determining factor for the number of Thorn S23i+ flame detectors on single loop?


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We are currently experiencing problems with the Thorn Flame Detectors model: S231i+.

The Thorn 131+ models have been changed out to the newer 231+ models due to the 131+'s susceptibility to radio interference. During this change out the End of line resistor was changed to suit the S231+'s quiescent electrical parameters.

Previously. with the 131+'s a loop could sustain up to 6 (six) detectors but now with the S231+'s, not more than 3 (three) detectors can be sustained without the ICS (Fire and Gas control system) being driven into fault.

Extensive experimentation has been carried out to date and it has been revealed that the S231+'s exhibited a significantly larger inrush current and thus when multiplied by six (six detectors on a loop), was sufficient to initiate the fault condition.

Since the ICS latches the fault, the fault condition caused by the inrush current on startup prevails, even though the quiescent (operating) parameters are in the normal operating range.

Changes to the code allowed the inhibit time on power up to be changed from 7 seconds to 10 seconds, thus preventing fault to be recognised and latched.

All in all what I need to know is what is the determining factor for the limit of detectors on a single loop? And thus what is the maximum number of detectors that can be placed on a single loop?

Currently an MTL 700 series type barrier is being used. Is there a recommended MTL 700 series barrier suitable for the detectors?