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1200 volts on a 600 volt system 2

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ghostbuster7

Electrical
Jun 25, 2007
80
At approx. 4 pm everyday the phase to ground voltage jumps up from 347 to 1200 volts on 1 phase of a 600 volt delta system in a plant.

15-20 hours later the voltage drops back to 347 volts phase to ground.

Several large motors and circuit boards have alrerady been damaged.

Will be at site this week to investigate further.Could this be an arcing ground??
 
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I have no doubt that what busbar quoted could be true, but I am certain that it would have moved all of the phase voltages well away from ground, note the "switchboard voltmeters (measuring line-to-line volts)... indicated no unusual conditions". The only way that could be true with one phase at 1200V plus to ground would be for the other two phases to be at least 720V to ground.
 
On reflection I have to agree with you, David. I stand corrected.
There is a measuring problem,
Yes!
But
Several large motors and circuit boards have alrerady been damaged.
Regardless of the measurement issue there is a serious situation that must be resolved.
I would concentrate on isolating the fault. We can speculate later as to the reason for the measurement issue.


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
This trouble can be avoided by proper grounding of the system, and other important benefits are also obtained.

Amen to that! It's just amazing that ungrounded systems still persist.
 
FOUND IT!

Outdoor 3 phase 600 volt power cable running up a wooden pole.A vehicle had backed into this cable and had damaged it.

Aside:
Our own calibrated voltmeter readings to-day indicated approx. 600 volts to ground on 2 phases and 0 volts on the third phase.With the damaged feeder disconnected,the voltage dropped to approx.347 volts to ground on all 3 phases.
 
gb7, now would be a good time to add high-resistance grounding to the 600-volt system.

Typically, it sized at ±1 ampere per MVA of serving transformer, with machine-tool control-power transformers with primaries connected grounded-wye, and secondaries in broken delta, with a resistor. For 1 ampere of system charging current, the transformers would be ±0.75kVA, and the resistor would be rated ±44? at 1.1kW. This assembly could be connected to any point on the 600V system—even a small branch circuit; minimizing the need for a system outage.

Voltage (or absence of it) across the resistor would be an indirect measure of 600V-system insulation quality.

John P Nelson has an outstanding paper on the subject regarding its use in the petrochemical industry. See—
 
Good suggestion busbar. LPS

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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