III phase 480V Delta Issue/Question

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winnie said:
For the CNC shop I'd recommend an isolation transformer to supply a properly grounded 480/277 wye system.

Jon
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This ^^^.

Do not try to put power to the CNC, lock it out.

The CNC is all drives to its motors and it will go poof quickly or at some point in the near future if you try to apply a 480 delta feed to it.

The CNC drives are looking for and need a solidly grounded Y supply to match their grounded Y, drive front end config. If you have not yet tried to apply power, call the CNC manufacturer and they will tell you this, that you need the delta to Y transformer (if the supply is delta).

If you've already applied the delta power to it, you're doing it the hard way.
 
Hi everyone that helped and made comments, I figured out the issue and I really don't know why this would make a difference but one of the fans on the roof top was bad and not working, I didn't dig into it much but I pulled the power to the fan and pulled wires from the capacitor to it and then my voltage went back to normal? 277 to ground on the phase that was previously 12V to ground. Any clue on why that would make a difference? basically if that fan is hooked up that phase got to 12V to ground I unhook it and it's back to 277 to ground? Thank you. Jeff
 
Sarge590 said:
277 to ground on the phase that was previously 12V to ground. Any clue on why that would make a difference? basically if that fan is hooked up that phase got to 12V to ground I unhook it and it's back to 277 to ground? Thank you. Jeff
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That is easy. You have an ungrounded system, and there was a ground fault in the capacitor.

Most people are familiar with solidly grounded electrical systems. These are systems where one terminal of the system is intentionally connected to 'ground'. This _single_ connection to ground doesn't cause a short circuit because it isn't a circuit; there is not a closed loop that current can flow in. But with this intentional connection to ground, a second unintentional connection to ground (a 'fault') creates a short circuit, lots of current flows, and a breaker trips.

Your facility has a ungrounded system. There is no first intentional connection to ground. Instead the system 'floats' until a fault connects some part of the system to ground. Because the first fault creates the first connection to ground, there is no short circuit, and the system can continue to operate until the necessary repair can be safely accomplished.

One aspect of ungrounded systems is that any voltage measurement to ground is extremely sensitive to the state of the system. When everything is normal, phase to ground voltages can be just about anything. When there is a ground fault, the faulted phase to ground voltage falls to close to zero, exactly what you experienced.

This requires a maintenance staff that understands the system, and knows that as soon as the phase light is on there is a problem to be fixed. This sort of system is only appropriate where a shutdown due to a fault causes more safety risk than leaving a system energized with a fault. The code specifically spells out "Conditions of maintenance and supervision...."

IMHO @Sarge590 , you are going to need to learn more about this system, or push the folk with the dollars to get the system converted to a solidly grounded system.

-Jon
 
Jon, thank you very much explained perfectly now I get it. I've ready so many forums and comments about similar issues and I received many different answers but this one sounds and is explain more clearly and totally make sense. Thanks for taking your time to respond to me, I know time is valuable. I really do appreciate the help. Take care. Jeff
 
Capacitive coupling and high impedance meters, I see that confuse the youngsters a lot. That is when old men like myself break out one of these
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LarryFine said:
Not me. I use a solenoid tester for trouble-shooting, unless I need to know the exact voltage.
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Solenoid tester is even better. Believe it or not we don't even have one anywhere in the maintenance department. I only have a Simpson 260 because I found it on a really dusty shelf in the boiler house. I keep it hidden so nobody throws it out.
 
winnie said:
One aspect of ungrounded systems is that any voltage measurement to ground is extremely sensitive to the state of the system.
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Does that include the presence of a meter measuring L-G? On a real-world 480V delta ungrounded system, can you get measurements of 480V L-G on each leg (or maybe 0V L-G on each leg) due to the presence of the meter shifting the system potential to ground? High impedance meter or low impedance meter?

Cheers, Wayne
 
wwhitney said:
Does that include the presence of a meter measuring L-G? On a real-world 480V delta ungrounded system, can you get measurements of 480V L-G on each leg (or maybe 0V L-G on each leg) due to the presence of the meter shifting the system potential to ground? High impedance meter or low impedance meter?

Cheers, Wayne
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Maybe if the meter has a very inductive impedance, but I rather doubt it.

I suspect that the 480V L-G in the original post is either a measurement error, or some issue with harmonics creating measurements that are not possible with pure 60Hz sine waves. But I don't give these measurements much thought. The issue with the ground fault was discovered and resolved.

-Jon
 
Check the rooftop heating unit. I suspect it has one or more heaters per phase. They should be wired phase-to-phase (480V across each heating element). One of them might be wired phase-to-ground (to essentially reduce its output based on an ASSUMED 277V). It is not uncommon for heating elements to be rated at different heat outputs at different voltages.
 
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