LVQ Panel Questions - Ground Fault Detection

Explain what a ground fault is

An unintentional electric path between source of current and grounded surface. They Occur when current is leaking somewhere, essentially electricity is escaping to ground

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

Voltage, conductor size, integrity of connection, grounding scheme of the separately derived grounding system are the factors. Amount of resistance between the fault and the transformer. Size of transformer. Size of the conductors going to fault.

Explain the difference between an arc fault and a bolted fault

An arc fault is an intermittent air gap fault. Jumps through the air.

A bolted fault is a continuous connection across phases-(For example, a hand tool forgotten in MCC bucket that is bridged across conductor phases). Fault that goes through conductive material.

Draw Schematic for the three types of system ground configs

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 in this facility

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

Solidly Grounded

Advantage – Finding faults is easy, stable voltages.

Disadvantage – High ground fault currents can potentially damage equipment.

(Our lighting transformers are grounded this way).

Lighting panel, power panel, phase to neutral loads.

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.

High-Resistance

Advantage – Can continue to run equipment with existing ground fault.

Disadvantage – Everyone that works on it must be trained and qualified.

(2nd fault on that substation transformer will trip substation breaker, and will stay off until ground fault(s) are repaired. This will affect all MCC racks on that sub.

(Our 3 phase motors are grounded this way).

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

Our resistors are 5 amps.

R = V/I so, (live phase conductor to neutral wire = 277v) 277v/5a = 55.4 ohms

W = V times I so, W = 277v times 5 = 1,385 watts

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 conductors will read 480v. (That is how motors continue to operate when there is a ground fault). The phase with the fault will read near zero when a phase to ground reading is taken.

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

Need to have it done.

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

An intermittent ground fault that may come and go- phases may have various voltages (ok, one minute, then bad the next).

The phase that has the “low-grade” ground connection will have lower than 277 V (measured Phase to ground) and the other two

Explain how ground fault annunciation works in your department.

There may be lights in substation or meters to indicate ground fault.

PC&IS C-3 will inform the proper people.

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

The fault doesn’t affect another substation because they are 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.

Motor termination box of the motor (peckerhead)

Any immersed heaters (heat liquid)

Portable welders

Electric Brake

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 480 volts until another ground fault occurs

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-MCC fuses blow, etc.

Find a Single Line Diagram for an MCC in your area.

Use Epic Resources!