Tyco MX4428 - Manual (Page 73)

Document: LT0273 MX4428 MXP Engineering / Technical Manual
Analog Loop Design Considerations
Issue 1.5 24 March 2006 Page 4-5
4.2.3 STAR CONNECTION OF ANALOGUE LINES
It is not always necessary to connect addressable systems as loops, especially if an existing
conventional detector system is being converted to addressable detectors. As the existing
detector zone cables probably already terminate at the main panel, it is possible to connect
these in a star connection to the MXP as shown in Figure 4.2.
The two line terminals should be joined together as shown in Figure 4.2. The total length of
cable connected to the MXP should not exceed 2000m.
Because shorting the cable in one line will short out all the other lines connected to the same
MXP, it is recommended that 5BI Isolator Bases be fitted at the start of each line and then
placed every 20 - 40 devices along each line. (Refer to section 3.2.4 for the calculations to
determine where the isolator bases must be positioned.) The cabling from the MXP to the
initial Isolator Bases should be as short as possible. In fact 5BI Isolator bases without
detectors can be used at the star junction point.
Note: The Star Connection is not recommended for new installations. A loop configuration
should be used as it offers open circuit fault protection and with Isolator Bases, short circuit
protection.
4.2.4 SPURS
Both the loop topography described in section 4.2.2 and the star topography described in
section 4.2.3 can have "spurs" attached. (Spurs on a spur for the star topography.)
Any such spur should be connected to the loop or its parent spur with an isolator base.
However spurs are not recommended for new installations as an open circuit will disconnect
all detectors further away from the MXP than the open circuit, and a short circuit on a spur
will disconnect the whole spur.
In any case, to comply with standards, all the detectors on a spur should be in the same
zone.