Circuit Protection Options in Security and Fire Alarm Systems | #ESC_LLC #PTC #Fuse #Power #LifeSAfety | PTC Resettable fuses offer circuit protection for security and fire alarm systems to meet changing design needs. Security and fire alarm systems can frequently encounter high fault currents and cause potential safety hazards.
These potentially fatal overcurrent events can originate from improper system installation, inappropriate replacement of back-up batteries, or deliberate external shorting of sensors or wiring in an attempt to disable the system. These system vulnerabilities force designers to choose a circuit protection solution from options such as: non-resettable fuses, circuit breakers, and PTC resettable fuses.
Standard non-resettable fuses are typically seen as the solution to such fault currents. Their ability to provide precise and accurate protection, at a low component cost, makes them an obvious choice. The disadvantage is that each time the fuse blows it requires replacement, unlike circuit breakers or PTC resettable fuses that are used repeatedly in a circuit.
Circuit breakers are also seen as a known technology that provides exact calibrating for a specific reaction in a given environment. It’s ability to be manually reset may also be desired in a given fault range to allow for further investigation of the fault origin before resetting. The disadvantages to using circuit breakers is that they are generally a more expensive solution and more difficult to package.
The answer for many security applications today is resettable fuses. Growing in popularity over the past 10 years, resettable fuses have become a preferred solution. These overcurrent protection devices are placed in line with each power bus in fire alarm or security systems to protect the power supply from a fault current event.
Resettable fuses are best described as Polymer Positive Temperature Coefficient (PPTC) fuses and are made from a conductive polymer that is very low in resistance during normal circuit operation. When a fault condition occurs, they heat up and exhibit a logarithmic, step-like increase in resistance. This increase in resistance limits the current flow to the power supply and protects the circuit from damage. The devices remain in the high resistance state until the fault condition is removed. Once the fault is removed, the devices reset to their low resistance state allowing the circuit to operate normally.
The resettable characteristic of the PPTC over non-resettable fuses reduces the warranty cost to the manufacturer by preventing unnecessary returns or nuisance service calls, and increases system reliability by eliminating system down-time. Several regulatory agencies have developed standards that require these systems to be able to withstand overload and endurance testing without damage.
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