Sunday, January 25, 2009

Selective Coordination

Selective coordination is the application of circuit protective devices in series such that under overload or fault conditions, only the upstream device nearest the fault will open. The rest of the devices remain closed, leaving other circuits unaffected. In the following example a short circuit has occurred in the circuit fed by branch circuit breaker “C”. Power is interrupted to equipment supplied by circuit breaker “C” only. All other circuits remain unaffected.

Circuit Breaker Coordination
Time current curves are useful for coordinating circuit breakers. If the trip curves of main breaker “A”, feeder breaker “B”, and branch breaker “C” are placed on the same graph, there should be no overlapping, indicating the breakers are coordinated. The three circuit breakers in the following example have been coordinated so that for any given fault value, the tripping time of each breaker is greater than tripping time for the downstream breaker(s). In the following illustration, circuit breaker “C” is set to trip if a 400 amp fault current remains for .04 seconds. Circuit breaker “B” will trip if the fault remains for .15 seconds, and circuit breaker “A” if the fault remains for .8 seconds. If a 400 amp fault occurs downstream from circuit breaker “C”, it will trip first and clear the fault. Circuit breakers “A” and “B” will not trip.

Series-Connected Systems

When selecting circuit breakers, it is extremely important to know both the maximum continuous amperes and the available fault current. NEC® article 110.9 states:

Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment.
Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.

There are two ways to meet this requirement. The first method is to select circuit breakers with individual ratings equal to or greater than the available fault current. This means that, in the case of a building with 65,000 amperes of fault current available at the service entrance, every circuit breaker must have an interrupting rating of at least 65,000 amperes.

The second method is to select circuit breakers with a series combination rating equal to or greater than the available fault current at the service entrance. The series-rated concept requires the main upstream breaker to have an interrupting rating equal to or greater than the available fault current of the system, but subsequent downstream breakers connected in series can be rated at lower values. For example, a building with 65,000 amperes of available fault current might only need the breaker at the service entrance to have an interrupting rating of 65,000 amperes. Additional downstream breakers can be rated at lower values. The series combination must be tested and listed by UL.

Time-Current Curves

Time-current curves:
similar to the one shown on the following page, are used to show how fast a breaker will trip at any magnitude of current. The following illustration shows how to read a time-current curve. The figures along the bottom (horizontal axis) represent multiples of the continuous current rating (In) for the breaker. The figures along the left side (vertical axis) represent time in seconds.
Time in Seconds Multiple of In

To determine how long a breaker will take to trip at a given multiple of In, find the multiple on the bottom of the graph and draw a vertical line to the point where it intersects the curve. Then draw a horizontal line to the left side of the graph and find the time to trip. For example, in this illustration a circuit breaker will trip when current remains at six times In for .6 seconds. Note that the higher the current, the shorter the time the circuit breaker will remain closed. Time-current curves are usually drawn on log-log paper. Many time-current curves also show the bandwidth, tolerance limits, of the curve.
From the information box in the upper right hand corner, note that the time-current curve illustrated on the next page defines the operation of a Siemens MG frame circuit breaker. For this example, operation with an 800 ampere trip unit is shown, but, depending upon the specific breaker chosen, this circuit breaker may be purchased with a 600, 700, or 800 amp continuous current rating.

Overload Protection:
The top part of the time-current curve shows the continuous
Current performance of the circuit breaker. The black line shows the nominal performance of the circuit breaker and the gray band represents possible variation from this nominal performance that can occur even under specified conditions.

Using the example of an MG breaker with an 800 amp continuous current rating (In), note that the circuit breaker can be operated at 800 amps (1.0 times In) indefinitely without tripping. However, the top of the trip curve shows that an overload trip will occur in 10,000 seconds at 1000 amps (1.25 times In). Additionally, the gray area on either side of the trip curve shows the range of possible variation from this response.

Keep in mind that this trip curve was developed based upon predefined specifications, such as operation at a 40°C ambient temperature. Variations in actual operating conditions will result in variations in circuit breaker performance.

Instantaneous Trip:
The middle and bottom parts of this time-current curve show
Instantaneous trip (short circuit) performance of the circuit breaker. Note that the maximum clearing time (time it takes for the breaker to completely open) decreases as current increases. This is because of high-speed contact designs which utilize the magnetic field built up around the contacts. As current increases, the magnetic field strength increases, which speeds the opening of the contacts?

