In any large organisation, such as hospitals, hotels, mines, aircraft hangers, factories and large buildings, the fire alarm system is a critical piece of equipment. A fully functional fire alarm panel is a requirement by many insurance companies and is non negotiable for insurance purposes.
As these panels become more sophisticated, so their likelihood of being exposed to lightning and overvoltage damage increases. It is a known fact that lightning can cause fires, and it is therefore important, that during a storm, you are certain that the system will be functional and undamaged due to possible induced surge currents.
The fire alarm panels have loops feeding to the individual zones, which are exposed to lightning and overvoltage damage, even though they do not leave the building. These cables are often relatively long, up to 100 metres in length, which act as long antennas to induced surge currents. These induced currents can be as high as 200 V per metre of cable should lightning strike within 1 km of the building, rising to as high as 2000 V per metre should lightning strike the building directly or within 100 metres of the building. This means that any electronics, generally running on less than 24 V, will be exposed to voltages in excess of 500 V which will cause damage to the equipment.
The fire alarm panels are normally powered by a 220 V a.c. supply which is stepped down to the lower voltages of say 24 V, which further exposes the system to damage. The fact that the power is fed from outside the building, Eskom supply, you are exposing the panel to surges from Zone 0 into Zone 1, 2 or 3.
It is a misconception that the transformer will reduce the surge in proportion to the ratio of the windings, as due to the high frequency component of lightning, the transformer is seen as a capacitor, thus only reducing the surge slightly.
On most large sites, the panels in the individual buildings are all linked to a master panel or control room, normally via an RS422 or RS485 backbone. These protocols can be run over very long distances on unscreened cables which further exposes them to lightning and overvoltage damage. Once again, you are exposing the cable and equipment and subjecting the panel to surges from Zone 0 into Zone 1, 2 or 3. These protocols run on very low voltages, typically 5 V, and are thus very susceptible to lightning and overvoltage damage.
Many individuals have tried using off-the-shelf protected power outlets and protected plug tops believing that these could offer adequate protection. Unfortunately, most of these protectors are generally Class 3 arresters, and in terms of SANS 10142-1:2003 these need to be protected with upstream Class 2 arresters. Under severe surge and lightning conditions, these Class 3 arresters become overstressed, allowing too much energy to pass through, thus damaging the panel.
To overcome this problem, it is recommended that Copa HDO 280/T 40 kA Class 2 arresters be installed on the 220 V a.c. supply feeding the panels. The Copa HDO 280/T far exceeds the SANS requirement of a 5 kA for power surge arresters. It ensures adequate energy handling capability, lowers clamping voltage and ensures a longer life span. The Copa HDO 280/T is a compact surge arrester which is easy to mount in parallel with the existing power cable on the termination block.
To protect the loops feeding the zones, the recommendation is that the Copa 8-wire protection modules be installed. These units are designed so that they add limited impedance to the line and have no effect on the addressable panels. The units have been tested for compatibility with various other fire panel manufacturers' products and were found to be acceptable from an interference perspective. The Copa 8-wire protection modules are designed to repeatedly withstand peak surge currents of 10 kA and limit the voltage to acceptable levels.
In order to protect the RS422 or RS485 backbone, the Copa 8-wire protection modules, designed specifically for these protocols, should be installed. Due to the fact that these systems can run on unscreened cables between buildings, the exposure to lightning and overvoltage surges is further increased. However, the modules are designed with this factor in mind.
This protection concept has been used on fire alarm panels throughout South Africa, and more recently in Kenya, Uganda and Nigeria, with excellent results.
For more information contact Paul van As, Surge Technology, 011 792 1303, [email protected], www.surgetek.co.za
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