Electrical EquipmentElectricity 101

What are Circuit Protection Devices

Circuit protection devices automatically prevent dangerous or excessive amount of current or a short circuit in an electrical conductor. This article will look at the various circuit protection devices available so you can figure out which one will work best in your electrical application.

How Circuit Protection Devices Work

When thinking about circuit protection, there is a certain measure of electron flow through a conductor. If the electron flow is excessive, it can melt and burn the wire insulation and surrounding materials. Every wire has an allowable amperage that determines the electron flow that can safely pass through the wire. However, voltage drop must also be a consideration. This is the amount of voltage consumed as voltage pushes the amperage through the resistance of the wire. Sometimes the allowable amperage will determine the size of wire needed and other times this will be determined by voltage drop. Whatever the case, the wire must be the larger of the two.

Even if the wire is properly sized, the circuit can be accidentally grounded and allow a dangerous amount of amperage to flow. If a circuit is grounded by a wire’s insulation, chafing can occur due to the failure of equipment or accidental grounding during maintenance. If this is the case, circuit protection devices can break the circuit before any damage is done.

Types of Circuit Protection Devices

There are many types of circuit protection devices. These include the following:

Circuit Breaker: This is an automatically operated electrical switch which can protect an electrical circuit from damage caused by excess current which can result in an overload in the circuit. It works by interrupting the current flow after a fault is detected. Unlike a fuse, which has a similar function, circuit breakers can be reset and reused to resume normal operation. They are made in varying sizes including small devices that protect low current circuits on household appliances to large switchgear that can protect high voltage circuits.

Circuit Breakers can be found in the following applications:

  •         Home power
  •         Lighting
  •         Appliances
  •         Power supplies
  •         Uninterruptible power supplies
  •         Power tools
  •         High inrush motors
  •         Transformers
  •         Reactive loads
  •         Medical diagnostic appliances
  •         HVAC

Electronic Fuses: Electronic fuses are low resistance resistors that provide protection in the event of a load overcurrent caused by device failure or overloading. When it detects an overcurrent, the metal wire in the fuse melts causing an interruption in the circuit. Fuses are available in a variety of types and sizes including industrial electric fuses, slow blow fuses, electrical high voltage fuses or electronics fuses.

Electronic Fuses can be used in the following applications:

  •         Laptops
  •         Cell pones
  •         Game systems
  •         Printers
  •         Digital cameras
  •         DVD players
  •         Portable electronics
  •         LCD monitors
  •         Scanners
  •         Battery packs
  •         Hard disk drives

ESD Protection and Diode Array Devices: ESD or electrostatic discharge, is an energy transfer between two objects that are differently charged. This can short out or cause damage to electronic devices making ESD circuit protection devices necessary. A diode array is an electronic component that consists of several diodes in a semiconductor package where the diodes may share a common cathode of anode, or where each may be separate. There are several different kinds of ESD protection and diode arrays which can be categorized by the number of channels, directional type, maximum breakdown voltage, maximum clamping voltage, capacitance, operating temperature range and packaging type.

ESD and Diode Arrays can be found in:

  •         LED Printing
  •         Scanners
  •         Flashlights
  •         USB power and data line protection
  •         12C bus protection
  •         Video line protection
  •         Portable electronics
  •         Microcontroller input protection
  •         WAN/LAN equipment

Fuse Holders: Fuse holders can be just as important as fuses when it comes to circuit protection devices. They are used to contain, protect and mount fuses. They come in two basic types, open or fully enclosed. Open fuse holder types include fuse clips, fuse blocks, socket and plug on cap varieties. The fully enclosed variety may use a fuse carrier that is inserted into a holder or have other means to fully enclose the fuse.

Fuse Clips and Blocks: These are designed to provide a tight enclosure between the clip and the actual fuse.

Gas Discharge Tubes: This is a glass enclosed circuit protection device that contains a special gas mixture that is trapped between two electrodes. They conduct electrical current after they become ionized by a high voltage spike and they can conduct a relatively high amount of current for their size. Because of this, they can handle very large transients or several smaller transients. They take a long time to trigger permitting a higher voltage spike to pass through before conducting a significant amount of current.

These are commonly used on high frequency lines like in telecommunications equipment and can also be used in industrial and consumer electronics like in surge protectors and alarm systems. Other applications include instrumentation circuits, power supplies and medical electronics.

Power Thyristor: A thyristor or silicon controlled rectifier (SCR) is a solid state component used to switch and control electric current flow. They are often used in high current flow applications. They conduct current when they receive a certain voltage on its gate terminal and continue to conduct current even after the voltage is removed from the gate terminal. Because of these characteristics, and their wide power rating range, they are used as current controllers.

Power thyristors are used in applications where high voltages and currents are present. They are also used to control alternating currents and can be used as the control elements of phase fired controllers.

Protection Thyristors: These are solid state components that can be used to switch and control electric current flow. Because they are so robust, they are often used in high flow current applications. They start to conduct current when they receive a certain voltage on its gate terminal and continue to conduct current after that voltage is removed. Because of these characteristics, as well as their wide protection rating range, they are used as current controllers for overvoltage protection.

Protection Thyristors can be used in the following applications:

  •         Temperature control
  •         Light control
  •         Speed control
  •         Process control
  •         Warning systems where reliability of operation is important

Resettable Fuses: Also known as PTCs, these circuit protection devices will come in handy if your circuit tries to draw more than 250mA of current. If this happens, the PTC heats up and trips, allowing only a small leakage current. The leakage may still cause damage to some electronics, but it minimizes it as compared to the 3A the short would have allowed. Once the short is removed, the PTC resets, allowing for current up to 250mA to flow again. It is good for use on battery powered devices that need to protect against high current accidental discharges.

TVS Diodes: A TVS, or transient voltage suppression diode, works to divert or shunt voltage spikes from a circuit as a protective measure. They are also used to protect circuits from electrostatic discharge. Silicon-avalanche diodes (SAD) and Zener diodes are commonly used for circuit protection in TVS applications. SADs see temperature increase as voltage decreases whereas Zener diodes see a temperature increase as voltage decreases.

TVS Diodes can be found in:

  •         Computers
  •         Printers
  •         Wireless communication systems
  •         Medical equipment
  •         Security systems
  •         ATM machines
  •         HFC systems
  •         Network systems
  •         FireWire applications

MOV/MLV Varistors: A varistor is a resistor that has a non-ohmic current; They act as a spark gap, protecting circuits from excessive voltage. An MOV (metal oxide varistor) is the most common type of varistor, containing a mass of zinc oxide grains in a matrix of other metal oxides and sandwiched between two metal plates that act as electrodes. A multi-layer varistor, or MLV, provides electronic circuits with electrostatic discharge protection.

MOV and MLV varistors can be used in a variety of electronic applications including the following:

  •         Computers and peripherals
  •         Set top boxes
  •         Cell phones
  •         Digital still cameras
  •         Medical equipment
  •         DVD players
  •         Printers, scanners and copiers
  •         Multimedia players
  •         External storage
  •         LCD monitors
  •         Modems

There are several types of circuit protection devices that can keep your electronic equipment safe from damage in the event of a short circuit or overcurrent. It is a good idea to familiarize yourself with the different types of CPD’s available so that you can make every effort to avoid electrical hazards. Good luck keeping safe while working in your home or office.

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