This circuit breaker has an adjustable instantaneous trip point from 3250 to 6500 amps, which is approximately four to eight times the 800 amp continuous current unit rating. This adjustment affects the middle portion of the trip curve, but not the top and bottom parts of the curve. The breaker shown in this example has a thermal-magnetic trip unit. Circuit breakers with solid-state trip units typically have additional adjustments

True RMS Sensing

Some solid state circuit breakers sense the peak values of the current sine wave. This method accurately measures the heating effect of the current when the current sine waves are perfectly sinusoidal. Frequently, however, the sine waves are harmonically distorted by non-linear loads. When this happens, peak current measurement does not adequately evaluate the true heating effect of the current.

Siemens solid state trip unit circuit breakers incorporate true root-mean-square (RMS) sensing to accurately sense the effective value of circuit current. True RMS sensing is accomplished by taking multiple, instantaneous “samples” of the actual current wave shape for a more accurate picture of its true heating value.

The microcomputer in Siemens solid state trip unit breakers samples the AC current waveform many times a second, converting each value into a digital representation. The microcomputer then uses the samples to calculate the true RMS value of the load current. This capability allows these circuit breakers to perform faster, more efficiently and with repeatable accuracy.

Being able to monitor true RMS current precisely is becoming more important in today’s electrical distribution systems because of the increasing number of power electronic devices being used that can distort the current waveform.

Adjustable Trip

Curves One of the key features of solid state trip unit circuit breakers is the ability to make selective adjustments to the circuit breaker’s time-current curve. The time-current curve shown here is for a circuit breaker in the SJD6-SLD6 family.

Solid State Circuit Breaker

Adjustments The type of trip unit included in a circuit breaker determines the specific time-current curve adjustments available. . The following illustration and associated table describes the adjustments available.

Circuit Breaker Ratings

Voltage Rating:
Circuit breakers are rated according to the maximum voltage they can handle. The voltage rating of the circuit breaker must be at least equal to the circuit voltage. The voltage rating of a circuit breaker can be higher than the circuit voltage, but never lower. For example, a 480 VAC circuit breaker could be used on a 240 VAC circuit. A 240 VAC circuit breaker could not be used on a 480 VAC circuit. The voltage rating is a function of the circuit breaker’s ability to suppress the internal arc that occurs when the circuit breaker’s contacts open.
Some circuit breakers have what is referred to as a “slash” voltage rating, such as 120/240 volts. In such cases, the breaker may be applied in a circuit where the nominal voltage between any conductor and ground does not exceed the lower rating and the nominal voltage between conductors does not exceed the higher rating.

Continuous Current Rating
Every circuit breaker has a continuous current rating which is the maximum continuous current a circuit breaker is designed to carry without tripping. The current rating is sometimes referred to as the ampere rating because the unit of measure is amperes, or, more simply, amps.

The rated current for a circuit breaker is often represented as In. This should not be confused with the current setting (Ir) which applies to those circuit breakers that have a continuous current adjustment. Ir is the maximum continuous current that circuit breaker can carry without tripping for the given continuous current setting. Ir may be specified in amps or as a percentage of In.

As mentioned previously, conductors are rated for how much current they can carry continuously. This is commonly referred to as the conductor’s ampacity. In general, the ampere rating of a circuit breaker and the ampacity of the associated conductors must be at least equal to the sum of any non-continuous load current plus 125% of the continuous load current.

Circuit breakers are rated on the basis of using 60° C or 75° C conductors. This means that even if a conductor with a higher temperature rating were used, the ampacity of the conductor must be figured on its 60° C or 75° C rating.

Frame Size:

The circuit breaker frame includes all the various components that make up a circuit breaker except for the trip unit. For any given frame, circuit breakers with a range of current ratings can be manufactured by installing a different trip unit for each rating. The breaker frame size is the highest continuous current rating offered for a breaker with a given frame.

Interrupting Rating:
Circuit breakers are also rated according to the maximum level of current they can interrupt. This is the interrupting rating or ampere interrupting rating (AIR). Because UL and IEC testing specifications are different, separate UL and IEC interrupting ratings are usually provided.
When designing an electrical power distribution system, a main circuit breaker must be selected that can interrupt the largest potential fault current that can occur in the selected application. The interrupting ratings for branch circuit breakers must also be taken into consideration, but these interrupting ratings will depend upon whether series ratings can be applied. Series-connected systems are discussed later in this course.

The interrupting ratings for a circuit breaker are typically specified in symmetrical RMS amperes for specific rated voltages. As discussed in Basics of Electricity, RMS stands for root-mean-square and refers to the effective value of an alternating current or voltage. The term symmetrical indicates that the alternating current value specified is centered around zero and has equal positive and negative half cycles. Siemens circuit breakers have interrupting ratings from 10,000 to 200,000 amps.

