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Innovative Thinking

Types of Circuit Protection

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Circuit protection should always be used to prevent damage to systems in the case of a power surge or a short circuit. Buildings have breaker boxes that help protect and distribute power. Plug strips and other devices fed from these breakers have their own protective components to provide local control and protection for the devices plugged into them. In control panels, protective devices are usually the first device in the circuitPower is then safely distributed to all other components in the system, and there are usually smaller protective devices further on in the panel for protection of specific devices. There are different types of protection for different loads and applications. 

There are two types of circuit protection which cover the two aspects of power, overcurrent, and overvoltage protection. Overvoltage protective devices protect from voltage surges from events such as static electric discharge all the way up to lightning strikes. Some examples of voltage protective devices are: 

  • Metal Oxide Varistors 
  • Transient voltage suppression diodes 
  • Protective thyristors 
  • Discharge suppressors 

In most cases, control panels don’t need overvoltage protection because the power that feeds them is “clean power” which means there aren’t significant voltage fluctuation issues. 

Overcurrent devices are much more common in control panels, and usually there are multiple types in a single panel. Below are some examples of overcurrent devices: 

Fuses: Small devices that are sacrificed during a single overcurrent event. Need replaced afterwards. Comes in many current ratings, and trip ratings (Slo-blo, fast acting, time delay, etc.) 
Circuit Breakers: An automatic electrical switch that opens, or trips, when overcurrent is detected. After one of these events the breaker can be reset but remember that a breaker is only rated for so many overloads and short circuit trips. Like fuses these come in a wide variety of sizes, but have many more accessories like lug kits, extra contacts, and disconnects 
Thermal Overload Devices: 
  • MSP (Motor Starter Protector)- Like breakers except they have a thermal overload release that prevents the circuit from closing until it returns to an acceptable temperature. Has an adjustable trip setting. 
  • Overload Relays- Used after a contactor and has a thermal overload release just like the MSP 


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