When discussing an overcurrent protection device (OCPD for short), you will often hear people mention a fuse size or circuit breaker size. Are these terms being used interchangeably for the same thing? Is there an actual difference between the two, and if there is, which one do we use for which application? In order to answer this question, we must first understand what each device is and their intended purpose.
A fuse is a type of overcurrent device. It functions to protect the wire or conductors connected to it from overheating and catching on fire and/or melting the insulation in the case of a short. A fuse is usually made of a very small metal filament or wire inside of a casing that can be made of plastic, metal, or even glass. The metal filament or wire inside the case is designed to burn or melt at a certain ampacity to prevent overheating of the wire connected to it. Once this happens, the electrical flow is stopped, since it is now an open circuit. The metal filament inside the fuse is rated to pass just enough current for proper operation without melting. It’s a simple device that has been around for a very long time. Of course, once the metal filament is ‘blown out, you have a broken fuse; so it must be replaced with a completely new one.
Circuit breakers function in a similar way to fuses, and provide the same type of overcurrent protection – however, they are a little more involved. A circuit breaker acts as a switch, but typically has a bi-metallic strip and an electromagnet inside that operates the opening and closing of the device. When the circuit breaker is flipped to the on position during normal operation, current will flow through the electromagnet and out of the terminal where the conductor is terminated. The current flow will magnetize the electromagnet inside the breaker. The electromagnet is designed so if current flow exceeds a certain amount, it will be strong enough to pull the lever of the circuit breaker to the open (off) position. This electromagnetic trip mechanism is used to open circuits instantaneously in the event of a high amount of overcurrent, as would happen in a short-circuit or ground fault.
Most breakers also have a bi-metallic strip inside that is connected to the spring of the handle. When the bi-metallic strip senses a rise in temperature beyond the normal level that the device is intended for, it will open the circuit. This is to protect the circuit from overheating and melting during overloads. There are some more advanced circuit breakers that involve power electronics, that allow more precise control and tuning of when they will trip. Fundamentally, they function the same way. The one big difference between circuit breakers and fuses is that circuit breakers can be reset and used over and over if they trip or open, whereas fuses are disposable.
Circuit Breaker vs Fuses
So, if circuit breakers are reusable, and fuses have to be replaced once they are blown, why would we even consider using fuses anymore? Aren’t circuit breakers the latest evolution in overcurrent protection? Here are a few key aspects that make fuses attractive:
- Fuses trip faster than circuit breakers.
- Fuses are generally much cheaper than breakers.
- Fuses can offer very high interrupting capacity for very little cost. Many standard rated fuses can be rated to interrupt over 200,000 Amps. Circuit breakers capable of this are very expensive and take up a lot of space.
- Fuse offer easy selective coordination without complicated calculations.
Let’s discuss items #3 and #4 above. There are times when high fault-current will exist on a service. This is the case in places with large underground utility systems in high-density areas such as Manhattan in New York. The utility may parallel three, four, or even five transformers together which would provide a single service into the building. Doing this significantly increases the available fault-current the utility will provide to the customer. It can be as high as 200,000 amps in some instances. Switchboards with fused switches are ideal for these types of applications since they are much smaller in size, cheaper and more readily available than circuit breakers, as well as providing a 200,000-amp fault-current rating. A 200,000-amp fault-current-capable circuit breaker would have to be installed in large switchgear, which is much larger and more expensive than conventional switch and fuse switchboards.
The other item to discuss is selective coordination. Selective coordination is the concept of being able to control which overcurrent device will open in an electrical system, in what order. Let’s say we have a 400-amp fuse feeding a 150-amp fuse downstream. This system would be considered selectively coordinated since if a fault were to occur after the 150-amp fuse, that would blow before the 400-amp fuse. This can be important in many situations, as it is ideal to not have a single fuse take out an entire electrical system! NEC 700.27 requires selective coordination for emergency life safety systems. By using fuses, maintaining a 2:1 ratio will always allow selective coordination. As an example, using this method, an 800-amp fuse will coordinate with a downstream 400-amp fuse. Compared to a circuit breaker, it is impossible to tell if an 800-amp circuit breaker would coordinate with a 400-amp circuit breaker, without doing complicated calculations or asking the manufacturer. When designing electrical systems, fuses offer an attractive solution to this issue.
It is recommended to use fuses rather than circuit breakers anytime we have a very high fault current rating from a utility company (100,000-amps or more). Also anytime we need to consider selective coordination or need to lower fault current, or need to gain space within a piece of equipment such as switchgear.