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8.8 5704 Fire Card The 5704F fire input circuit operates from the system dc input supply (21 to 32V) but has a built-in voltage limiter that limits the maximum loop voltage to +24V to protect the detectors from damage. When the system supply is less than +24V the limiter has no effect and the loop will see the true input supply voltage. The loop current is determined by measuring the voltage across a 220 ohm current sense resistance. A link selectable 330 ohm barrier equivalent resistance is incorporated for use when an external IS barrier is NOT fitted. For fault monitoring purposes, an end of line (EOL) resistor must be fitted in or after the last detector on the loop. The typical value for the end of line resistance is 5.1k ohms although this may need to be reduced when many detectors are fitted onto the loop. An equivalent circuit of one fire input together with an example detector connection is shown below:- 8.8.1 Line Resistance Detectors should be located such that the line resistance of the cable required does not prevent correct operation. As a general guide and for a typical installation of twenty low quiescent current detectors, the loop cable resistance should be kept below 100 ohms total (50 ohms per core). The table below gives a quick guide to the maximum cable lengths permitted in this case: Maximum Cable Length (m) Detectors Conductor Cross Sectional Area AWG 21 19 18 16 14 mm2 0.50 0.75 1.00 1.50 2.50 20 low quiescent (100µA) detectors 1300 2000 2700 4100 6500 For further details of line resistance calculation refer to the 5704F Fire Card manual 05704- M-5002. 14 System 57 Quick Start Guide 05701-M-5026 MAN0839 Issue 1 8.8.2 Typical Loop Powered Detector Connections Loop powered detectors (e.g. most smoke, heat and manual call points) have two wire connection. The detector documentation will indicate the positive and negative loop connections. Multiple detectors may be connected in parallel on a single loop input provided the loop operational limit for quiescent current is not exceeded. At the System 57 end of the field cables, the two detector wires should be connected to the appropriate channels IN+ and IN- terminals of the Hex Relay Interface Card that is attached to the required 5704F Control Card. The loop current always flows from the IN+ terminal and returns via the IN- terminal. For fault monitoring purposes an end of line (EOL) resistor must be fitted in or after the last detector on the loop. The typical value for end of line resistance is 5.1k ohms. The detector cable screen, or steel wire armour or braid as appropriate, should be connected to the system protective earth. This can be achieved where the cable enters the cabinet by using a metal cable gland, or by other suitable means, and avoiding any screen tails within the cabinet. Where the cable consists of a separate screen sheath and wire armour or braid, the armour should be connected at the cabinet entry to the protective earth and the screen sheath should be connected to the GROUND terminal of the Hex Relay Interface Card or to a suitable instrument earth point. EOL 5.1k - + IN- - + 23 21 Detectors IN+ 5704 Hex Card The diagram shows the detector connections for Ch