Because special considerations need to be taken when using circuit breakers with motors, we will dedicate this section on their particular characteristics and applications.
Most faults on a motor circuit are caused by a breakdown of the insulation within the motor windings. The initial fault current is usually low when compared to the overall system capacity. However, because it causes an Arcing condition, it could cascade and short out more and more of the motor windings. If the fault is allowed to continue, serious motor and starter damages occur, increasing repair costs. Although fusible switches and thermal magnetic breakers can provide motor branch circuit protection, the level of protection is not as effective against this type of fault.
For this reason, the motor circuit protector was developed. A motor circuit protector (MCP) operates on a magnetic only principle. It has a specially designed current sensing coil in each of its three poles to provide sensitive low level protection. It can clear a fault faster than a fusible device. It does not, however, provide overload protection for the motor. As a result, a contactor with an overload relay or motor starter must be used in conjunction with the motor circuit protector. (See Module 19 for information on contactors, overload protection and starters.)
Fuse vs. Circuit Breaker
This chart shows the typical fault Clearing Time (Fuse) of a dual element fuse on Low Level Faults. It required from 12 to 84 cycles to clear the fault. This test was based on a 3 hp motor with an FLA of approximately 4.2A. The short circuit was 150A, or 35 times the full load current. The fusible device had a 5A dual element fuse, sized at 120% the FLA. The NEC allows fuses to be sized up to 175% of motor full load current.
Fault Clearing Time of Dual Element Fuse
When an MCP rated at 7A was used, it was able to clear the fault in less than one cycle. It was set to trip at 51A, or approximately 12 times the full load current. The MCP also did not create a single-phasing condition, whereas the dual element fuse did. The fuse cleared the fault in phase B in only 12 cycles, but did not clear it in phases A and C until 22 cycles had elapsed.
Fault Clearing Time of MCP
In a single-phasing condition, the current to a motor in the remaining phases increases significantly above normal. This can lead to severe equipment damage. Because the motor circuit protector cleared the fault in all three phases in less than one cycle, the motor failure did not result in starting an electrical fire.
Components
The components of motor circuit protectors are very similar to molded case circuit breakers. They are:
• Molded Case
• Operating Mechanism
• Arc Extinguishers
• Contacts
• Trip Mechanism
• External Adjusting Mechanism
Motor Circuit Protector Components
How It Operates
Motor circuit protectors disconnect the motor load from an electrical supply under three conditions. They are:
• When the handle is switched to OFF.
• When an automatic trip operation occurs.
• When a manual trip is initiated with a push-to-trip button.
As with the molded case circuit breakers, the operating mechanism is a spring-loaded toggle that provides quick-make, quick-break and trip-free operation.
Its design provides an increased air gap between the stationary and moveable contacts when in the tripped position. This air gap results in greater arc extinguishing during contact opening and provides higher interrupt ratings.
The magnetic trip unit operates when a fault current exceeds the magnetic pickup setting. It consists of an electromagnetic coil and plunger assembly. Certain HMCPs also have a transient inrush trip suppression device. This allows the startup of energy efficient motors without nuisance tripping the sensitive short circuit protection of the current sensing coil.
A tuned spring introduces a time delay of approximately 8 ms into the trip sequence under normal conditions. It allows the HMCP to ignore the initial high inrush current during the first half-cycle of start-up. A true fault current would supply a magnetic force to override the spring action and provide instantaneous tripping of the device.
Transient Inrush Suppressor
Larger HMCPs (600A) and above may use electronic trip units to provide an adjustable three-phase instantaneous trip setting. At start-up, an electronic time delay acts as the inrush trip suppressor. However, any current in excess of the predetermined setting level, such as with a short circuit, would override the time delay and trip the HCMP.
A trip setting adjustment allows for precise motor protection. Press in on a cam and turn the arrow until it is aligned with the required trip setting shown on the nameplate.
However, in keeping with NEC requirements, they cannot be set at more than 1300% of the motor full load current rating.
Trip Setting Adjustment
Applications
Motor circuit protectors can be used in combination starter units within a motor control center. They allow for protection against both low and high level fault currents without requiring current limiters. They can also be applied in standalone combination starters.
When properly sized, they can provide short circuit protection for resistance welding devices. The normal high welding currents can flow, but the HMCP trips instantaneously if a short circuit develops.
HMCPs can be used in panelboards. You can have both distribution branch circuit protection and protection of the motor circuits within the same enclosure.
