Understanding Discrimination with LV Power Circuit-Breakers

Introduction

In any electrical installation, loads are connected to power sources through a succession of protection, isolation, and control devices. When a fault occurs, the primary goal is to disconnect only the faulty part of the network while keeping the rest of the installation energised. This principle is known as discrimination (or selectivity), and it is essential for ensuring both safety and continuity of service.

Schneider Electric's Cahier Technique No. 201 provides a comprehensive examination of discrimination techniques specifically for low-voltage (LV) power circuit-breakers. These devices, characterised by their high ratings (800 A to 6300 A), are typically located at the head of an LV installation, directly downstream of an MV/LV transformer – a position that demands strict discrimination requirements.

What is Discrimination?

Discrimination is the ability of a protection system to isolate a fault by opening only the circuit-breaker directly upstream of the fault, while all other circuit-breakers remain closed. In a radial distribution layout, the objective is to disconnect only the faulty load or feeder, maintaining power to the rest of the installation.

Discrimination can be:

• Total – guaranteed for all fault current values up to the maximum available in the installation
• Partial – ensured only up to a current value lower than the prospective short-circuit current

Types of Faults and Protection

Different fault types require different protection strategies:

Overloads – Currents between 1 and 10 times the normal service current. Their elimination must occur within a time compatible with the thermal withstand of the conductors. Protection uses inverse-time delay (long-time delay) trip units. Discrimination in the overload zone is achieved if the ratio Ir1/Ir2 is greater than 1.6.

Short-circuits – High-magnitude fault currents that require instantaneous or short-time delayed tripping. Because of their magnitude, and especially the presence of electrical arcs which generally accompany them, these circuits should be interrupted almost instantly, in less than a few hundred milliseconds. Circuit-breakers must be capable of safely interrupting these currents up to their breaking capacity (Icu).

Earth leakage currents – Discrimination must also be considered for insulation faults to prevent main devices from tripping unnecessarily. For low or very low current values (typically between 30 mA and 30 A), residual protection (vigi) is used; for higher leakage currents, ground fault protection is employed.

Voltage dips or loss of supply – These can be generated by short-circuits and lead to tripping of main devices equipped with undervoltage trip units. The solution is to use time-delayed undervoltage trip units with a reaction time longer than the short-circuit trip time of downstream equipment.

Discrimination Techniques for Short-Circuits

Improving discrimination generally comes down to "restraining" tripping by the circuit-breaker concerned as opposed to the circuit-breakers located downstream. The Cahier Technique describes several methods:

Current Discrimination (Amperometric)

Creates a difference between trip thresholds. For upstream devices, the short-time threshold (ICR1) must be set above the instantaneous threshold of downstream devices (Iins2).

Time Discrimination (Chronometric)

Uses intentional time delays – downstream devices trip instantaneously while upstream devices have short-time delays (ST) of several hundred milliseconds. This solution can only be used if the device can withstand the short-circuit current during this time delay.

Pseudo-Time Discrimination

When a limiting circuit-breaker is used downstream, the magnitude and duration of the actual short-circuit current is significantly reduced. The upstream trip unit therefore detects a much weaker current, assisting discrimination.

SELLIM (Energy-Based Discrimination)

A more sophisticated technique that allows both discrimination and current limitation. It is based on detection of the form of current waves (∫i²dt) and is implemented on ranges like Compact NS.

Zone Selective Interlocking (ZSI)

Uses communication between circuit-breakers – when a fault is detected, the downstream device sends a signal to the upstream device to inhibit its tripping, allowing total discrimination without time delays.

Circuit-Breaker Characteristics

For power circuit-breakers, key characteristics affecting discrimination include:

• Breaking Capacity (Icu) – the highest short-circuit current intensity the circuit-breaker can interrupt at a given rated operating voltage
• Short-time withstand current (Icw) – the maximum short-circuit current (RMS value) the circuit-breaker can withstand for a defined period (0.5, 1, or 3 seconds)
• Current limiting capability – when interrupting a short-circuit, the circuit-breaker limits the current to a value considerably less than the prospective current
• Cascading – using the limiting capacity of the upstream circuit-breaker to increase the actual breaking capacity of downstream units

Practical Application

The document provides worked examples for selecting circuit-breakers to achieve discrimination in typical LV installations, including:

• Dimensioning protective equipment
• Selecting breakers to ensure discrimination
• Variants incorporating zone selective interlocking
• Scenarios with multiple incoming lines

Conclusion

Proper discrimination in LV power distribution is essential for maximising service continuity and protecting equipment. Cahier Technique No. 201 provides engineers with the practical knowledge needed to select and coordinate circuit-breakers effectively. By understanding the various discrimination techniques and their applications, professionals can design installations that are both safe and reliable.

This article summarises key concepts from 'Discrimination with LV power circuit-breakers' (Cahier Technique No. 201) by Jean-Pierre Nereau, Schneider Electric.

Download the full free document here: Discrimination with LV power circuit-breakers – Cahier Technique No. 201 (PDF)

Disclaimer: This article provides a summary of the 'Discrimination with LV power circuit-breakers' technical publication by Schneider Electric. While efforts have been made to accurately represent the content, please refer to the official document for complete technical details. Final design, specification, and implementation decisions should be made by qualified electrical engineers and competent professionals in accordance with all applicable regulations and standards.