What is the Best Circuit Protection for Motor Circuits?

Summary: The choice of overcurrent circuit protection depends on various factors.  The right choice depends on the application and functions required.

Introduction

Many, many years ago, there were two overcurrent protection options... breakers and fuses.  Over time new circuit protection options have appeared on the scene.  Millions of electric motors are in operation in the US, and each one needs overcurrent protection.  These circuit protection devices play a very important role, not only in opening motor circuits, but also in how the control panel is designed, built, installed, and operated.  In this article we will cover the main options, their main benefits and disadvantages, as well as where they often function the best.

What are the options?

There are several different types of circuit protection used in motor control circuits, each bringing its own mix of value and use.  The following are the most popular:

  1. Fuses - These devices use a self-sacrificing approach in the event of a sustained or serious overcurrent event.  This is the source of their greatest strength and greatest weakness.  Fuses can operate fast, especially in a high short-circuit event.  However, they must be replaced when they open the circuit.
  2. Molded Case Circuit Breakers - These devices employ a mechanical means to open a circuit and are resettable.  They come in a wide variety of sizes and ratings but may also have a higher purchase price.
  3. Miniature Circuit Breakers - These devices are similar to molded case circuit breakers but are in a compact size.  While smaller, they are also have limited ratings compared to molded case circuit breakers.  Miniature circuit breakers can be rated to UL489 like molded case circuit breakers, but be sure to apply these correctly.  Many miniature circuit breakers have a version for branch protection and a version for supplementary protection. 
  4. Type E Combination Motor Controllers - These devices (or assemblies) are comprised of UL 60947 (formerly 508) motor control devices, which in of themself are not rated to provide overcurrent short-circuit protection. However, these devices can be approved by UL to serve this function when tested as a Type E combination motor controller.  You may also find these listed as a Type F combination motor controller, which most often adds a contactor to the mix. Type E/F combination motor controllers are compact and very cost effective, but may have limited voltage, current and short-circuit ratings compared to circuits using fuses or breakers.
There are other circuit protection derivatives that provide unique features, or combine the best of two worlds.  I consider these the extended family of circuit protection.
  • Motor Protective Circuit Breakers - These devices are the marriage of molded case circuit breakers and motor overload relays.  They eliminate the need for a separate breaker and overload relay. Many motor protective circuit breakers also have very wide motor full load amp setting ranges, which help reduce the number of part numbers needed for a panel builder.  While motor protective circuit breakers require fewer devices to install and wire, they may be larger than other circuit protection types depending on the size.
  • Motor Circuit Protectors - These devices are a cousin to the molded case circuit breaker, in that to the casual observer they look very similar.  However, motor circuit protectors have a unique twist.  They often have adjustable magnetic ranges to optimize sizing for motor in rush.  Motor in rush is the temporary spike in current during motor start up.  This spike has the potential to cause a trip in a standard molded case circuit breaker.  One other important difference to note is that the thermal overload protection is often removed in motor circuit protectors.  They are intended to be used with motor starters (such as contactor + overload relay) where the thermal protection is provided by the motor overload protection device.  As such the NEC requires motor circuit protectors only be used as per an approved combination.  In terms of UL that would be a Type D combination motor controller.


🏆 Which one is best? 

There is no universal single right answer to this question.  Each application has a set of functional requirements. Each designer, installer, operator, and troubleshooter has their own set of needs as well.  Your choice may vary from application to application.  Here are some common use cases though to help you see where they shine.

  • Purchase Price - this is often a key priority of those that build panels.  You will find different opinions and options in this space.  Some like fuses due to a perceived low purchase price.  Miniature circuit breakers could be a good fit here also.
  • Uptime - If this is important to you then reset-ability is a key feature.  All of the overcurrent protective devices are resettable except for fuses.  However, a short-circuit event can also mean the contactor or overload relay devices will need to be replaced, even though the overcurrent protective device did its job of opening the short circuit.  If no downtime is important to you, consider using a Type E device, or an overcurrent protection device type used in an assembly that has Type 2 coordination.  These are rated to clear the short circuit without sustaining damage to any of the devices.
  • Space - If you are limited on space, or don't want to end up with a huge industrial control cabinet, then consider starting with a Type F combination motor controller.  In most cases this offers a complete motor branch circuit in two core devices.  However, many designs are limited to 65 amps.  If you have larger motors, consider motor protective circuit breakers.  These will have the same two-device approach as Type F assemblies, but cover much higher amperages.
  • Wiring Reduction - Because wiring can be time consuming to install and troubleshoot, some will appreciate devices that reduce or eliminate it.  The most popular option for this is the Type F combination motor controller and miniature circuit breakers.  Most manufacturers offer busbar accessories that quickly distribute power to adjacent devices.
  • High SCCR - The overcurrent protection used likely will depend on the level of SCCR desired to be achieved.  Control devices often have 100kA SCCR with the use of fuses, and some have 100kA with molded case circuit breakers and others.  However, this level is not often required and may result in higher material costs or a larger control panel. There are many options that reach 65kA, but you likely may find inconsistency across the range of a particular offer where the SCCR is lower.  In the case of SCCR, you may need to consider different offers from different manufacturers to find the one that best fits your application.
Some situations may ultimately dictate the circuit protection used, regardless of your application needs or preferences.  For example, an adjustable frequency drive may require the use of a certain fuse or specific circuit breaker part number.

Know your application

Given there is no single universal choice for best circuit protection for motors, it is important to know ahead of time what is most important to you and your business.  Having a good understanding of your customers' needs, equipment functional needs, code requirements, and financial needs is key.  Once you are clear on the priorities, choosing the right circuit protection for your motor becomes much clearer.

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