Engineering Tutorials

The testing required and the standards that must be met by a low voltage power circuit breaker depend on the area of the world where the circuit breaker is applied. To play a major international role, low voltage power circuit breakers should be able to meet the requirements of ANSI, UL and IEC .

As you continue through this module, you will become well aware that the required testing is the key factor in defining the type of circuit breaker. In a very general and simplistic way, low voltage power circuit breakers undergo a sequence of four rigorous tests.

1. The first sequence consists of a temperature rise, an overload, and a short-circuit test.
2. The second sequence is a series of short-circuit tests.
3. The third sequence is an endurance test.
4. The fourth is a momentary rating test.

Molded case circuit breakers, for example, are subjected to tests similar to numbers 1, 2 and 3. The fourth test sequence, momentary rating test, is specific to power circuit breakers and to some IEC molded case circuit breakers.
Specific testing details will not be covered in this module. It should be pointed out, however, that the momentary rating test just mentioned (test sequence 4) subjects the circuit breaker to tremendous physical forces and severe heating effects. Very simply speaking, the circuit breaker is subjected to its full short time current rating for two (2) time periods up to 30 cycles each. The short time rating indicates what magnitude of current the circuit breaker can stand with its contacts closed for a short period without being damaged. The circuit breaker's short time rating is often equal to its 600 volt interrupting capacity. A low voltage power circuit breaker must be strong enough to survive this test and function properly after completion.

Helping the Customer
Selection of the proper low voltage power circuit breaker for a specific application is not a difficult process. There are some important questions, however, you must be able to answer. Fortunately, the most difficult part of the job has already been done by other qualified individuals when they determined the requirements of the system.
This includes determining things like:
• Circuit Breaker type required
• Application voltage
• Maximum fault current system could see
• Continuous current for the system and each branch
• System frequency
• Types of trip unit capabilities
• Programmable functions
• Accessory needs
• Mounting configuration
• Special requirements

Your job is to make sure these types of questions are answered. The more familiar you are with what a particular circuit breaker line has to offer, the easier the task. Let's start by looking at what circuit breaker manufacturers do to help.
Manufacturers normally provide a great deal of assistance in the way of printed material, computer accessible information and direct contact. This does not mean, however, you should not put forth that extra effort to know personally what is available. Learn to use all the information provided.
Most selection factors fall into one of two categories:

• Standard selection factors
• Special selection factors

Standard Selection Factors

Standard selection factors normally are associated with the circuit breaker's ratings/standards, operation method, accessory items, and how the breaker will be mounted. The most common points to consider will be discussed.
1. Standards - Applicable standards were discussed in this module and earlier modules. You should be told or it will appear in a written specification what standards the circuit breakers must meet. Newer low voltage power circuit breakers meet a wide array of standards which will make them acceptable in most parts of the world. In addition, make sure you are aware of any special local requirements and/or standards.
2. Ratings - This is a critical part of the selection process. You should already know what is required. Now you must determine what specific circuit breakers will meet the rating requirements. Manufacturers normally provide easy to read tables outlining the ratings of every circuit breaker frame. Keep in mind there could be more than one table. This is especially true for newer circuit breaker designs that meet both ANSI and IEC requirements. A manufacturer might choose to present it as one combined table or two tables. If you know what is required, you will be able to make a selection from the tables under normal circumstances.

ANSI Example
Let's take a look at a typical type ANSI table for a low voltage power circuit breaker and see what it has to offer (Figure 35). The table used in this example will not cover every circuit breaker rating for a particular design.
Enough of the table is presented to give you a good working knowledge on how to proceed. Each area of the table that is discussed is identified by a circled letter to simplify the discussion. One last important point should be made before beginning. Always read footnote references carefully. They provide important information and could be critical to the proper selection.



A: The "Breaker Type" is usually the name given to the circuit breaker by the manufacturer along with some general information about the ratings of that specific circuit breaker type. In the example ratings table shown, XYZ-508 is the first circuit breaker listed. The XYZ is the circuit breaker's name. The first number "5" gives you a general idea what the interrupting rating is of the XYZ-508 circuit breaker at a voltage of 480 volts. This is a common presentation method because the widest used application voltage domestically is 480 volts. The last two numbers, "08" in this case, usually tell you the maximum continuous current rating of the circuit breaker. XYZ-508 can, therefore, carry 800 amperes continuously, and interrupt 50,000 amperes at 480 volts.
B: This column outlines specifically the maximum continuous current the circuit breaker will carry. Notice that circuit breaker type XYZ-616 in the example table will carry a maximum continuous current of 1600 amperes. Notice also that the last two numbers of the circuit breaker type XYZ-616 ("16") give you the same information, with 16 meaning 1600. Take the time to make this same comparison with circuit breaker type XYZ-632.
C: Notice that the rest of this table is devoted to the interrupting capabilities in amperes of the different circuit breaker types at different application voltages. Also notice that the application voltages listed are:
• 208-240 volts
• 480 volts
• 600 volts
The nominal voltage range for the ANSI market is 208 to 600 Volts AC at a frequency of 50 or 60 hertz. Get comfortable with seeing these voltages when talking about ANSI rated low voltage power circuit breakers.

D: You will notice that these two columns are labeled differently. The first column entitled "With Instantaneous Trip" outlines the interrupting capabilities of each circuit breaker frame at the different application voltages. These ratings are applicable when the circuit breaker's trip unit provides instantaneous protection. In other words, the circuit breaker can be applied to safely handle faults of the magnitudes shown.