Circuit breaker Contact design:

Straight-through contact
The current flowing in a circuit controlled by a circuit breaker flows through the circuit breaker’s contacts. When a circuit breaker is turned off or is tripped by a fault current, the circuit breaker interrupts the flow of current by separating its contacts.
Many circuit breakers use a straight-through contact arrangement, so called because the current flowing in one contact arm continues in a straight line through the other contact arm.

Blow-Apart Contacts:
As an improvement over the straight-through contact design, Siemens developed the blow-apart contact design now commonly used by circuit breakers with higher interrupting ratings. With this design, the two contact arms are positioned parallel to each other as shown in the following illustration. As current flows through the contact arms, magnetic fields are set up around each arm. Because the current flow in one arm is opposite in direction to the current flow in the other arm, the two magnetic fields oppose each other. Under normal conditions, the magnetic fields are not strong enough to force the contacts apart.

When a fault develops, current increases rapidly causing the strength of the magnetic fields surrounding the contacts to increase as well. The increased strength of the opposing magnetic fields helps to open the contacts faster by forcing them apart.

By reducing the time required to open circuit breaker contacts in the event of a fault condition, the blow-apart contact design exposes the electrical equipment protected by the circuit breaker to less damaging heat.

Arc Chute Assembly :
As the contacts open a live circuit, current continues to flow for a short time by jumping the air space between the contacts in the form of an arc. When the contacts open far enough, the arc is extinguished and the current flow stops.

Minimizing the arc is important for two reasons. First, the arc can damage the contacts. In addition, the arc ionizes gases inside the molded case. If the arc isn’t extinguished quickly the pressure from the ionized gases could cause the molded case to rupture.

Circuit breakers commonly use an arc chute assembly to quench the arc. This assembly is made up of several “U” shaped steel plates that surround the contacts. As the arc is developed, it is drawn into the arc chute where it is divided into smaller arcs, which are extinguished faster.

Types of Circuit Breakers

Instantaneous Magnetic Trip-Only Circuit Breakers -
As the name indicates, instantaneous magnetic-trip-only circuit breakers provide short circuit protection but do not provide overload protection. This type of circuit breaker is typically used in motor control applications where overload protection is provided by an overload relay.

For example, in the circuit shown below, a three-pole instantaneous magnetic-trip-only circuit breaker provides short circuit protection while the overload protection for the motor is provided by an overload relay which is part of a motor starter.

Thermal-Magnetic Circuit Breakers,

This type of circuit breaker is called a thermal-magnetic circuit breaker because it has a trip unit that senses heat to detect an overload and senses a magnetic field generated by current to detect a short circuit.

As described in the Circuit Breaker Design portion of this book, this type of circuit breaker trips immediately when a short circuit occurs, but delays an appropriate amount of time before tripping in the event of an overload.

Interchangeable Trip Ckt breaker

The user cannot change the trip unit on many circuit breakers, but some circuit breakers have an interchangeable trip feature. This feature allows the user to change the continuous current rating of the breaker without replacing the breaker. This is done by replacing the trip unit with one of a different rating
Note: Care must be exercised when considering interchangeable trip circuit breakers. A circuit breaker may be listed by Underwriters Laboratories, Inc.® (UL®) for a specific interchangeable trip unit only. Circuit breaker frames are usually designed to prevent the installation of an improper trip unit size or type.

Current Limiting Circuit Breakers

Many electrical power distribution systems can deliver large Short circuit currents to electrical equipment. This high current can cause extensive damage. Current limiting circuit breakers protect expensive equipment by significantly reducing the current flowing in the faulted circuit.

One way to accomplish current limiting is with an additional set of contacts that feature two moveable arms. These are referred to as dual-pivot contacts, which separate even more quickly than the single-pivot contacts. The dual-pivot contacts are connected in series with the single-pivot contacts. As with the single-pivot design, current flows in opposite directions through the contact arms, creating a magnetic repulsion. As current increases, the magnetic repulsion force increases.

In an overload condition where current may only be one to six times normal current, the contacts remain closed until the breaker trips. When a short circuit occurs, fault current is extremely high and both sets of contact arms open simultaneously, generating high impedance arcs. The contact gap of the dual-pivot contacts increases more rapidly, therefore generating arc impedance more rapidly. Once the arcs are extinguished, the dual-pivot contacts close on their own due to spring tension. The single-pivot contacts are held open by the breaker mechanism, which will have tripped during the fault and must be manually reset.

The frame on current limiting circuit breakers of this design is extended to allow room for the dual-pivot set of contacts. Siemens current limiting breakers can handle fault currents of up to 200,000 amps.

