Earth Fault Protection

What is Earth Fault?

An Earth Fault, also known as a Ground Fault, arises when unexpected fault current runs from an energized conductor to the ground, potentially causing hazardous conditions. 

Earth Faults are more likely to occur in electrical installations with the high earth impedance. These faults can occur under a wide range of conditions.

Earth Fault Protection

Earth fault protection is necessary for identifying insulation failures or accidental contacts between live conductors & the ground. 

Such faults might result in 

• Equipment damage, 
• Fire dangers, and 
• Personnel safety concerns. 

To solve these issues, various types of earth fault prevention techniques are used in electrical systems. 

What is stand by Earth Fault Protection?

Standby Earth Fault Protection is a type of protection that is mostly used as a backup to other protection. 

The protection comprises of a CT and a simple relay, with the CT located as close as possible to the earth path return to a transformer’s star point.

Types of Earth Fault Protection

Earth fault protection can be classified into different types:

1). Derived Earth Fault (50N/51N)

2). Measured Earth Fault 

• Residual Ground Current Measuring (50N/51N)
• Sensitive Earth Fault (50G/51G)
• Standby Earth Fault (50SBF)

3). Restricted Earth Fault (64REF)

1). Derived Earth Fault Protection (50N/51N) 

The vector sum of 3-phase currents (R, Y, & B) that are recorded using phase CTs is that which is used to determine the earth fault current through the use of numerical methods.

Working Principle:

In a balanced state, 

I_R + I_Y+ I_B = 0

During an earth fault, the vector sum deviates from zero, suggesting an imbalance caused by current flowing to the ground.

CT Connection:

CTs are provided in three lines.

The calculation for the residual current, often known as the zero-sequence current, is as follows: 

I_Residual = I_R + I_Y + I_B

Protection Code:

• 50N: Instantaneous earth fault (no delay).
• 51N: Inverse-time earth fault (with time delay).

Applications:

1. Common in systems in which Neutral is not readily quantifiable.
2. Ideal for feeders, incomers, & bus couplers.

2). Measured Earth Fault Protection

The earth fault current is calculated directly from a Neutral CT (or) Residual CT rather than being derived.

2.a). Residual Ground Current Measurement

A separate CT is put around each three-phase conductor (core-balance CT or ring CT).

Residual Ground Current Measurement is used to measures the genuine residual (zero-sequence) current flowing to the earth.

Working:

If the currents in each phase are balanced, then the vector sum will be equal to zero.

If a fault occurs, an imbalanced current runs through the earth and is recognized by the CT.

Advantage:

• More reliable detection compared to the derived technique, particularly in noisy systems.

2.b). Sensitive Earth Fault Protection (50G/51G)

Sensitive Earth Fault Protection (50G/51G) is a type of measured earth fault protection that is more sensitive than conventional earth fault protection.

It is used to detects extremely low-level earth leakage currents, usually in the milliamp to low ampere range.

Protection Code:

• 50G: Instant ground fault detection.
• 51G: Detecting earth faults with a time delay.

CT Ratio:

Frequently use low-ratio CTs (Ex: 100/1, 50/1) to improve sensitivity.

Applications:

1. Used in grounded systems, particularly motor feeders, transformer protection, and control circuits, where insulation defects must be detected early.

2.c). Standby Earth Fault (50SBF)

Standby Earth Fault, often referred to as Backup Earth Fault Protection, is also abbreviated as 50SBF.

If the first protection fails to isolate the issue, this adds a second layer of defense.

Working:

Monitors residual current (or) earth current downstream from the primary protection.

Operate with a time delay to prioritize main protection.

Application:

1. Most commonly utilized in breaker failure cases.
2. If the local breaker fails to operate, it can be wired to trip an upstream breaker and isolate the fault.

3). Restricted Earth Fault Protection (64REF)

Restricted earth fault protection are the high-speed zone-based earth fault protection.

It operates on internal earth faults within a protected zone, such as transformer windings or generator stators.

Working Principle:

When it comes to fault protection, Restricted Earth Fault Protection utilizes the differential principle.

CTs are located at either end of the protected zone.

It measures the distinction between incoming & outgoing currents, as well as neutral current.

Operating Condition:

An internal earth fault is diagnosed when the total of line currents exceeds the neutral current.

External faults do not trigger this protection.

Advantages:

• Extremely rapid, selec tive & sensitive to interior earth faults.
• Does not detect external defects or load imbalance – extremely discriminatory.

CT Connection:

Matching CTs are required on both sides of the protected equipment (for example, HV and LV transformers).

Applications:

1. Power transformers (particularly star-wound).

• Generators
• Motors
• Busbars

Typical Settings:

• Pickup is extremely low (Ex: 10% of rated current).
• Operating time: around 30 ms (rapid).

Earth Fault Protection Comparison Table

Type	Relay Code	Detection Method	Sensitivity	Speed	Zone	Typical Application
Derived Earth Fault	50N/51N	From 3-phase CT sum	Moderate	Medium	Feeder/Incomer	General feeder protection
Measured Earth Fault	50N/51N	Via core-balance CT	High	Fast	Feeder/Incomer	Better sensitivity than derived type
Sensitive Earth Fault	50G/51G	Core-balance CT (Low CT)	Very High	Fast	Feeder/Transformer	Leakage detection, early fault detect
Standby Earth Fault (Backup)	50SBF	Via residual CT	Moderate	Delayed	System-wide	Breaker failure backup protection
Restricted Earth Fault	64REF	Differential + Neutral CT	Very High	Very Fast	Transformer/Gen.	Internal zone fault only

Summary

For the purpose of safety, reliability, and durability, earth fault protection is an absolutely necessary component of power networks.

Choosing the proper scheme depends on:

• Equipment importance
• Protection zone
• Required sensitivity
• Coordination with upstream and downstream relays

REF (64REF) provides fast, selective internal protection, whereas Sensitive Earth Fault (50G/51G) is efficient at detecting small leakage currents. Derived and Measured Types are for general-purpose, whereas SBF (50SBF) provides backup isolation.