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Electronic Trip Unit

The second type of trip unit is the electronic trip unit. It is generally temperature insensitive and more expensive. It is used on low voltage circuit breakers beginning at 400A and on medium voltage circuit breakers. The trip unit is integrally mounted on the low voltage and externally mounted on the medium voltage.

Figure 22. Electronic Trip Units

This unit is rapidly replacing the thermal magnetic trip because of its increased accuracy, repeatability and discrimination. It also has an optional built-in ground fault protection. In addition, it offers other capabilities such as programming, monitoring, communication, system coordination and testing.

In general, electronic trip units are composed of three components, which are internal to the trip unit. These components are the current transformer, circuit board and flux-transfer Shunt Trip.

Figure 23. Components of Electronic Trip Unit

The current transformer is used in each current phase to monitor and reduce the current to the proper input level.

The circuit board is the brains of the system. It interprets input current and makes a decision based on predetermined parameters. A decision to trip sends an output to the flux-transfer shunt trip.

The flux-transfer shunt trip is the component that trips the circuit breaker.

There are two types of electronic trip units: Analog and Digital.

Figure 24. Digital and Analog Trip Units

The analog trip unit was developed first and considered the conventional approach. It functions by looking at all the points on a particular curve and responding to peak values. This can cause a problem because peak sensing can cause false tripping. The unit is also sensitive to harmonics.

Figure 25. Analog Peak Sensing

The digital trip unit functions by looking at selected discrete points on a particular curve and making a summation of those discrete points. The result is an RMS value that is more accurate because you are using all of the values instead of just peak values. This method correlates better with the thermal characteristics of conductors and equipment.

Figure 26. Digital Peak Sensing

4-Arc Extinguishers

An Arc Extinguisher is the component of the circuit breaker that extinguishes an arc when the contacts are opened. An arc is a discharge of electric current crossing a gap between two contacts. Circuit breakers must be designed to control them because arcs cannot be prevented. There are four techniques to extinguish an arc and there are several arc control methods. In this topic, you will be introduced to those methods.

Figure 27. Arc Extinguishers

What is an Arc?

The arc is defined as a discharge of electric current crossing a gap between two contacts.

Arcs are formed when the contacts of a circuit breaker are opened under a load. Arcs can be very destructive and vary greatly in size and intensity. The size of the arc depends on the amount of current present when the contacts are pulled apart. For example, an arc that forms when normal load current is broken is insignificant compared to the arc that forms when a short circuit is broken. Because arcs cannot be prevented, circuit breakers must be designed to control them.

The heat associated with an arc creates an ionized gas environment. The more ionization, is the better conditions for the arc to be maintained and grow., the more heat created,  increases ionization.

The important thing to remember here is that the ability of the circuit breaker to control the arc is the key to its short circuit interrupting capability. This is a critical factor for selecting circuit breakers.

A short circuit is the most devastating overcurrent condition.Current Zero or Zero Point is a very important aspect to arc extinguishing. At current zero, conditions are optimal for preventing an arc from continuing. The current is said to be "Current Zero" when the sine curve is at 0°, 180° and 360°.

Figure 29. Current Zero

Voltage is also a very important consideration because it is the pressure that keeps the current moving.

Circuit breakers take this process into account by simultaneously opening the contacts and extinguishing the arc. The successful extinguishing of the arc depends on the Dielectric Strength of the gap between the contacts. The dielectric strength is the maximum voltage a dielectric can withstand without breaking down. A Dielectric is any insulating material between two conductors. In these discussions, the circuit breaker contacts are the conductors and the insulating material can be air, gas or a vacuum. If the dielectric strength is greater than the voltage trying to re-ignite the arc, the arc extinguishing will be successful.

Figure 30. Extinguishing an Arc

Arc Control Techniques

Each approach has made improvements to its initial concept in an effort to extinguish arcs more efficiently. Arc control methods utilize one or more of the following general techniques:

1-Stretching Arc - The arc is produced when the contacts part. As the gap widens, the arc is stretched and cooled to the point where it is extinguished.

Figure 31. Stretching Arc

2-Breaking Arc into Smaller Pieces - The arc is produced when the contacts part. The arc moves up into the arc divider and splits, cools and is extinguished.

Figure 32. Breaking Arc

3-Blowing Out Arc - In this method, a high-pressure gas blows the arc into an arc divider to be extinguished.

Figure 33. Blowing Out Arc

4-Enclosing Contacts - In this method, the contacts are housed in an oxygen-free enclosure with a dielectric such as a vacuum, gas or cooling oil. Without oxygen, the arc cannot sustain itself and the arc is extinguished.

Figure 34. Enclosing Contacts

Arc Control Methods

There are six methods used around the world today to deal with arc control. The two most commonly used methods are arc chute and vacuum interrupter. The other four methods are SF₆, minimum oil, magnetic coil and puffer.

1-The arc chute method only uses the Breaking Arc into Smaller Pieces technique. Arc chutes are normally associated with low voltage circuit breakers due to efficiency and cost. In general, an arc chute will confine, divide and cool an arc, resulting in the arc being unable to sustain itself. There is one arc chute for each set of contacts.

Figure 35. Arc Chute Method

2-The vacuum interrupter method uses the Enclosing Contacts technique to extinguish arcs. The vacuum enables the contacts to be smaller and eliminates the divider, making this method the most cost effective and efficient above 1000V. Arcing takes place within a sealed evacuated enclosure. The contacts are located inside and arcing occurs when the contacts are separated. Because the environment inside the interrupter envelope is a vacuum, an arc cannot be easily sustained. It will not reach the intensity possible with an arch chute. One vacuum interrupter is provided for each set of contacts.

3-The SF₆ method also uses the Enclosing Contacts technique. It was a precursor to the vacuum interrupter and used SF₆ gas as the dielectric. The heat energy created by the arc works to break apart the SF₆ molecules. The larger the arc, the greater the breakdown of the gas which aids in extinguishing the arc. The technology is related more to European manufacturers of medium and higher voltage circuit breakers.

4-The minimum oil method also uses Enclosing Contacts with oil as the dielectric. The arc energy is absorbed as it rips hydrogen away from the oil molecule. The oil itself also helps to cool the arc. As current zero is approached, more oil is drawn into the system, further cooling and Deionizing the arc. It is used today in low voltage situations and potentially explosive environments where an arc chute is not desirable.

5-The magnetic coil method uses the Breaking Arc into Smaller Pieces technique. It is very similar to the arc chute method. The natural movement of an arc is upward, in this instance, into an arc chute. A coil, called a blowout coil, is located in the center of the arc chute. The arc is broken into two. The arcs are lengthened and cooled as they rise higher. The cooling reduces the rate of ionization. When the ionization drops below the level necessary to sustain the arcs, they extinguish at current zero. Prior to vacuum interrupter technology becoming the method of choice with medium voltage power breakers for extinguishing arcs, the magnetic coil method served well for many years.

6-The puffer method uses the Blowing Out Arc and Enclosing Contacts techniques. It uses SF₆ gas as the dielectric. It is the most efficient and cost effective method above 38 kV. This type interrupter is basically a pair of separable contacts, a piston and a cylinder, mounted in a reservoir of gas. As the contacts part, the piston moves up to drive the gas through the arc to interrupt it. It also utilizes coils and takes advantage of natural magnetic affects to create a force sufficient to extinguish the arc.

Figure 39. Puffer Method

As you have seen, there are several techniques to effectively deal with extinguishing arcs and improvements continue to be made.

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