Most faults on a motor circuit are caused by a breakdown of the insulation within the motor windings. The initial fault current is usually low when compared to the overall system capacity. However, because it causes an Arcing condition, it could cascade and short out more and more of the motor windings. If the fault is allowed to continue, serious motor and starter damages occur, increasing repair costs. Although fusible switches and thermal magnetic breakers can provide motor branch circuit protection, the level of protection is not as effective against this type of fault.
For this reason, the motor circuit protector was developed. A motor circuit protector (MCP) operates on a magnetic only principle. It has a specially designed current sensing coil in each of its three poles to provide sensitive low level protection. It can clear a fault faster than a fusible device. It does not, however, provide overload protection for the motor. As a result, a contactor with an overload relay or motor starter must be used in conjunction with the motor circuit protector. (See Module 19 for information on contactors, overload protection and starters.)
Fuse vs. Circuit Breaker
This chart shows the typical fault Clearing Time (Fuse) of a dual element fuse on Low Level Faults. It required from 12 to 84 cycles to clear the fault. This test was based on a 3 hp motor with an FLA of approximately 4.2A. The short circuit was 150A, or 35 times the full load current. The fusible device had a 5A dual element fuse, sized at 120% the FLA. The NEC allows fuses to be sized up to 175% of motor full load current.
Fault Clearing Time of Dual Element Fuse
When an MCP rated at 7A was used, it was able to clear the fault in less than one cycle. It was set to trip at 51A, or approximately 12 times the full load current. The MCP also did not create a single-phasing condition, whereas the dual element fuse did. The fuse cleared the fault in phase B in only 12 cycles, but did not clear it in phases A and C until 22 cycles had elapsed.
Fault Clearing Time of MCP
In a single-phasing condition, the current to a motor in the remaining phases increases significantly above normal. This can lead to severe equipment damage. Because the motor circuit protector cleared the fault in all three phases in less than one cycle, the motor failure did not result in starting an electrical fire.
Components
The components of motor circuit protectors are very similar to molded case circuit breakers. They are:
• Molded Case
• Operating Mechanism
• Arc Extinguishers
• Contacts
• Trip Mechanism
• External Adjusting Mechanism
Motor Circuit Protector Components
How It Operates
Motor circuit protectors disconnect the motor load from an electrical supply under three conditions. They are:
• When the handle is switched to OFF.
• When an automatic trip operation occurs.
• When a manual trip is initiated with a push-to-trip button.
As with the molded case circuit breakers, the operating mechanism is a spring-loaded toggle that provides quick-make, quick-break and trip-free operation.
Its design provides an increased air gap between the stationary and moveable contacts when in the tripped position. This air gap results in greater arc extinguishing during contact opening and provides higher interrupt ratings.
The magnetic trip unit operates when a fault current exceeds the magnetic pickup setting. It consists of an electromagnetic coil and plunger assembly. Certain HMCPs also have a transient inrush trip suppression device. This allows the startup of energy efficient motors without nuisance tripping the sensitive short circuit protection of the current sensing coil.
A tuned spring introduces a time delay of approximately 8 ms into the trip sequence under normal conditions. It allows the HMCP to ignore the initial high inrush current during the first half-cycle of start-up. A true fault current would supply a magnetic force to override the spring action and provide instantaneous tripping of the device.
Transient Inrush Suppressor
Larger HMCPs (600A) and above may use electronic trip units to provide an adjustable three-phase instantaneous trip setting. At start-up, an electronic time delay acts as the inrush trip suppressor. However, any current in excess of the predetermined setting level, such as with a short circuit, would override the time delay and trip the HCMP.
A trip setting adjustment allows for precise motor protection. Press in on a cam and turn the arrow until it is aligned with the required trip setting shown on the nameplate.
However, in keeping with NEC requirements, they cannot be set at more than 1300% of the motor full load current rating.
Trip Setting Adjustment
Applications
Motor circuit protectors can be used in combination starter units within a motor control center. They allow for protection against both low and high level fault currents without requiring current limiters. They can also be applied in standalone combination starters.
When properly sized, they can provide short circuit protection for resistance welding devices. The normal high welding currents can flow, but the HMCP trips instantaneously if a short circuit develops.
HMCPs can be used in panelboards. You can have both distribution branch circuit protection and protection of the motor circuits within the same enclosure.
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