Circuit breaker is the most common short circuit and over current protection. It is a simple electrical device but very important to safety. The most common question is what must be the current rating of the breaker to safely protect devices during abnormal condition. Basically, this is not the only thing to consider in selecting a circuit breaker. Below are some inputs on how to select a circuit breaker.
1. Setting the Typical Breaker Current Rating
The first thing to consider in selecting circuit breaker is the typical trip current. A word typical is referring to nominal conditions. Nominal condition is considering and ambient temperature of 20C-25’C. Also, it will refer to the usual level of the voltages in the circuit and not considering yet the effect of any tolerances.
Why emphasizing the typical condition? Is there a scenario other than this?
The answer is yes. The ambient temperature where the breaker installed could vary. The breaker electrical properties will be affected by too high or too low temperatures. Extreme temperatures are mostly a concern of an outdoor applications. It is also a concern in any application where the breaker is exposed to a higher environment temperature.
Line voltage is also not ideal. For instance, in 480Vac 3phase line, it can go as high as +10% and as low as -10%.
For an ordinary people, these explanations could be too much. So, it is a good idea to stick to the typical conditions and from it add some margin.
The Rule of Thumb
Size the typical breaker current rating using an 80% utilization. This means, when the expected maximum current of the circuit where the breaker to be installed is 100A, then the breaker current rating must be 125A (100A/80%). In other words, always add 25% margin (100A X 1.25 = 125A).
This ensures that the breaker tripping current is not so close to the circuit typical current to avoid false tripping. With this setting also, the circuit in series with the breaker do not need to be rated with too high current.
This selection strategy is somewhat related to the fuse. In some cases, fuse selection is based on 75%-80% utilization or 25%-33% margin. For more about fuse selection, read How to Select a Fuse for Specific Application.
2. Voltage Rating
Circuit breaker is a device use to protect current related abnormality. Then, why care about the voltage rating in circuit breaker selection?
For breakers, the voltage rating is also associated to safety. This rating guarantees that a circuit breaker can safely trip during any fault condition. The breaker voltage rating must be at least the same or higher than the open circuit voltage. If the voltage rating is low, arcing or explosion may occur when contacts open especially when there is short circuit.
So, for instance the application is to put a breaker in series to 480Vac, the voltage rating must be at least 480Vac. 480Vac rated breakers are also tested to +10% or even higher during component development. This information is available in the datasheet.
3. Short Circuit Current Rating or Interrupting Capacity (unit in kAIC)
Another important to consider in circuit breaker selection is interrupting rating or the short circuit current rating. This is the maximum short circuit current the breaker can safely interrupt at a certain standard test conditions.
What will happen if the actual short circuit current is higher than the interrupting rating?
The breaker short circuit current rating is ideally higher than the actual short circuit current. Otherwise, there is no guarantee of safe interruption. I have personal experience about this where arcing happened during short circuit with a breaker interrupting rating not enough.
To select this rating properly, it is important to have idea on the level of the fault current. This is possible by doing deep analysis. For a breaker in the AC line, like 480Vac, an SCCR of 65kAIC is a good value.
For branch circuit, the local SCCR requirement must be known. Most common, 20kAIC is already sufficient.
If in doubt, consult an expert!
Below table is taken from Eaton datasheet of family of breaker. It shows the interrupting capacity at 480Vac.
Below is a good explanation of breaker interrupting ratings.
4. Operating Temperatures
Operating temperature is important. If the target maximum ambient temperature where to use the breaker is 50C, the breaker must at least can handle 50C. On the other hand, if the minimum ambient temperature of application is minus 30, then the breaker must be rated to at least minus 30C. Failure to follow this requirement, the breaker may trip ahead or late as set.
During breaker selection, also check the current versus operating temperature curve. Breakers current rating will de-rate or decrease in level at higher operating temperature. It is very important to select a breaker that has very good de-rating curve. Which means, the current will only de-rate slightly. Otherwise, the 80% utilization or 25% margin setting for typical current may not anymore work at higher temperatures.
5. Ability to Detect Ground Fault
Ordinary circuit breaker does not have built-in ground fault circuit interrupter (GFCI). GFCI will add extra protection against faults to ground. GFCI is sensitive to ground faults, usual trip level is up to 30mA. This is a safe level of current thus ensure human to be protected from electrocution.
6. Safety Certificates
To ensure a qualify breaker, consider buying the one with safety compliance certificates like from UL, CE, CSE or CCC. Below is an example from Eaton.
7. Line Configuration Specific
AC grid can be configured as wye (Y) or delta (star). In most cases, breakers can be used in both. There are however few instances that a breaker is rated at voltage level at wye configuration only. To cite an example, a breaker is rated at 400Vac, but it can be used up to 480Vac considering wye type of grid only. Always refer to the datasheet or consult the vendor.