Circuit Protection Solutions for Security and Fire Alarm Systems

Page Contents:   [ Featured Products | Datasheets | Application Notes | Buy Now ]

Security and fire alarm systems are designed for operation within specified current and voltage ratings. If these ratings are exceeded due to short-circuit or voltage transients, components may sustain permanent damage and the equipment may fail. Power supplies and circuit traces must also be protected against faults that may occur during installation or in the case of a shorted back-up battery.

 

Modems are frequently utilized in security systems to automatically alert the fire or police department in an emergency. These telecom lines must be protected from damage caused by lightning strikes, power line crosses or AC power induction. Alarm systems must comply with the UL864 standard, which mandates that soldered-in fuses are not allowed to qualify a power supply as inherently limited. If the system has provisions for connecting to a telephone line, it must comply with UL60950 and TIA 968-A in North America, as well as ITU K.21 in Europe and other parts of the world.

Tyco Electronics offers a broad range of devices suitable for circuit protection in security and fire alarm systems. A number of options are shown in the following circuit diagram.

 

 

Resettable Overcurrent Protection

Fuses have typically been used for overcurrent protection in security and alarm systems. However, UL864 and UL60950 pose difficult challenges for these devices, which can fatigue under certain test conditions. More importantly, fuses are singleuse components that must be replaced after a fault occurs.

Many equipment manufacturers prefer resettable PolySwitch™ device protection. Unlike fuses, these devices do not generally require replacement after a fault event and allow the circuit to return to normal operating condition after the power has been cycled and/or the overcurrent condition is eliminated.

 

Overvoltage Protection Options

A variety of methods can be used to help protect security and alarm systems from overvoltage damage caused by switching or lightning transients. For example, SiBar™ thyristors are in use around the world, helping equipment manufacturers comply with major standards such as GR-1089 Core, ITU K.20/K.21 and UL60950.

The ROV metal oxide varistor's high current handling and energy absorption capability, fast response and low cost, make it suitable for overvoltage protection in line voltage applications. Pairing the ROV device with a PolySwitch LVR device can help provide a completely resettable circuit protection solution for power supplies and control board transformers.

Tyco Electronics’ PolyZen™ device also helps protect sensitive electronics from damage caused by inductive voltage spikes, voltage transients, incorrect power supplies and reverse bias. The PolyZen device is a polymer-enhanced, precision Zener diode micro-assembly. It consists of a low resistance, precision Zener diode that is thermally coupled to a PolySwitch element.

In operation, extended overvoltage or reverse bias conditions will cause the PolySwitch element to “trip” as the diode begins to heat up. A “trip event” causes the PolySwitch element to transition from a low to high-resistance state, helping protect downstream electronics by generating a series element voltage drop. Also, by limiting the current, it helps to prevent Zener diode failure caused by thermal runaway.

To help shunt electrostatic discharge (ESD) away from sensitive circuitry in electronic devices, Tyco Electronics’ PESD devices are another suitable option. These devices offer a very low capacitance of 0.2pF, which is useful in high frequency applications.

 

Integrated Overcurrent/Overvoltage Protection

The new 2Pro™ device integrates overcurrent and overvoltage circuit protection technology and is designed to help equipment meet the UL60950, TIA-968-A and ITU K.20 and K.21 telecom protection requirements. The device’s small footprint, resettable functionality and coordinated protection capabilities make it suitable for a wide range of telephony and security system applications.

The 2Pro device incorporates PolySwitch overcurrent technology with an MOV component into one thermally protected device that helps provide current limiting during overcurrent events and voltage clamping during overvoltage events. This single-device approach helps reduce component count and improve equipment reliability.

 

 

To view all of Tyco Electronics’ circuit protection offering, please click here.