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Grounded L1 leg? 1

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jcromwell

jcromwell

Electrical
Feb 12, 2008
7
Ok here it goes we have a 3 phase machine that uses 4 three phase motors and one single phase motor there was a short in the receptacle that caused a phase to ground short after replacing the receptecle I megged the motors and checked each power phase from the contactor to ground I had a zero reading with a vom from L1 to machine ground this machine is using one leg L1 and ground to supply the single phase motor the power circuit for the single phase motor goes through a relay circuit not a contactor the only way the ground to L1 clears is if I disconnect the power feed wire for the 120Vac circuit from the L1 leg disconnecting the single phase motor makes no difference so I am wondering if this is normal why when i had the single phase motor electrically disconnected and the relay block for this motor power feed circuit removed from the enclosure but electrically connected i had a 15 meg reading L1 to ground. keepin mind this machine is a birds nest and there is now documentation on the wiring and controls.
 
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I'm sorry, your single horrible run-on sentence is just too much for me to sort out. Next time remember this is a professional engineering forum, and we expect some professionalism in the questions.

Keith Cress
Flamin Systems, Inc.-
 
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what do you mean smoked are you not smart enough to understand written text or is it the technology that you do not understand if that is so i recommend that you find a textbook on electricity there are many and you probably can find one in the city library have you ever considered that? there are by the way grounded corner grids i wonder if this is one of them they are not so unusual in some states but we do not have them so i would also be surprised to measure zero ohms from l1 to ground wouldn't you, too?

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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jcromwell; Please copy your question out add some periods "." and some CAPITALS and plaster it back in here. We really do want to help, we just aren't into masochism. (Well Skogs might be..)



Keith Cress
Flamin Systems, Inc.-
 
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Uh the meaning of your post changes somewhat depending on where I guess where the periods should be.
Help

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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If you posted that from a Balckberry and are sick and tired of having to use the Alt key for punctuation, you can hit the space bar twice at the end of your sentence. It automatically puts in the period and capitalizes the next word.
 
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Ok here it goes we have a 3 phase machine that uses 4 three phase motors and one single phase motor. There was a short in the receptacle that caused a phase to ground short after replacing the receptacle, I megged the motors and checked each power phase from the contactor to ground. I had a zero reading with a vom from L1 to machine ground. This machine is using one leg L1 and ground to supply the single phase motor. The power circuit for the single phase motor goes through a relay circuit not a contactor. The only way the ground to L1 clears is if I disconnect the power feed wire for the 120Vac circuit from the L1 leg. Disconnecting the single phase motor makes no difference. I am wondering if this is normal? If so why when I had the single phase motor electrically disconnected and the relay block for this motor power feed circuit removed from the enclosure but electrically connected I had a 15 meg reading L1 to ground.
Hope this is better my apoligies.
 
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To start with, it is illegal in most of North America to use a ground connection to feed any load. I don't know about the codes in your location.
Many normal AC loads will appear to be a short circuit when measured with a VOM. It is an instance of the difference between resistance and impedance. The VOM measures resistance but the impedance limits the current in an AC circuit.
The impedance is typically many times greater than the resistance at 60 Hz. Is the machine blowing the feed breaker after the repair? What is the current draw of L1 with the motor and relay connected but nothing running?

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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There are many things in this facility that are not to code or even industry accepted practices. I am trying to put out one fire at a time. I have not powered this machine back up yet. I was looking for some clarity of what I was seeing. However I belive you have explained what I was seeing. My hesistation to power this machine back up is do to the fact that there is not a single protection device except for the MCCB at the load center. No overloads no fuses no CB's in the machine. One contactor to switch 4 motors and one relay for the other motor.
 
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Thanks for the remodel.

As was mentioned any load like a lamp or a relay coil can appear to have nearly zero resistance with a meter (using DC).

Can you measure the from the "power feed wire" to case ground with your meter set to a very low range? So we can see the actual low resistance? Is it 1.3 Ohms? 0.4 Ohms? 10 Ohms? What exactly are you getting?

If you can tell us 2 or 3 ohms, or more, we can suspect it is a load. If it is less, than likely it is an unintentional short.

Keith Cress
Flamin Systems, Inc.-
 
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Presto I am must be tired the reading was .4 Ohms if I had an A/C HI-POT I would have found it, just kidding it was my to many years of working on above ground circuitry using ground relay detection and protection were you will never encounter a low megger or Ohm reading from any circuit or load except the ground relay detection coil. Thanks for jogging my brain or my rear. Machine works well I won't hesitate to post another thread with puncuation...of course.
 
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Yes. Welcome - with dots and commas and all that!

Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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A bit? A lot. I am in the process of upgrading and retrofiting the facility, it is unbelievable that this place has not been shut down or someone has not been killed.
I have rectified several dangerous situations and will continue to do so as budget and time permits. This process will take a little while but I believe management sees the overall value. Thanks for the insight.
 
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