Undervoltage Coil: Application Differences Between Time-Delay Type and Instantaneous Type

In low-voltage power distribution systems, the undervoltage release (UV or UVT) is an often-overlooked but very important protection accessory. Its function is: when the supply voltage drops below a set threshold (typically 35% to 70% of rated voltage), it automatically triggers the Circuit Breaker to trip, preventing equipment from operating abnormally under low voltage conditions.

 

However, in practical engineering, undervoltage releases come in two types: instantaneous and time-delay. Although the only difference is a "time delay," the application scenarios and consequences are completely different.

 

Choosing the wrong type can result in:

- Tripping when it shouldn't (a brief grid fluctuation causes a large-area blackout)

- Not tripping when it should (voltage drops to a dangerous level but the breaker doesn't open)

 

Below, we compare these two types of undervoltage coils from four perspectives: operating principle, time characteristics, typical applications, and selection recommendations.

 

 

 

1. Operating Principle Review

 

First, let's briefly understand the basic principle of an undervoltage release to better grasp the differences.

 

An undervoltage release contains an electromagnetic coil connected in parallel to the power supply. Under normal conditions, the voltage is high enough that the coil's magnetic force overcomes the spring force, keeping the release in the attracted state and allowing the circuit breaker to close.

 

When the voltage drops below the set threshold:

- Insufficient magnetic force → spring returns → strikes the trip mechanism → circuit breaker opens

 

Therefore, an undervoltage release is a protection device that automatically disconnects when voltage is lost or too low. It is commonly used for:

- Preventing motors from stalling and burning out under low voltage

- Preventing equipment from unexpectedly self-starting when voltage returns (requiring manual reset)

- Meeting certain industry code requirements (e.g., cranes, elevators)

 

 

 

2. Instantaneous Undervoltage Coil

 

 2.1 Definition and Time Characteristics

 

Instantaneous undervoltage release: When the voltage drops below the set threshold, it triggers the circuit breaker to trip with virtually no delay (typically within tens of milliseconds).

 

- Operating time: ≤ 0.1 second (typical)

- Recovery threshold: Typically above 85% of rated voltage (re-closing condition)

 

 2.2 Application Scenarios

 

Because it is extremely sensitive, the instantaneous undervoltage coil is suitable for applications where any voltage sag cannot be tolerated:

 

Typical applications:

 

1. High-risk equipment protection

   For example: cranes, hoists, elevators. A voltage sag may cause insufficient motor torque, resulting in a falling load—immediate sHutdown is required.

 

2. Equipment that cannot self-start

   Some equipment, if it suddenly starts on its own after voltage returns, could cause personal injury or equipment damage (e.g., woodworking machinery, centrifuges).

 

3. Applications with very low tolerance for process continuity

   In some industrial processes, if the voltage is unstable, it's better to shut down than to continue operating under fault conditions.

 

4. Coordination with automatic transfer switches (ATS)

   When the main power supply experiences undervoltage, the incoming main breaker trips instantly so that the backup power supply can be quickly engaged.

 

 2.3 Main Risk

 

The biggest issue: too sensitive.

 

Common momentary voltage sags on the grid (e.g., from starting large motors, lightning strikes, fault clearing on the grid, capacitor bank switching) may last only tens to hundreds of milliseconds—but that is enough to trigger an instantaneous undervoltage coil and cause the breaker to trip.

 

Consequences:

- All equipment on a production line shuts down simultaneously

- Re-closing requires personnel to go on-site

- Unnecessary production loss

 

> Many users complain that "the breaker trips as soon as the voltage flickers"—this often happens because an instantaneous undervoltage coil was chosen while grid stability is less than ideal.

 

 

 

3. Time-Delay Undervoltage Coil

 

 3.1 Definition and Time Characteristics

 

Time-delay undervoltage release: When the voltage drops below the set threshold, the circuit breaker does not trip immediately. Instead, it starts timing. If the voltage returns to normal within the set delay time, the trip is canceled. Only if the voltage remains low after the delay time elapses does it trigger a trip.

 

- Common delay times: 0.1s, 0.5s, 1s, 3s, 5s (adjustable or fixed)

- Voltage behavior during delay: Can be designed as "trip only if undervoltage persists" or "ignore momentary dips"

 

 3.2 Application Scenarios

 

The time-delay undervoltage coil is suitable for applications where you want to ride through brief grid fluctuations:

 

Typical applications:

 

1. Main incoming breakers in factory workshops, data centers, and commercial buildings

   If a brief grid fluctuation causes the main breaker to trip, it results in a large-area blackout with significant losses. A time-delay type can "tolerate" the dip—if voltage recovers quickly, it won't trip.

 

2. Circuits with predominantly motor loads

   Starting a large motor causes a momentary voltage dip at the bus (typically 80%~90% of rated voltage). An instantaneous type would trip falsely; a time-delay type can ride through the starting process.

 

3. Coordination with standby generators or UPS

   When utility power fails, a generator typically takes several seconds to start and build voltage. A time-delay undervoltage coil can hold off tripping during those seconds, waiting for backup power to engage.

