or is this common in certian applications

 

[jrwb4gbm]

Your system most likely is a 480 delta system which has no ground to reference to, unlike a 480 wye (Y) system which does. 277v to ground readings are only valid on wye systems. The reason for the two phases reading 480 volts to ground is that the other phase that is reading 0 volts to ground has an accidental or on purpose ground on that 3rd phase. No, it is not common to do this on purpose except on certain trolleys, hoists, or railway systems, generally speaking.

 

[e27978]

Would this be done Because it is a hazardous location
class 1 div 1

 

[milldrone]

e27978,

Do you know of a panel, display or box that might be called "ground lights" or something similar?

 

[jrwb4gbm]

You should probably check the NEC code book to get the exact correct answer regarding the hazardous location, somewhere in the 500's section I think. It would be too risky to just guess on this.

 

[e27978]

I don't remember seeing any ground lights or any panels this is a large plant and I have only been there about 8 months.This was my first time working on any thing electrical in this area.this motor control center has been there for many years before. I came there.at least 20 years

 

[Derek]

Ungrounded Deltas were all the rage in earlier times.
Off the top of my head without digging out a code book the
NEC requires that they only be used in occupancies
where there are trained personal to constantly supervise them (3 light bulbs). Doubt they would be used in a hazardous location.

 

[jrwb4gbm]

480v delta systems (ungrounded) were fairly common up until the 1960's but have been phased out for new installations since then. There are existing power company systems like this that have not been changed out or replaced.

 

[e27978]

Checked the plant map this is actually outside of the class 1 div 1 area.and there are operators in the area when equipment is used maybe I just have not seen the lights.Could of asked some of the senior guys but didn't want to look bad for not knowing if it were common thanks for your help

 

[milldrone]

e27978,

If this is an ungrounded delta it is common to have the ground lights near the service entrance. Is this a single metered facility? If it is and you have just one high voltage (12,000V or thereabouts) to 480 transformer. Then my guess is that you have this condition everywhere in your plant.

Corner grounded delta's do exist but they are very rare.

The big question is what kind of grounding scheme does your plant have?

 

[milldrone]

Could of asked some of the senior guys but didn't want to look bad for not knowing if it were common thanks for your help

This might be a very serious matter!!!!!!! Do not hesitate to ask this question!!!!!!!!!!!!

 

[e27978]

I know there places in the plant that are not this way but they are newer parts.Parts of the plant have been here since the 40's

 

[Derek]

I would get on this ASAP with an orderly shutdown to determine
what is going on. Now that you know about it, your are in a liability
position if anything happens.

 

[milldrone]

Parts of the plant have been here since the 40's

This is what I was suspecting. The ungrounded or floating delta systems were very common in that time. Unfortunately this also probably means the disconnecting - protecting gear is this old also. If this is truly an ungrounded delta and one leg is grounded. The next ground will announce itself with a big boom! Like blow the doors off cabinets!

 

[e27978]

Im am sure that this is known by the Plant electricians and management as our services are checked by high voltage maintenance yearly.Every company I have worked for prior to this had relativity new facility's and I have just never seen it before.Now that I know what it is I will find out more information from them and why it is still used