Earth leakage Vs Residual and Zero-sequence Current – What’s the Difference

Although earth leakage, residual current, and zero-sequence current are all related to unbalanced current, they are distinct from one another. This article will provide a detailed introduction to the differences between earth leakage, residual current, and zero-sequence current.

What are earth leakage, residual current, and zero-sequence current?

Earth leakage current typically describes current that unintentionally flows from an energized component (or a part that should be insulated) to ground. This can happen accidentally through a person, an equipment enclosure, or even just the insulating material itself. It also includes situations where current flows directly from a live wire to the earth. We generally use “leakage current” to refer to this unintended current escaping within an electrical system or piece of equipment. You’ll find and detect this leakage current when there’s an insulation breakdown, a ground fault, or simply an ongoing leakage issue within the power system.

Residual current mainly refers to the vector sum of the current flowing from the live wire into the neutral wire and from the neutral wire into the live wire. In an ideal power system, when all currents are in a balanced state in the circuit, the current flowing in and out will completely cancel each other out, so the residual current is zero. However, once the system is in an unbalanced state, such as an insulation fault to ground (where current flows to the ground through the insulating medium), this balance is broken. At this time, the residual current is no longer zero. This non-zero residual current is the basis for the judgment and action of the residual current protection. When earth leakage current occurs, its most direct manifestation is the increase of residual current. Therefore, the residual current itself includes leakage current. This is why in daily communication, people sometimes directly refer to “residual current” as “leakage current”.

Zero-sequence current is a current component related to the symmetrical components of the three-phase current of the power system. The three-phase current can be decomposed into positive sequence, negative sequence, and zero sequence components, among which the zero-sequence current is the average component of the three-phase current, specifically defined as: I0=(IA+IB+IC)/3, where: IA, IB, IC are the instantaneous values of the three-phase current respectively. The zero-sequence current usually occurs when the three-phase system is unbalanced or a single-phase grounding fault occurs.

Earth leakage, Residual and Zero-sequence Current – What’s the Difference?

Item	zero-sequence Current	Earth Leakage	Residual Current
cause	Single-phase grounding fault, load imbalance, transformer or line asymmetry, etc.	Insulation aging, equipment leakage, grounding faults, etc. cause current to flow into the ground.
Current flow	Flows through the neutral line or grounding line and finally flows into the neutral point or the earth.	Flow to the ground or grounding point through the leakage current path.
Measurement method	Usually measured by zero-sequence CT	Measure the ground current of a specific device or path	Zero-sequence current transformer (through all phase lines and neutral line) measures the current vector sum.
Protection application	Grounding fault protection is applied to three-phase three-wire systems in the medium and high voltage part, that is, the section between the power station and the user transformer	Applied to inverters, solar energy, energy storage systems, DC systems with multiple signal interference, generators, transformers, switchboards, cables, and other power systems	Used on the 380V low-voltage side, suitable for three-phase four-wire, N-wire occasions.
Function	When a fault occurs, quickly cut off the fault circuit or take other protective measures	Assess equipment safety, guide, and protect insulation maintenance	Prevent personal safety hazards and electrical fires caused by low-voltage side leakage to the ground.
Protection relay	Zero sequence current protection relay	Earth leakage relay	Residual current monitor