You will also notice in the column entitled "Without Instantaneous Trip" that some of the interrupting ratings are somewhat lower than the left column under 208-240 volts. These ratings are the magnitudes that the circuit breaker can tolerate safely for a short delay period of time (30 cycles) before opening at the short delay current ratings shown. This might sound like a contradiction. It really is not for a number of reasons. Consider the following points.

1. You will recall from material presented earlier that a low voltage power circuit breaker's short time rating is normally the same as its interrupting rating. The key word here is normally. The partial ratings table being considered here already indicates that there are some very limited times when a low voltage power circuit breaker could have a higher interrupting rating if it has instantaneous protection versus just short time protection and no instantaneous. This was probably the result of a conscious decision to meet some very specific application requirement for a particular customer or industry, knowing the fact that a circuit breaker had to have instantaneous to be applied at these somewhat higher ratings.

2. The fact still remains that low voltage power circuit breakers must be and are only applied in keeping with their nameplate rating. This, in almost all cases, shows the interrupting rating and the short time rating to be the same. When electrical systems are being considered, fault calculations are done to determine the maximum fault current a system can experience. Low voltage power circuit breakers are then selected with ratings that are able to deal successfully and safely with the worst case fault scenario calculated. In other words, if a low voltage power circuit breaker with an adequate short delay current rating is applied, it will stay closed for the appropriate short time no matter what. This is true because it will not see (experience) more that it was designed to safely handle. End of that part of the story.

3. On the other hand, a low voltage power circuit breaker, which is already in the open position, will trip open instantaneously if an attempt is made to close the breaker on an existing fault. This safety feature prevents damage that could result from closing on a fault. Today, this feature is normally accomplished through circuitry which is part of the trip unit. This self protecting circuitry is often called a discriminator circuit or may be called a making current release in newer designs like Magnum DS. Its purpose has nothing to do with a circuit breaker that is already closed and functioning.

For now, how this feature is technically accomplished will not be discussed. Just be aware that such a feature exists with low voltage power circuit breakers. Future training material specific to a particular low voltage power circuit breaker design will discuss just how it is accomplished.
Remember:

• Low voltage power circuit breakers are applied at their nameplate ratings.
• Low voltage power circuit breakers are sized and selected for application to handle the maximum fault that could be encountered where they are applied.
• Low voltage power circuit breakers are provided with a means to trip (open) instantaneously if they are closed in on an existing fault.

E: Let's just briefly take a look at the footnote. It tells you that these ratings are also the short time ratings of the circuit breaker. The material in D was discussed as though we already knew these were short time ratings, and we did. Suppose we did not know that fact and failed to read the footnote. We would not be as informed as we should be for the proper circuit breaker selection. It could be like making the selection blindfolded. Be sure to read the footnotes.

IEC Example

IEC Example - An IEC example similar to the one just presented will not be offered here. Ratings tables and their appearance as to how data is presented can change from country to country and even manufacturer to manufacturer. The information presented, however, is usually similar. You should be aware of some of the noticeable differences in the presented data, and start now to become familiar with IEC rated breakers. For now, consider the following to get started:
• The voltage range for the international market is 380 through 690 Volts AC at a frequency of 50 or 60 hertz.
• The general continuous current range for low voltage power circuit breakers is 800 through 6300 amperes.
• The voltage and current abbreviations and names are different, such as:
Ue - application voltage, such as 380 or 690 volts.
In - rated current such as 800 or 6300 amperes.
Ics - rated service short circuit breaking capacity.
Icu - rated ultimate short-circuit breaking capacity.
Icw - rated short time withstand current (similar to the ANSI short time rating and the circuit breaker is expected to function properly again after having dealt with the Icw).

ANSI and IEC Example
Let's make a quick comparison from a presentation standpoint. Keep in mind, the important things are:
1. Will the circuit breaker being considered do the job?
2. Will the circuit breaker being considered meet the standards in effect where the circuit breaker is to be used?
It is not possible to simply take a product designed and tested to one standard (ANSI or IEC) and certify it to the other standard. A manufacturer must undertake a concerted design effort to satisfy both standards individually.

This is by far not an all inclusive example. It is only intended to draw some simple ANSI and IEC comparisons between some of the most common selection points that must be considered when selecting low voltage power circuit breakers. You can see that although not exactly the same, it is primarily a matter of familiarization.

3. Operation Method - As discussed earlier, low voltage power circuit breakers are either manually or electrically operated. You must always specify the method of operation. At some point, you will need to know the secondary control voltage being used for an electrically operated circuit breaker. Even if the circuit breakers are manually operated, it is a good idea to find out the secondary control voltage. The control voltage is necessary for the final selection of a number of items, not just electrically operated circuit breakers.
4. Accessory Items - Many of the common accessory items associated with low voltage power circuit breakers were discussed earlier. You have to be alert for these items in a specification or ask the customer if any are required. A determination can then be made if a compatible accessory is available to meet the need. Make a list of the required accessories and the specific requirements that are appropriate for them, such as control voltage, number and types of contacts and overall function.
5. Mounting Method - You will need to know whether the breakers will be fixed mounted or drawout. Always check to see if there are any special requirements for either configuration.
Special Selection Factors
There may not be special conditions to consider, although this should be determined as soon as possible. You may be able to deal with certain special conditions and others might call for assistance from the manufacturer. Do not hesitate to ask for help. Some conditions or requirements to look for that might not be considered standard are:

• High or low ambient temperatures
• Moist or corrosive atmospheres
• Altitude
• High shock conditions
• Unusual circuit breaker mounting conditions