Fire Friday #8: Redundancy

What role does system configuration and redundancy play in LHD reliability?

The reliability of a fiber optic Linear Heat Detection (LHD) system can be positively influenced not only by design, component selection, and intensive testing, but also by clever setup and use of the system itself. In this way, fault resilience is achieved in both system components as well as the overall system.

Class-A Setups

The simplest case is the use of a measurement loop – also known in the industry as a Class-A setup. Here, the fiber optic sensor cable is not laid in a "single-ended" configuration, but instead looped back to the evaluation unit. By using a second optical channel, the sensor cable can be measured from both sides. In the event of a cable break – caused, for example, by a truck and its superstructure – the fiber optic sensor cable is still fully operational for detection up to the point of breakage. With a four-channel system, up to two Class-A loops can thus be installed per system. However, this redundancy concept falls short when considering the evaluation unit itself.

Full Redundancy

In the above consideration, an error that leads to a failure of the evaluation unit automatically leads to a failure of the entire system. Due to the wide coverage possible with fiber optic LHD, large areas are left unmonitored in the case of a system failure. To achieve single fault resilience, two evaluation units can be used, each installed at the ends of the fiber optic sensing cable. All AP Sensing fiber optic sensor cables contain two optical fibers, hence each interrogator measures on a dedicated fiber independently. In the event of a fiber break, the evaluation units continue to measure from each end into the cable up to the point of breakage. This way, the entire area is still monitored. On the other hand, if one evaluation unit fails, the other still covers the entire area. Thus, with such a  fault tolerant design, full redundancy is achieved.

With these considerations, multiple LHD evaluation units can be used with multiple sensor cable lines, rendering virtually infinite coverage.

Under certain conditions, AP Sensing's LHD solution even achieves fault tolerance for two simultaneous faults – failure of one evaluation unit and a fiber break at the same time. A combination of two sensor cables and two LHD units enables very high fault resilience, meeting requirements for certain demanding projects.


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Response time, spatial resolution, and alarm zone configuration: what is important when selecting a Linear Heat Detection system? Find out more in our weekly series.

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