Designing overvoltage protection for fire detection equipment

Author: Sonja Brown, EPCOS, a TDK Group Company

The analyst firm Gartner predicts there will be nearly 25 billion IoT-enabled devices by 2020 – many of which will take the form of home automation, safety, security, appliance and entertainment equipment. A recent article on Smart Home technology cited a Coldwell Banker Real Estate survey that found 56% of consumers said safety devices such as fire alarms, smoke detectors, carbon monoxide detectors are what makes a home smart.

By this definition, many consumers already live in smart homes – 96% of Americans have a smoke alarm in their home. By the early 1990s, most state and local laws required smoke alarms in homes, yet the National Fire Protection Association says that “three of every five home fire deaths resulted from fires in homes with no smoke alarms (38%) or no working smoke alarms (21%).” The NFPA also reports that fires caused more than $14.3 billion in property damage in 2015. In new buildings – both residential and commercial – smoke detectors must be hooked directly to the electrical wiring, be interconnected to other smoke alarms and have a battery backup.

So, what happens if there is an electrical surge that causes the smoke detector to fail due to overvoltage? This scenario is all too real; so overvoltage protection is of particular importance to those designing fire alarm systems as lives and property are on the line.

Fire alarm systems are complex. Individual fire detectors are generally linked over a network to a controller which scans fire or smoke detectors regularly – the system’s primary function. If a detector is actuated, the controller will activate the audible and visual alarms in the building, close fire doors and notify the fire service over the LAN or mobile communications network. To do this, both ring and star network topologies may be used, depending on building structure, layout, and systems.

Depending on building codes and owner or other requirements, there are several sensors that operate in fire detection systems. These include optical sensors that detect smoke, temperature sensors that identify rising temperatures and gas sensors that recognize the presence of carbon monoxide (CO) and carbon dioxide (CO₂).

Most fire alarm systems are powered though 24-V power supplies and have an uninterruptible power supply (UPS) that guarantees a voltage supply during a power outage – even when power is down for extended periods. Because these are safety-related systems, they often include motion detectors and glass breakage sensors.

Over-voltages may arise on both data and power supply cables and are magnified because of the substantial cable lengths involved in fire alarm systems. The five most common over-voltage causes include:

• Electrostatic discharge (ESD)
• Lightning surges
• Electrical fast transient bursts (EFT)
• Switching of inductive loads
• Temporary overvoltage (TOV)

There are several industry standards to help address the first four scenarios, including IEC 61000 4-2, IEC 61000 4-5, IEC 61000 4-4, ISO 7637-2, respectively. Because temporary over-voltage causes are often unknown, no standard specifically addresses this scenario.

To deal with over-voltage, TVS diodes may be the first response. However, these diodes exhibit derating by 150°C and therefore won’t withstand the hotter temperatures that may be necessary for fire detection equipment.

In most scenarios, the use of a varistor will help to mitigate the effects of overvoltage. There are two types of varistors: monolithic and multilayer.

Monolithic varistors are specifically designed for high currents and high voltages, which is one reason they are used as the primary input of power supplies. Should the varistor overheat, an integral fuse isolates the varistor from the network and prevents any fire on the printed circuit board or damage to components located near the varistor.

Multilayer varistors can be used for the power supply bus of fire alarm systems and within the fire detectors themselves. This type of varistor can protect against lightning surges and EFT bursts as specified by IEC 61000 4-5. Not only can multilayer varistors can be manufactured for different rated voltages, they can also provide the electrostatic protection specified by IEC 61000 4-2.

Voltage considerations

Fire detection systems employing optical sensors need several internal voltage levels for the infrared transmitter (10 V), the receiver (10 V) and the microcontroller. The input is typically supplied by a 24-V dc bus. A varistor should protect this bus from over-voltage injection and should sit as close as feasible to the connecting terminals. A parallel-switched capacity can suppress any high-frequency interference.

To boost safety in the 10-V output transmitter/receiver circuit, a varistor may be switched in parallel with a zener diode to prevent excessive voltage from reaching the rest of the system in the case of diode failure. Ceramic capacitors can also be integrated to stabilize and suppress noise.

Data cables also need protection – particularly from electrostatic discharge events. A ceramic-based high-speed diode – like the EPCOS CeraDiode from TDK – works well in this application without affecting the integrity of the signal thanks to its low self-capacitance (as low as 0.6 pF) and fast response time (no more than 0.5 nsec).

Because of their design, a short circuit in a fire detector will paralyze the entire line section. This happens because they run parallel to the supply bus. To prevent such a failure, each detector is equipped with a fuse. An alternative is the use of a PTC thermistor that acts as a self-resetting fuse. Room temperature (cold) thermistors have low resistance/impedance. But if they short-circuit or if a current flows that is above the specified maximum range, the thermistor heats up and switches to a high-impedance state, reducing current flow back to normal levels. Once the short circuit disappears, the thermistor cools down to its low-resistance state.

While missing, disconnected or dead batteries are responsible for almost half (46%) of smoke alarm failures, over-voltage is still a significant reason for failure. Engineers cannot force consumers and businesses to put batteries in their smoke detectors, but they can protect these systems from over-voltage. The result could be nearly a thousand lives saved annually and the prevention of property damage totaling in the billions of dollars.