Circuit Breakers: Overview and Applications

Circuit breakers are fundamental elements for a safe and code-compliant electrical installation. Conductors and electrical equipment are exposed to damage and malfunction, and there is always a risk that someone will connect a device incorrectly or use it for the wrong application. These conditions can cause a device to draw current above its rated value and the corresponding circuit breaker to trip to disconnect the fault.

Before providing an overview of circuit breakers, it is important to understand the difference between the two main current conditions that cause a circuit breaker to trip.

• An overload current occurs when a device draws current above its rated value, but not by a drastic margin. For example, a motor rated at 60 Amps but consuming 75 Amps is likely experiencing an overload condition.
• Fault current is orders of magnitude higher than the rated current of a circuit and occurs when an energized conductor touches another with a different voltage (short circuit) or a conductive surface (earth fault). There is a high magnitude current in both cases, since the low resistance contact is established through a voltage difference. For example, a residential circuit that normally carries 20 Amps may experience a few thousand Amps during a fault.

A circuit breaker should trip under both conditions, but the ideal trip response is different for each case:

• The response to an overload current must have a delay. Some types of equipment draw current above their rated value for short periods of time as part of their normal operation. For example, electric motors consume a starting current of up to 8 times the rated current when they start.

• The response to a fault current must be instantaneous. These currents are not normal under any operating conditions and must be eliminated immediately when detected.

Given this combination of performance requirements, most circuit breakers actually have two protection mechanisms in a single device. There is a thermal protection mechanism that responds to overload current and a magnetic protection mechanism that responds to fault currents.

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Thermal and Magnetic Protection

The thermal protection mechanism in a circuit breaker is based on an expanding contact: the circuit is broken when the contact expands beyond a certain point. The circuit breaker is calibrated so that the contact does not open below the rated current, but any current condition exceeding it may eventually cause a trip. Since current is the heat source that expands the contact, more severe overload conditions cause faster expansion and shorter trip time.

The magnetic protection mechanism is based on induction. The current passes through a coil inside the circuit breaker, creating a magnetic field that opens the connection. The field is too weak to trip the circuit breaker under normal operating conditions, but high magnitude currents cause a strong magnetic field that forces the circuit breaker open.

Main types of circuit breakers

Most circuit breakers found in residential and commercial buildings are miniature circuit breakers (MCB) or molded case circuit breakers (MCCB). MCBs are more compact as their name implies, but MCCBs are available in much higher current ratings and come with additional performance features. MCBs are typically available with a current rating of up to 100 amps, while MCCBs reach up to 2,500 amps.

You probably won't find MCCBs in small homes and businesses, but they are common in larger buildings, like the multifamily and office buildings found throughout New York City.

Miniature Circuit Breakers

Miniature circuit breakers come in two main versions: DIN rail mountable MCBs can be installed together with other protection and control devices that also use DIN rails, while plug-in MCBs are inserted into load centers with specially designed slots . Keep in mind that DIN rail MCBs are designed for standard rails, while plug-in MCBs only fit corresponding load centers from the same manufacturer.

DIN rail MCBs (left) and plug-in MCBs (right).

Plug-in MCBs have one to three poles, depending on the number of energized conductors in the circuit being protected. DIN rail circuit breakers can have up to 4 poles, to disconnect the neutral conductor along with the energized conductors. Regardless of the type of circuit breaker, it is important to select a circuit breaker suitable for rated current and breaking capacity.

• The current rating is determined by the circuit being protected. Any value above this eventually disables the thermal protection mechanism.
• The breaking capacity is the largest fault current that the unit can interrupt without suffering permanent damage. If a fault exceeds this value, there is a final interrupting capacity where the circuit breaker can still clear the fault, but is permanently damaged. Any fault above the ultimate interrupting capacity cannot be cleared by the circuit breaker and must be handled by a higher capacity protection system connected upstream.

Miniature circuit breakers are also classified into three types based on their response to fault currents: Type B, C, and D. The type determines the threshold where the magnetic protection takes control of the thermal protection, causing an instantaneous trip. The following table describes the response for each type:

For example, if you have a device that draws 400% of the rated current during startup, a Type B circuit breaker will trip, but a Type C or D circuit breaker will not. Another load that draws 800% of the rated current during startup would trip the Type B and C breakers, leaving Type D as the only option.

Molded case circuit breakers

MCCBs are bulkier than MCBs and are available with higher current ratings. Many models also feature adjustable travel settings, allowing for very precise protection response if a specific load requires it.

Some MCCBs also come with a removable trip unit that can be replaced with a smaller capacity unit to retrofit the circuit breaker for a reduced current load. However, you cannot upgrade to a larger trigger unit that exceeds the MCCB frame size.

There are modern MCCBs that do not use the conventional thermomagnetic mechanism, but rather an electronic circuit that measures the current and simulates the trigger response. This allows very precise adjustment of protection settings.

Two MCCB subtypes are designed specifically for the protection needs of electric motors: motor protective circuit breakers (MPCB) and motor circuit protectors (MCP). The main difference is that an MPCB includes thermal and magnetic protection, while an MCP comes with magnetic protection only and needs an external overload relay to offer full protection.

Conclusion

Selecting the right type of circuit breaker is very important to ensure the safe operation of building systems that include electrical components. Undersized circuit breakers continually trip and interrupt equipment operation, while oversized circuit breakers do not provide reliable protection against overload current. If an overload is not stopped, the heating effect can damage the conductor insulation and eventually cause a ground fault or short circuit.

In New York City, your installation must be designed in accordance with the New York Electrical Code, which also includes the National Electrical Code NFPA 70. The New York Energy Conservation Code does not directly affect circuit breaker selection, but the Using more efficient equipment leads to lower current consumption and possibly the use of smaller circuit breakers.