 

4. Applications requiring "ridethrough" capability (anti-sag)

   "Voltage sag" refers to a brief voltage drop (typically 0.1~1 second). A time-delay type is one of the basic methods for achieving ridethrough.

 

 3.3 Points to Note

 

- During the delay time, the circuit breaker has not yet opened. If the voltage remains low and the equipment cannot tolerate it, motor overheating may occur.

- The delay accuracy of a time-delay undervoltage coil is affected by ambient temperature (especially electromagnetic types). Electronic types are relatively more stable.

- Delay time cannot be arbitrarily long—excessive delay may violate code requirements for undervoltage protection operating time.

 

 

 

4. Direct Comparison: Instantaneous vs. Time-Delay

 

5. Selection Decision Tree: Which One to Choose?

 

Follow this logic:

 

Step 1: Does your equipment or applicable code explicitly require "trip immediately when voltage drops"?

 

- Yes → Choose Instantaneous type

  - Typical: cranes, elevators, certain machine tools

- No → Go to Step 2

 

Step 2: Is it possible to have brief voltage fluctuations on the grid (motor starting, lightning, large load switching, etc.)?

 

- Yes, and the loss from a trip would be significant → Choose Time-delay type

- Almost never (e.g., very small system or dedicated transformer) → Can choose Instantaneous type

 

Step 3: Is coordination with a generator or backup power transfer required?

 

- Yes → Must choose Time-delay type (or a "delay + instantaneous" combination)

- No → Judge based on the previous steps

 

Step 4: Does the equipment require that it cannot self-start after voltage returns (manual re-closing required)?

 

- Both instantaneous and time-delay types have this feature (once tripped, manual reset is required)

- However, a time-delay type avoids "unnecessary waiting for self-start" caused by momentary dips

 

 

 

6. Typical Application Case Studies

 

 Case 1: Main circuit of a tower crane

 

- Requirement: Voltage drops → immediate shutdown required to prevent falling load

- Choice: Instantaneous undervoltage coil

- Reason: Cannot wait. Even if it's just a grid fluctuation, tripping is necessary—safety first

 

 Case 2: Main incoming breaker in a factory workshop

 

- Requirement: Cannot have a whole-plant blackout due to a brief external grid flicker

- Choice: Time-delay undervoltage coil (delay 0.5~1 second)

- Reason: The vast majority of voltage sags recover within 1 second. A time-delay type effectively prevents a "whole-plant blackout"

 

 Case 3: Utility incoming breaker in a data center

 

- Requirement: When utility fails, wait for the generator to start (typically 10~15 seconds)

- Choice: Time-delay undervoltage coil (delay 10 seconds or more) or a delay + instantaneous combination

- Reason: The generator needs time to build voltage. An instantaneous type would trip within 3 seconds, failing the transfer

 

 Case 4: Fire pump control panel

 

- Requirement: Per code, fire-fighting equipment typically does NOT have an undervoltage release (unless specifically required)

- Note: Many codes explicitly state that fire-fighting loads should not trip due to short-term voltage dips—otherwise, water supply could be interrupted during a fire

 

 

 

7. Common Misconceptions and Important Notes

 

 ❌ Misconception 1: Undervoltage coil is the same as loss-of-voltage coil

There is actually a difference. "Loss of voltage" typically refers to complete voltage absence (0V), but an undervoltage coil may trip when voltage drops below 70% of rated. The terms are often used interchangeably, but when selecting, check the threshold parameters.

 

 ❌ Misconception 2: Longer delay is always better

Incorrect. Too long a delay can cause a motor to run under low voltage for an extended period, potentially overheating and burning out. The delay time should be set based on the equipment's ability to tolerate undervoltage.

 

 ⚠ Important notes:

 

1. Electromagnetic vs. Electronic

   - Electromagnetic undervoltage coil: No auxiliary power required, but delay accuracy is poor and affected by temperature

   - Electronic undervoltage coil: Requires auxiliary power, delay is precisely adjustable, but does not function when auxiliary power is lost

 

2. Difference from shunt trip coil

   A clear distinction for users: Shunt trip = actively apply power to cause tripping; Undervoltage = passively trip when power is lost (or voltage is too low)

 

3. Cannot use two undervoltage coils on one breaker

   A single circuit breaker can only accommodate one undervoltage release (which can be either instantaneous or time-delay type)

 

4. Re-closing conditions

   - Instantaneous type: Voltage must recover above 85% before manual or electric closing is possible

   - Time-delay type: Voltage must recover to normal, and the delay circuit must reset before closing is possible

 

 

 

8. Summary

One-sentence summary:

 

- Instantaneous type: Trips as soon as voltage drops—protects equipment safety, but does not preserve supply continuity.

- Time-delay type: Doesn't trip immediately on low voltage—waits first. If voltage recovers, it won't trip—preserves supply continuity.

 

When selecting, ask yourself one question: "If the grid just flickers for a moment, do I want the breaker to trip?"

- Yes, trip → Instantaneous type

- No, don't trip → Time-delay type

 

The answer to this question determines which undervoltage coil you should choose.