Low voltage grounding systems are essential components in power systems, ensuring electrical safety and stable operation. In power transmission and distribution, these systems effectively connect the metal enclosures or other conductive parts of electrical equipment to the ground, thereby ensuring electrical safety. Beyond short-circuit protection and overload protection, low voltage distribution line protection must include ground fault protection to prevent indirect electric shocks, electrical fires, line damage, and other accidents. This document details the requirements for ground fault protection in low voltage grounding systems, providing a reference for electrical design personnel.

Ground Fault Protection Methods and Requirements for TN Systems:

Using Overcurrent Protection for Ground Fault Protection: 

When overcurrent protection meets sensitivity requirements, using a circuit breaker’s overcurrent trip mechanism for short-circuit instant protection is the most economical and convenient method. However, for longer distribution lines, increased line impedance can result in a smaller ground fault current (Id), making it difficult to meet sensitivity requirements with instant short-circuit protection.

In such cases, consider using a circuit breaker with a short-time delay overcurrent trip for ground fault protection. The short-time delay protection setting value (Isd) of the circuit breaker is often only 1/5 to 1/3 of the instant protection setting value (Ii), making protection easier to achieve. However, this method requires an electronic trip unit, which is more expensive than conventional thermal-magnetic trip units and is rarely used, typically reserved for primary distribution feeder breakers.

Both methods must meet the following requirement:

Id ≥ 1.3 x Ii (or Isd)

The following two methods utilize circuit breakers with ground fault protection function attachments, providing alternative protection options:

Using Zero-Sequence Current (Three-Phase Unbalanced Current) Protection for Ground Fault Protection: 

When the sensitivity requirements are not met by instant and short-time delay overcurrent protection, zero-sequence current protection can be used. This method is suitable for TN-C, TN-C-S, and TN-S systems but not for distribution circuits with high harmonic currents. This protection is realized through the ground fault protection function (Ig) of the electronic trip unit in the circuit breaker.

In design, this method is employed at the main incoming circuit breaker on the low voltage side, where the breaker has four protection functions: long-time delay (Ir), short-time delay (Isd), instant short-circuit (Ii), and ground fault protection (Ig).

Zero-sequence current In is defined as:

In = Iu + Iv + Iw

Under normal conditions, the three-phase loads in distribution circuits are nearly balanced, and with minimal harmonic currents, the current flowing through the neutral line (N) is small. However, during a single-phase ground fault, zero-sequence current In significantly increases.

The zero-sequence current protection setting value (Ig) must be greater than the sum of the maximum three-phase unbalanced current, harmonic current, and normal leakage current flowing through the PEN or N line during normal operation. This method must meet the following requirements:

Ig ≥ 2.0 x In (Equation 1)

Id ≥ 1.3 x Ig (Equation 2)

Practical application: In normal operation, the zero-sequence current value In generally does not exceed 20% to 25% of the calculated line current. The zero-sequence current protection setting value Ig can be set between 50% to 60% of the circuit breaker’s long-time delay (Ir), ensuring compliance with Equation 2.

Using Residual Current Devices (RCDs) for Ground Fault Protection:

 When overcurrent protection does not meet sensitivity requirements, RCDs can be used. This method is commonly applied in secondary and tertiary distribution levels.

Residual current Ie is defined as:

Ie = Iu + Iv + Iw + In

This formula includes the vector sum of the neutral line current, providing a smaller and more accurate current measurement, essentially the ground fault current through the PE line. Hence, residual current protection is more sensitive than zero-sequence current protection. It is only suitable for TN-S systems and not for TN-C systems. When using RCDs in TN-C-S systems, the RCD should not cover the PEN line, and the convergence point of PE and PEN lines should be on the supply side of the RCD to prevent malfunction.

This method must meet the following requirement:

Ie ≥ 1.3 x Ia (where Ia is the residual current protection trip value)

Additional considerations:

To avoid false trips, the RCD setting value should be 2.5 to 4 times greater than the sum of the leakage currents in the protected circuit during normal operation.

For handheld or mobile equipment and socket circuits, RCDs should be installed with a rated trip current of ≤30mA; for medical equipment, swimming pools, and bathroom lighting, the trip setting should be 10mA; and for preventing electrical fires, the trip current should not exceed 300mA.

When multiple RCDs are installed, ensure selective coordination of their rated residual trip current and trip times.

Do not route PE lines through RCDs.

During residual current protection trips, disconnect both the phase line and neutral line of the protected circuit.

2. Ground Fault Protection Requirements for TT and IT Systems:

TT System Ground Fault Protection Requirements: 

In TT systems, the system ground on the power supply side (ground resistance Rb not exceeding 4Ω) and the protective ground on the load side (ground resistance Ra generally between 10-30Ω) are not electrically connected. In the event of a single-phase ground fault, the fault loop impedance is high and fault current is small, making it difficult for overcurrent protection to meet sensitivity requirements. Therefore, RCDs should be used for ground fault protection, with considerations similar to those in TN systems.

IT System Ground Fault Protection Requirements: 

In IT systems, the first single-phase ground fault generates a very small fault current (in the milliamp range), equal to the vector sum of the capacitive currents to ground of the other two phases. The fault voltage on exposed conductive parts remains below the touch voltage limit, posing no harm to humans, and power does not need to be cut off. Ground fault protection usually involves monitoring and alarming the insulation of the distribution system. Faults should be cleared by on-duty personnel within 2 hours to prevent secondary phase-to-phase short circuits and avoid power supply interruptions.