1 Rated short-circuit breaking current2 Generated short-circuit current 3 The rated working sequence of the circuit breaker 4 Intended for use with short-term current.
The four fault ratings of a circuit breaker are: 1 Rated short-circuit breaking current 2 Generated short-circuit current
Short-circuit breaking capacity is expressed in amperes and refers to the maximum load that will not cause the circuit breaker to open its contacts. Short-circuit current is the highest rate of current flow through any section of a circuit breaker's path as long as the circuit breaker remains closed. The term "rated" means that if the circuit breaker was used on a circuit carrying more current than specified, it would likely fail prematurely due to overheating or other problems caused by the higher current load. Circuit breakers are designed to handle their rated values for no longer than an hour and usually less than 10 minutes. In general, the larger the breaker, the lower its operating temperature will be and the longer it will last before requiring replacement.
A circuit breaker's short-time current rating is its capacity to resist the effects of the rated short-time current level over a certain time period. It exhibits the breaker's capacity to remain closed for a certain period of time under high fault current circumstances. The shorter the time, the higher the rating. For example, a 200-amp, 15-minute breaker would be more resistant to burning out than a 100-amp, 60-minute breaker if both breakers opened at the same current level.
The short-time rating should be used with care. It assumes that the breaker will open after the short time and allow the current to pass through it. If it does not, then it has failed its intended purpose and should be replaced before it causes damage to itself or other equipment.
Short-time ratings are usually indicated on circuit breaker boxes as well as on each breaker itself. They vary from manufacturer to manufacturer but can be found in the instructions manual that comes with the product.
For example, a 200-amp, 15-minute breaker would mean that it could withstand opening and closing 200 times without burning out. This means that if the breaker did not open after being subjected to this amount of current for 15 minutes, then it should be replaced so that it can perform its function properly.
Circuit breakers are rated based on the typical current they are anticipated to carry as well as the maximum short-circuit current they can safely interrupt. Testing determines the maximum short-circuit current that a breaker can interrupt. The two main types of testing are magnetic and thermal. Magnetic testing involves using a magnet to determine the strength of the magnetic field that would be produced by any wiring defects or other problems with the breaker. Thermal testing uses heat sensors to check for broken wires or other damage to the breaker.
Magnetic testing is generally done when you buy a breaker. If there are any indications that it may not be fully functional, such as a weak magnetic pull, you should ask why this breaker is less than 100 percent effective. A reduced magnetic force could be caused by corrosion of the wire inside the wall, which can lead to an open circuit if enough of the wires are exposed. An open circuit cannot be detected by measuring the magnetic force so this type of problem can only be found by performing a thermal test.
Thermal testing is usually done when you have already purchased and installed your circuit breaker. You will need a voltmeter and some thin wire (with a minimum insulation thickness of 14 AWG or larger).
The circuit breaker's rating is determined by the functions that it performs. This refers to enhancing the circuit breaker's capacity. It must be capable of securely carrying the problem for a limited period of time until the other breaker clears the issue. This refers to a circuit breaker's short-term capacity. A general rule is that the larger the number, the greater the capacity.
There are two types of circuit breakers: magnetic and electrical. Magnetic circuit breakers use a magnetic force to open the circuit, while electric circuit breakers use an electrical signal to open the circuit. The rating of a magnetic circuit breaker will indicate the maximum load that it can handle without opening the circuit. The rating of an electric circuit breaker will indicate the maximum load that it can handle before shutting off the power completely.
For example, if you were to connect a motor load to a circuit breaker with a 20 amp rating, the circuit breaker would likely fail soon after you connected the motors because more than 20 amps will try to go through one circuit breaker at a time. If another way could be found to feed the motor, such as using two 10 amp circuits instead of one 20 amp circuit, this would allow the first circuit breaker to carry its full load and not trip. A motor load with a 40 amp rating could then be plugged into the second circuit without any problems.
Magnetic and electric circuit breakers have different requirements in terms of voltage and current they can handle.
All circuit breakers have a current rating (In), which is the amount of current that may be carried continuously by the breaker. The curve type (B, C, D, and so on) specifies the instantaneous trip current range, or the amount of current at which the breaker will trip without generating a time delay. For example, a 20-amp circuit breaker with a curve type of B would give a 20-minute trip time if subjected to a surge current of 2 amps for 1 minute followed by a normal load current of 15 amps for 19 minutes and 49 seconds.
The term "curve" comes from the fact that these currents are rated in terms of how long they can be carried before the breaker will act to disconnect the circuit. A circuit breaker's current rating is found on its label or sticker. Modern breakers use a symbol system to indicate their current rating: a lowercase letter indicates a range of current values, while an uppercase letter indicates the highest current value within that range.
This means that the breaker can carry a maximum current of 26 amps for 20 minutes before it acts to disconnect the circuit.