Solid State Circuit Breakers
Solid state circuit breakers function similarly to thermal-magnetic breakers and have a mechanical breaker mechanism but incorporate a solid state trip unit. The solid state trip unit allows this type of circuit breaker to have programmable features and a greater degree of accuracy and repeatability.

Similar to other types of trip units, the solid state trip unit:

• Senses magnitude of current flow

• Determines when current becomes excessive

• Determines when to send a trip signal to the breaker mechanism

The brains of a solid state trip unit are a microprocessor. Adjustments on the trip unit allow the user to select numerical values the microprocessor will use in performing protective functions. Current sensors mounted in the trip unit monitor the value of load current. The value of current is reduced to a low level and converted to a digital voltage, which is used by the microprocessor. The microprocessor continuously compares the line current with the value set by the user. When current exceeds a preset value for the selected time, the trip unit sends a signal to a magnetic latch. The magnetic latch opens the breaker’s contacts, disconnecting the protected circuit from the power source.

Types of over current Protective Devices

A fuse is a one-shot device. The heat produced by over current causes the current carrying element to melt open, disconnecting the load from the source voltage.

Nontime-Delay Fuse:
Fuses without time delay provide excellent short circuit protection. When an over current situation occurs, heat builds up rapidly in the fuse. Fuses without time delay usually hold 500% of their rating for approximately one-fourth second, after which the current carrying element melts. This means that these fuses cannot be used in motor circuits which often have inrush currents of greater than 500%.

Time-Delay Fuses:
Time-delay fuses provide overload and short circuit protection. Time-delay fuses usually allow five times the rated current for up to ten seconds to allow motors to start.

Circuit Breaker:

The National Electrical Code® defines a circuit breaker as a device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined over current without damage to itself when properly applied within its rating.

Circuit breakers provide a manual means of energizing and de-energizing a circuit. In addition, circuit breakers provide automatic over current protection of a circuit. A circuit breaker allows a circuit to be reactivated quickly after a short circuit or overload is cleared. Unlike fuses which must

be replaced when they open, a simple flip of the breaker’s operating handle restores the circuit.

All circuit breakers perform the following functions:

when an over current occurs.

• MEASURE the amount of over current.

• ACT by tripping the circuit breaker in a time frame necessary to prevent damage to itself and the associated load cables.

Circuit Breaker Operation
In the following illustration, an AC motor is connected through a circuit breaker to a voltage source. When the circuit breaker is closed, a complete path for current exists between the voltage source and the motor allowing the motor to run. Opening the circuit breaker breaks the path of current flow and the motor stops. The circuit breaker will open automatically during a fault, or can be manually opened. After the fault has been cleared, the breaker can be closed allowing the motor to operate.

Need for Circuit Protection

Current and Temperature:
Current flow in a conductor always
generates heat. The greater the current flow, the hotter the conductor. Excess heat is damaging to electrical components and conductor insulation. For that reason, conductors have a rated continuous current carrying capacity or ampacity. Over current protection devices, such as circuit breakers, are used to protect conductors from excessive current flow. These protective devices are designed to keep the flow of current in a circuit at a safe level to prevent the circuit conductors from overheating.


An overload occurs when too many devices are operated on a single circuit, or a piece of electrical equipment is made to work harder than it is designed for. For example, a motor rated for 10 amps may draw 20, 30, or more amps in an overload condition. In the following illustration, a package has become jammed on a conveyor, causing the motor to work harder and draw more current. Because the motor is drawing more current, it heats up. Damage will occur to the motor in a short time if the problem is not corrected or the circuit is shut down by the over current protector.

Conductor Insulation:

Motors, of course, are not the only devices that require circuit protection for an overload condition. Every circuit requires some form of protection against over current. Heat is one of the major causes of insulation failure of any electrical component. High levels of heat can cause the insulation to breakdown and flake off, exposing conductors.

Short Circuits:

When two bare conductors touch, either phase to phase or phase to ground, a short circuit occurs. When a short circuit occurs, resistance drops to almost zero. Short circuit current can be thousands of times higher than normal operating current.

Ohm’s Law demonstrates the relationship of current, voltage, and resistance. For example, a 240 volt motor with 24 Ω of resistance would normally draw 10 amps of current.

I =E/R = 240/24=10 Amps

When a short circuit develops, resistance drops. If resistance drops to 24 milliohms, current will be 10,000 amps.

I = 240/0.024=10,000 Amps

The heat generated by this current will cause extensive damage to connected equipment and conductors. This dangerous current must be interrupted immediately when a short circuit occurs.