Thanks to all. May I continue with a few supplementary questions?
An I right in deducing that a short circuit isolator is a completely passive device on the loop, that does not have its own identity or address and if it operates the fault will only be dentified by the failure to communicate with the devices on the isolated part of the loop?
And is the isolator an intelligent device that can monitor the potential conditions on the isolated part of the loop and reset itself when the short circuit appears no longer to exist or does it need to be manually reset like an MCB or replaced like a fuse?
I am having difficulty trying to visualise just how these isolators can monitor and isolate sections of a loop - thinking of Kirchoffs Law which said that the total current flowng in a circuit (loop?) = the sum of all the currents flowing in all parts of the circuit. I am now thinking that when you wire an alarm loop it must effectively look like a ladder with a main conductor ( Unprotected?)running up the strings of the ladder and across the top rung (we operational chaps call these rounds) and then each rung of the ladder having a number of devices ( ie a fire zone) with a short circuit isolator at either side of the rung (fire zone)?
Finally I sometimes see fault messages on systems such as "Loop split on +ve line". I have always assumed this to be a wiring fault due to a defective connection but from your last point Wiz it could be something else?
Thanks for your contribution to my continuing education. Wiz- I owe you a bottle of your favourite medicine!
Professor K. Please save the medicine for the next potting shed midnight meeting of the old codgers club. If Matron finds it in my room I'm in big trouble after all the palaver with the pavlova that Lucky 'borrowed' from the kitchen and hid in my bedside cabinet.
Graeme has suitably answered and confirmed some of your queries on the short-circuit isolator question.
I think your ladder analogy is probably not correct. Of course, I could have misunderstood it since I have a fear of heights! Instead, Just think of a two wire loop of cable where all your addressable devices are connected across the two wires. At suitable positions around the loop a number of short-circuit isolators are also connected across the loop. If a short-circuit occurs at any point of the loop then the isolators nearest (either side) the fault will automatically disconnect the loop at these two isolators. I.e. the section of cable with the short-circuit is now isolated from the control panel. The control panel now communicates along each of the ends of the loop still connected to the panel. Only the addressable devices on the section of the loop now isolated by the isolators are no longer functioning correctly. If the short-circuit is removed then the isolators automatically reset.
With respect to the other part of your question concerning our good friend Kirchoff (he has now been moved to room 4 on the first floor - next to the lift) I think I can understand your concerns. I believe your question asks why the current flowing through the isolator nearest the panel is not so great, even under non-short circuit fault conditions, that it doesn't automatically isolate?. This is because the short-circuit isolators don't actually measure the current flowing through the circuit. They are in fact looking for the normal operating voltage of the loop to drop quite drastically as an indication of a fault. I believe the voltage needs to drop to about 15 Volts for an isolator to operate. This voltage reduction will happen whenever there is a short-circuit on the loop because the maximum current that can be drawn by the loop is limited by the loop driver card circuitry and a consequence of this limting is a reduction in loop voltage during a short-circuit. Obviously a low resistance short-circuit will try to draw a lot of current.
My understanding is that this monitoring of loop voltage is carried out by the isolators a number of times a second and once the isolator nearest the short-circuit fault automatically disconnects the faulty section of the circuit, then the operating voltage will return to normal on the non-faulty sections.
I hope the above makes sense, (isolators measure voltage not current) if not please let me know and I'll try and find a better explanation.
Actually, this thread has gone way off target and I think this may have clouded the issue of the original promise of payment for a solution to the problem of connecting two fire panels together over a pair of wires. I need the money to pay for an escape attempt from the home. I understand that a tunnel is being dug from the sluice room in the East wing and I want to be included.
