Introduction

What are Stage 1, Stage 2, and Stage 3 overcurrent protections? Many electrical professionals are familiar with these concepts, but some may still find them confusing—especially beginners in the field.

We know that overcurrent protection is a critical measure to ensure the safe operation of equipment in power systems. The commonly used overcurrent protection methods are Stage 1 Overcurrent, Stage 2 Overcurrent, and Stage 3 Overcurrent protection. So, what distinguishes these stages? How should we understand them? This article explains the three-stage overcurrent protection mechanism, aiming to help electrical practitioners gain a clearer understanding.

01 What Is the Basic Principle of Overcurrent Protection?

Overcurrent protection refers to protecting against excessive current. The three-stage overcurrent protection mechanism consists of the following:

1.Instantaneous Overcurrent Protection (Stage 1): No intentional time delay.

2.Time-Delayed Overcurrent Protection (Stage 2): Includes a short intentional delay (0.3–0.5 seconds).

3.Inverse-Time Overcurrent Protection (Stage 3): Longer time delay (a few seconds) based on the load characteristics.

The three stages differ primarily in their protection range:

Stage 1: Provides immediate tripping (within 0 seconds) for near-zone short circuits, usually covering up to 85% of the protected line's total length.

Stage 2: Has a short delay (0.3–0.5 seconds) and covers the full length of the protected line plus 15% of the next section.

Stage 3: Offers long-term backup protection with a delay configured to avoid interference with the maximum load current, covering the entire protected line and often a significant portion of downstream circuits.

In case of a fault, short-circuit current is generated, significantly higher than normal working current. By measuring the current and its duration through current transformers and comparing it to set thresholds, the system sends a trip signal once thresholds are exceeded. The basic principle of overcurrent protection is to disconnect circuits when fault currents are detected.

02 What Is Stage 1 Overcurrent Protection?

Stage 1 Overcurrent Protection is commonly referred to as instantaneous overcurrent protection. The current setting for this stage is very high, with no intentional delay. In other words, the protection system operates immediately (in milliseconds) once the current exceeds the set value.

However, to ensure selectivity (prevent upstream protection from tripping for faults in downstream circuits), Stage 1 does not cover the entire line. While it is called "Stage 1," it does not act as the primary protection for the line.

03 What Is Stage 2 Overcurrent Protection?

Stage 2 Overcurrent Protection has a lower current setting than Stage 1 and includes a short intentional delay. The protection operates once the current exceeds the set value and persists for the defined time delay.

The current setting must ensure coverage of the entire protected line and extend into the front half of the next section. Stage 2 serves as the primary protection for the line and provides remote backup protection for downstream circuits.

It enhances fault protection, particularly for larger faults, though it is less effective for small faults.

04 What Is Stage 3 Overcurrent Protection?

Stage 3 Overcurrent Protection has the lowest current setting and the longest delay among the three stages. It ensures full coverage of the protected line and extends beyond the range of Stage

Stage 3 acts as backup protection for the line and provides remote backup protection for downstream circuits (and possibly circuits beyond that).

05 Configuration and Adjustment Principles of Three-Stage Overcurrent Protection

(1) Configuration Principles

According to national standards:

1.Lines with single-sided power supply or dual-sided power controlled by directional elements should use three-stage overcurrent protection.

2.For 35kV and above networks with non-directly grounded neutral points, three-stage protection is mandatory, but two-stage protection may suffice for terminal lines.

3.For 10kV non-directly grounded overhead or cable lines in radial networks, two-stage protection is allowed, with:

Stage 1: Instantaneous overcurrent protection.

Stage 2: Time-delayed overcurrent protection.

Specific scenarios allow for simplified configurations, such as omitting certain stages based on practical requirements.

(2) Setting and Range Principles

Instantaneous Overcurrent Protection (Stage 1):

The setting should be based on avoiding operation for maximum three-phase short-circuit currents at the line's end.

Reliability coefficient Kk>1.3K_k > 1.3Kk>1.3.

Time-Delayed Overcurrent Protection (Stage 2):

The setting ensures sensitivity to faults at the line's end, with a delay to coordinate with adjacent protection systems.

Delay time: 0.3–0.5 seconds.

Inverse-Time Overcurrent Protection (Stage 3):

Set to avoid tripping under maximum load conditions.

Must coordinate with downstream Stage 3 protection, adding 0.3–0.5 seconds delay for selectivity.

06 Summary

Overcurrent protection is a common protective measure in power systems, designed to safeguard equipment and personnel. The three stages—Stage 1, Stage 2, and Stage 3—differ in fault detection and response time. In practice, a combination of these stages enhances reliability and safety. Selecting the appropriate overcurrent protection scheme and fine-tuning the settings are crucial to ensuring the power system's secure and efficient operation.