Ground Fault Protection Methods

Types of Ground Fault Protection Methods

1. Earth leakage breaker method
2. Protective grounding method
3. Overcurrent breaker method
4. Insulated transformer method (non-grounded)
5. Earth leakage warning method

1.Earth leakage breaker method

Merits

• The high-speed high-sensitivity type is extremely effective for electric shock protection.
• The optimum sensitivity can be selected according to the working conditions and environment, scale of electrical circuit and importance, etc
• The current-operated type can protect all electrical circuits past the installation point.
• Labor-saving as the electrical circuit insulation resistance measurement can be omitted, and the inspection cycle can be extended.

Demerits

• If there is a ground fault accident, that circuit will be opened and the power will fail.
• Can be expensive if installed on each branch circuit to maintain the continuous power feed.

2.Protective grounding method

Merits

• Economical if soil, where protective grounding is to be provided, has a low resistance value.
• The contact voltage will not exceed the tolerable value.
• Relatively high reliability in terms of chronological degradation.

Demerits

• To limit the equipment’s contact voltage to less than the tolerable value with the TT method, the protective grounding resistance value must be much less than the power side’s grounding resistance value. However, since it is difficult to confirm the power side’s grounding resistance value, the low resistance grounding work is difficult. Thus, this method is not practical for low voltages.
• The ground fault accident itself cannot be detected or removed, and thus, fires cannot be prevented.

3.Overcurrent breaker method

Merits

• Highly reliable MCCB can swiftly and accurately remove ground fault accident circuits.
• Cost feasible as the electrical circuit’s metal pipes or building’s steel frame can be used for the grounding dedicated wire.

Demerits

• The tolerable contact voltage could be exceeded in the duration between generation of the ground fault accident to when􀀁MCCB functions and opens the circuit.
• Caution must be paid to the relation of the metal pipe and steel frame impedance and MCCB rated current.

4.Insulated transformer method (non-grounded)

Merits

• With the secondary side non-grounded method, there is no contact voltage to the human body during a ground fault or when the live section is contacted.
• There is no risk of power failure or fires caused by the ground fault accident.

Demerits

• Can be hazardous if the ground fault accident is not detected and a double insulation breakdown occurs for a long time.
• If a high voltage is reached due to the effect of the induction, or if there is a ground fault in one wire, the voltage to ground for the other wires may become higher than the grounded type electrical circuit. This is not suitable for large capacity applications.

5.Earth leakage warning method

Merits:

• Economical if soil, where protective grounding is to be provided, has a low resistance value.
• The contact voltage will not exceed the tolerable value.
• Relatively high reliability in terms of chronological degradation.

Demerits

• There is no self-protection function in respect to electric shocks.
•  Not effective if the load electric device is not grounded.
• Not effective if there is no communicant or administrator when alarm is issued.

  

Types of damage caused by ground fault accidents

1. Electric shock
2. Fire
3. Device Burning

1.Electric Shock

When a human body directly touches an electrical circuit or device’s conductive section, or touches a frame onto which voltage is inducted due to degradation of motor insulation, etc., the current passes through the human body and a ground fault current flows.

Size of ground fault current: Causes death is several 10mA or more.

2.Fire

When the insulation of the building where the wire passes through degrades and the ground faultcurrent passes through the thin easily heated conductor, such as the metal truss, the conductor heats and can cause the building material to ignite.

Size of ground fault current: Causes fire is several 1A or more.

3.Device burning

When the insulation of the electrical circuit or device is partially damaged and a large ground fault current flows, in most cases an arc occurs and burns the devices.

Size of ground fault current: Causes arcs is several 10A or more.