Solution To
"Overcurrent Protection"

The ground fault is occurring on a Delta system which is an ungrounded system. (The Delta system is determined by the transformer secondary connection. Since the secondary is connected in delta, the service is therefore a delta system.)

Consider the schematic of the delta system as shown below:

IN AN UNGROUNDED SYSTEM (DELTA OR UNGROUNDED Y), THE FIRST GROUND FAULT CAUSES NO SHORT CIRCUIT CURRENT TO FLOW.

EXPLANATION

Since the fault occurs from CØ to ground, there is no complete path for short circuit current to flow. If you trace the path of the short circuit (as shown) from CØ through the line conductor, through the ground fault, through the ground system, and back to any phase, you will notice that a return path is missing. Therefore, NO short circuit current will flow.

However, because one phase (CØ) is now grounded, the phase to ground voltage on the unfaulted phases (A & B) is now at full phase to phase voltage (i.e. 480 volts) instead of a more normal, but approximately, phase to "neutral" 277 volts.

In an ungrounded Delta system, there are stray capacitances inherently connected between each phase conductor to ground. These stray capacitances are not "solidly connected" but are innate to all systems. They are distributed pretty much evenly throughout the system, are usually equal in magnitude in all 3 phases, and cannot be removed.

(Note: Phase to phase stray capacitances also exist between all phases but these capacitances play no part in ground faults.)



It is these stray capacitances that charge up and maintain a voltage between any phase wire and ground. The ungrounded delta is not truly ungrounded. Rather it is considered to be "capacitively grounded." Notice in the schematic figure, that these stray capacitances are actually Y connected to the delta system, and their neutral point is connected to the ground.

Because of this Y connection, the "normal" phase to ground voltage in a delta is the voltage that appears across each phase to ground stray capacitance and would be measured at about 277 volts. Remember though that the ungrounded delta's Ø to ground voltage can vary since no real phase to ground connection actually exists.

Since the stray capacitances are usually equal, and are Y connected, the "normal" voltage that one would measure from each phase to ground would be around 277, which corresponds to approximately the phase to "neutral" voltage even though no neutral exists on the delta, but does exist in the Y connection.

When a ground fault occurs from CØ to ground, the stray capacitances connected from CØ to ground are shorted out. CØ is now grounded. If you measure the phase to ground voltage from AØ (or BØ) to ground, it would be the same as measuring from AØ (or BØ) to CØ. Thus, it can be seen that the phase to ground voltage on the two unfaulted phases instantly rises from 277 volts to 480 volts and maintains that voltage throughout the duration of the fault.

Because of this voltage rise, the stray capacitances on AØ and BØ to ground are now charged at 480 volts instead of 277. This causes the normal charging current of the system to increase by a factor of the square root of 3 or 1.732. The current that flows in the two unfaulted phases during the fault would be the normal load current plus the higher magnitude of charging current.

Note also, that any phase to ground insulation on the unfaulted A and B phases, in any device (transformer, motors, etc.) throughout the entire delta system, would be stressed at 480 volts instead of the normal 277, while the fault exists.

Phase to ground insulations on any ungrounded system must be specified for full line to line voltage. Lightning arresters on ungrounded systems must also be sized for full voltage.

Ground Fault Monitors make use of these stray capacitance phase to ground voltages to detect ground faults. One LED is hooked up on each phase to ground (total of three LED's are installed). The three LED's normally burn with equal brightness. When a fault occurs, the faulted phase light goes out, and the unfaulted phases' lights burn brighter.

Back To "Overcurrent Protection"

Select New Quiz Challenge