Selection and Adjustment of Circuit Breaker Trip Units

When selecting a circuit breaker, it is crucial to match its specifications to the operational context. This includes selecting the proper utilization category, rated operating voltage, rated current, and the trip unit's adjustable current values. Protection characteristics should be referenced from the manufacturer's technical curve charts, with further validation of short-circuit properties and sensitivity coefficients to ensure full coordination with the system.

(1) Air Circuit Breaker (ACB)

Air Circuit Breakers, also known as universal circuit breakers, are constructed within a metal frame that insulates and houses all components. They are typically open-type and allow for various accessory modules. Their design facilitates straightforward maintenance, particularly at the main power supply side.

Overcurrent trip units come in magnetic, electronic, or intelligent formats, and offer four segments of protection: long-delay, short-delay, instantaneous, and ground fault. Each can be adjusted within a defined range relative to the breaker's frame size.

Commonly operating in AC systems with rated voltages of 380V or 660V and rated currents from 200A to 6300A, ACBs are ideal for energy distribution and for safeguarding equipment against overloads, undervoltage, short circuits, and ground faults. These intelligent breakers support selective protection and can manage infrequent switching operations under normal conditions.

In practical installations, ACBs are often used as:

• Main switches on the 400V secondary side of transformers,

• Bus coupler switches,

• High-capacity feeder protection, and

• Master control switches for large motors.

(2) Molded Case Circuit Breaker (MCCB)

Molded Case Circuit Breakers, or MCCBs, are encapsulated in thermoset plastic housing, incorporating arc extinguishing chambers, terminals, trip units, and operating mechanisms. With a modular structure and compact design, MCCBs are rarely intended for field repair and are often used as branch protection devices.

Their overcurrent trip units come in electromagnetic and electronic versions:

• Electromagnetic MCCBs offer non-selective protection with only long-delay and instantaneous trip functions.

• Electronic MCCBs provide a full protection suite: long-delay, short-delay, instantaneous, and ground fault. Advanced models even feature zone-selective interlocking (ZSI).

Applications include:

• Feeder circuit protection in distribution systems,

• Transformer low-voltage output control,

• Power distribution at load terminals,

• Main switches for industrial equipment and machinery.

(3) Miniature Circuit Breaker (MCB)

Miniature Circuit Breakers are ubiquitous in building electrical distribution systems. Designed for systems up to 125A, they protect against short circuits, overloads, and overvoltages across single-phase and three-phase configurations (1P, 2P, 3P, 4P).

An MCB integrates manual or motorized actuation mechanisms, contacts, various trip elements, and arc extinguishing systems. Its trip units—thermal and magnetic—are connected in series with the main circuit. Undervoltage coils are parallel to the power source.

In civil engineering electrical applications, MCBs serve as:

• Protection and switching devices for short circuits, overloads, and undervoltages,

• Ground fault and leakage protection,

• Automatic transfer switches, and

• Occasional motor start protection.

Key Circuit Breaker Performance Parameters

(1) Rated Operating Voltage (Ue)

The nominal voltage under standard operating conditions. For systems ≤220kV, the highest operating voltage is 1.15×Ue; for ≥330kV, it's 1.1×Ue.

(2) Rated Current (In)

The maximum continuous current the breaker can carry at 40°C ambient temperature. For adjustable-trip models, this is the highest adjustable value.

(3) Overload Trip Current Setting (Ir)

Triggers delayed tripping when exceeded. It must exceed the load current (Ib) but remain below the permissible cable current (Iz). Ir can range from 0.7–1.0 In for thermal trip units and 0.4–1.0 In for electronic ones.

(4) Short-Circuit Trip Current Setting (Im)

Defines the threshold for instantaneous or short-delay tripping in the event of high fault currents.

(5) Rated Short-Time Withstand Current (Icw)

The maximum current the breaker can safely carry for a short, defined period without damage.

(6) Breaking Capacity

Denotes the maximum fault current the breaker can interrupt. Expressed as Icu (ultimate breaking capacity) and Ics (service breaking capacity), common values include 36kA and 50kA.

General Principles for Breaker Selection

1. Match application type and pole count to the system design.

2. Choose a rated current ≥ maximum load current.

3. Trip unit type and accessories must meet operational needs.

4. Voltage and fault current requirements:

• Ue ≥ line voltage

• Icu ≥ calculated load current

• Icu ≥ highest expected fault current

5. For motor protection, ensure trip settings align with starting currents:

• Single motor: Instantaneous trip ≥ 1.35×start current (DW series) or 1.7× (DZ series)

• Multiple motors: Trip current ≥ 1.3×largest motor’s start current + others' full load

6. For transformers, select:

• Breakers with Icu > low-side short-circuit current

• Trip unit current ≥ transformer rated current

• Short-circuit trip current = 6–10×transformer current

7. Coordination with upstream and downstream devices is essential to avoid misoperation and cascading failures.

Circuit Breaker Selectivity

Breakers can be:

• Selective, with tiered trip zones (long-delay, short-delay, instant), or

• Non-selective, offering singular protection response.

Selective coordination ensures only the faulted branch is isolated. Guidelines:

• Upstream trip current ≥ 1.1× downstream max fault current

• Short-delay settings ≥ 1.2× downstream instantaneous setting

• Time delay differential ≥ 0.1s between adjacent levels

• Use of upstream delay ensures orderly, tiered fault isolation

Cascading Protection (Backup Protection)

Cascading protection leverages the current-limiting characteristics of upstream breakers to shield downstream devices with lower rated breaking capacities.

When a downstream short circuit occurs:

• The upstream breaker's limitation reduces the peak fault current,

• Allowing the downstream device to survive and operate within its rating.

Requirements for successful cascading:

• No critical loads on adjacent branches,

• Compatible trip settings across devices,

• Manufacturer-verified coordination data based on testing.

Sensitivity of Circuit Breaker Protection

To guarantee responsiveness during minimal fault conditions (under weakest system conditions), the breaker must fulfill the sensitivity coefficient:
Sp = Ik.min / Iop ≥ 1.3

• Ik.min: Minimum fault current at the end of the protected line

• Iop: Trip unit setting (instantaneous or short-delay)

Only short-delay sensitivity needs verification if both short-delay and instantaneous units are present.

Trip Unit Settings for Circuit Breakers

(1) Instantaneous Overcurrent Trip Setting

To avoid nuisance tripping during equipment startup surges:
Iop(o) ≥ Krel × Ipk
Krel: Reliability factor
Ipk: Peak inrush current

(2) Short-Delay Overcurrent Trip Setting

Similarly:
Iop(s) ≥ Krel × Ipk
Short-delay times usually come in 0.2s, 0.4s, 0.6s tiers. Ensure upstream time > downstream time for coordination.

(3) Long-Delay Overcurrent Trip Setting

For overload protection:
Iop(l) ≥ Krel × I30
I30: Max load current
Trip delay must exceed the allowable temporary overload duration.

(4) Coordination with Cable Ratings

To prevent thermal damage or fire risks:
Iop ≤ Kol × Ial

• Ial: Cable’s permissible current

• Kol: Overload factor (typically 4.5 for instantaneous, 1.1 for short-delay, 1.0 for long-delay in overload-only scenarios)

Inadequate coordination demands a revised breaker setting or upsized conductor.