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Phase-to-Earth Short on Grounded Delta System

Phase-to-Earth Short on Grounded Delta System

(OP)
Hey Everyone! First post newbie here, so bear with me if I ask stupid questions :D

A coworker and I are having an argument about what happens if you have a direct earth short on one of your phases on a 480V, 3 phase motor if the factory is earth grounded. If the mill has a floating ground system, my theory is that the motor should continue to run if the phase is simply shorted (not burnt/cut in half) since the ground system because an extension of the 480 bus. However, if the factory is properly grounded, I say that the short would cause a massive voltage drop on the phase and would effectively single phase the motor (if the overloads didn't trip). Any thoughts on the matter would be much appreciated.

RE: Phase-to-Earth Short on Grounded Delta System

In a floating system, all three phases on a 480 Volt system typically are 277 Volts to ground. The system wiring has both high resistance and capacitance to ground. These act to for a very high impedance ground on the system.
If one phase is grounded, that phas will go to zero volts to ground and the other two phases will rise to 480 Volts to ground.
On a grounded system, if one phase goes to ground a high current flows from that phase to ground. The current should either trip out the supply circuit breaker or blow a supply fuse.
If a breaker trips, the circuit will be de-energized.
If a fuse blows a motor may continue to run on single phase and the back EMF will contribute to the fault current even though the system no longer contributes. The motor current will be excessively high as the motor transfers energy from the healthy phases to the faulted phase..

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

RE: Phase-to-Earth Short on Grounded Delta System

It depends- if the 480 v is connected delta and floating which is typical of these old systems. it is designed that way in the old days in case one phase of a motors grounds out then the building 480v system will stay running till a control shut down can be performed to correct the problem.

The system will then be a grounded delta system. The protective relays could alarm or shut down the system if they are using this type of detection or protection. In most cases this is not a desirable condition for the building system to be in and should be corrected as soon as possible

The motor may or may not fail depending where the short is.

RE: Phase-to-Earth Short on Grounded Delta System

I'm assuming its a grounded delta system. In that case the relays should open, attempt to re-close, then lock out the entire circuit.

If its not grounded then the faulted phase will go to 0 volts and the other phases will go from 277V phase to ground to 480V. Any motors on these phases may fail.

RE: Phase-to-Earth Short on Grounded Delta System

There are several ways of grounding a delta system.
Corner grounding.
Grounding the center tap of a 120/240 Volt four wire delta system.
Grounding with a zig-zag transformer.
Grounding with a small wye:delta transformer bank.
A phase to ground fault may cause the supply breaker to trip on overload (inverse time) or on over current (instant).
A phase to ground fault may be detected by the protection system which may then:
Send a shunt trip signal to the supply breaker instantly.
Send a shunt trip signal to the supply breaker after a set time.
Send an alarm signal.

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

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