021 - Ground Fault Detection

Determine the theoretical system voltages you should see in a three phase HRG system when one phase of this system experiences a solid ground fault.

- All phase to phase = 480V. (That is how motors continue to operate when there is a ground fault). - Faulty phase to ground = near zero

Explain the impact of a "low grade" ground fault (i.e. - resistance >0 Ohms) on system voltages.

- Intermittent ground fault that may come and go. Phases may have various voltages (ok, one minute, then bad the next). - Phase with "low-grade" ground connection will have lower than 277 V (measured Phase to ground)

Explain how ground fault annunciation works in your department.

- Lights or meters in substation - C3 will inform right people

Explain why a ground fault on one substation does not affect the other substations.

- Substations not physically connected due to physical break between the primary and secondary transformer winding. - Separately derived system.

Identify some common locations for ground faults in your department.

- Termination box of the motor - Portable welders - Electric Brake

Draw a schematic for the three types of system ground configurations.

- Ungrounded - Solidly grounded - Resistance grounded

Explain what an electrical ground fault is.

- Unintentional electric path between a current source and a grounded surface

Explain all of the factors that determine the amount of ground fault current capable of being generated in the event of a fault.

?????? - Voltage (higher voltage, higher fault current) - Conductor size (larger conductor, higher fault current) - Integrity of connection (better connection, higher fault current) - Grounding scheme (ungrounded = 0 ground fault current???)

Explain why a ground fault must be traced out and repaired even though the plant can continue to run.

A 2nd fault on another phase of the same transformer is a phase to phase short and will cause the substation breaker to trip or MCC fuses to blow

Explain the impact of a ground fault (if any) on a three phase motor connected to an HRG system.

No impact - All 3 phases continue to read 480V until another ground fault occurs

Deliberately induce a ground fault (under appropriate supervision) on an HRG system to verify the values determined in the previous skill demonstration.

???

Explain the benefits and disadvantages of a high resistance grounded system. Indicate the locations in your department where this type of system grounding is used.

Advantage - Can continue to run equipment with existing ground fault. Disadvantage - Everyone that works on it must be trained and qualified. 3 phase motors. Substations.

Explain the benefits and disadvantages of a solidy grounded system. Indicate the locations in your department where this type of system grounding is used.

Advantage - Finding faults is easy. Stable voltages. Disadvantage - High ground fault currents can damage equipment Lighting panel, power panel, phase to neutral loads.

Explain the benefits and disadvantages of an ungrounded system. Indicate the locations in your department where this type of system grounding is used.

Advantage - The first ground fault turns the system into a solidly grounded system. Disadvantage - Hazardous, unstable voltages, dangerous No ungrounded at Lima

Explain the difference between an arc fault and a bolted fault.

Arcing fault - jumps through the air Bolted fault - fault through conductive material

Determine the amount of fault current the HRG systems in your plant are designed to sustain. Determine the size of the resistor (in Ohm and Watts) required to deliver this fault current.

HRG = High Resistance Grounding Designed to sustain 5A R = V/I Phase to Neutral = 277V 277V/5A = 55.4 ohms W = IV = 277V*5A = 1,385 watts

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