Trip Time Delay


A circuit breaker is an electrical switch that automatically opens a circuit when certain electrical conditions are met. In general, circuit breakers open (trip) when the electrical current through a sensing device exceeds a pre-established current rating. Carling. Technologies' manufactures circuit breakers with a hydraulic/magnetic tripping element. Using the example of the most common circuit configuration, "Series Trip, the following information explains what makes hydraulic/magnetic circuit breakers trip, and how trip time delays are obtained.

What Makes a Magnetic Circuit Breaker Trip?

The hydraulic/magnetic circuit breaker "Series Trip" configuration consists of current sensing coil connected in series with a set of contacts. (Figure 1)

Figure 1

Inside the coil is a non-magnetic delay tube housing a spring-biased, moving magnetic core. An armature links the contacts to the coil mechanism, which functions as an electromagnet. When the contacts are open, there is no current flow through the circuit breaker, and the coil develops no electromagnetic energy. When the contacts are closed, current flow begins. (Figure 2)

Figure 2 - Rated Current or Less

As the normal operating or "rated" current flows through the sensing coil, a magnetic field is created around that coil. When the current flow increases, the strength of the magnetic field increases, drawing the spring-biased, movable magnetic core toward the pole piece. As the core moves inward, the efficiency of the magnetic circuit is increased, creating an even greater electromagnetic force. When the core is fully "in", maximum electromagnetic force is attained. The armature is attracted to the pole piece, unlatching a trip mechanism thereby opening the contacts.(Figure 3)

Figure 3 - Moderate Overload with Induced Delay

Under short circuit conditions, the resultant increase in electromagnetic energy is so rapid, that the armature is attracted without core movement, allowing the breaker to trip without induced delay. This is called "instantaneous trip". It is a safety feature that results in a very fast trip response when needed most. (Figure 4)

Figure 4 - Short Circuit Condition - No Induced Delay

How Various Time Delays are Obtained

The trip time delay is the length of time it takes for the moving metal core inside the current sensing coil to move to the fully "in" position, thereby tripping the circuit breaker. The time delay should be long enough to avoid nuisance tripping caused by harmless transients, yet fast enough to open the circuit when a hazard exists.

If the delay tube is filled with air, the core will move rather quickly, and the breaker will trip quickly. This is characteristic of the Ultrashort trip time delay. Solid state devices, which cannot tolerate even short periods of current overload, should use the Instantaneous trip time delay, which have no intentional time delay.

When the delay tube is filled with a light viscosity, temperature stable fluid, the core's travel to the full "in" position will be intentionally delayed. This results in the slightly longer Medium trip time delays that are used from general purpose applications. (Figure 5).

Figure 5

When a heavy viscosity fluid is used, the result will be a very Long trip time delay. These delays are commonly used in motor applications to minimize the potential for nuisance tripping during lengthy motor start-ups.

By use of magnetic shunt plates within the magnetic circuit, it is possible to divert the magnetic flux, thereby resulting in a higher inrush withstanding capability. These High Inrush trip time delays disregard short duration, high pulse surges (typically 8 ms or less and up to 25x rated current) characteristic of transformers, switching power supplies and capacitive loads.

Hydraulic delay protectors have the added advantage of tripping slightly sooner when operating in higher temperature conditions and slightly longer in lower temperature conditions. This characteristic mirrors the protection needs in most applications. Note that the time delay for tripping changes, but the current required to trip the breaker does not change.

Delay Curves

Each trip time delay type can be graphed to display trip time delay curves. These curves represent the relationship between the percent of rated current and the trip time in seconds. See the example below.

Trip Time delays are available in PDF format for each Carling Technologies' circuit breaker product series. These PDF files contain trip time delay curves and tables, along with other specifications can be found on the specific series' product page within this web site