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Document: LT0273 MX4428 MXP Engineering / Technical Manual
Device Information and Programming
Issue 1.5 24 March 2006 Page 3-9
3.2.4 ISOLATOR BASE LOADING
If isolator bases are being used, calculate the sum of the “IB Units” from Table 3-2 for each
section of cable between isolator bases (or between the last isolator base and the end of a
cable spur). Include only one of the detectors at the ends of the section. The sum for any
section must not exceed 100.
See also section
4.1.3 for details of AS1670 requirements and section 4.1.4 for details of
NZS4512 requirements.
3.2.5 EXAMPLE
Consider an MXP monitoring 200 * 814PH detectors with 10 814SB Sounder Bases set to
High, on a 1300 metre long loop, using 1.5mm
2
wire. The cable is divided (with 9 Isolator
Bases) into 10 segments with 1 Sounder Base and 20 detectors on each segment.
(i) Calculate DC Load
IA = 195 x 275uA (No. of detectors in NORMAL)
+ 5 x 3.0mA (No. of detectors with Alarm LEDs turned on, assume limited to
5 max by MXP)
+ 10 x 15mA (Number of 814SB Sounder Bases)
+ 9 x 80uA (Number of Isolator Bases)
(Ref
Table 3-2. Note 1mA = 1000uA)
= 220mA which is well under 400mA
For the voltage drop calculation, assume the worst case in the first instance, i.e. that
all devices are at the far end of 1300 metres. The loop resistance of 1.5mm
2
wire is
25Ω per 1000m and the isolator base resistance is 0.25Ω.
Total R = 25Ω x 1.3 + 9 x 0.25Ω
= 34.75Ω.
Voltage drop = 34.75 x 0.220 = 7.7V, which is well under the maximum allowable of
16V.
(ii) Calculate AC Load
AC Units = 200 x 1 (detectors)
+ 10 x 2.4 (Sounder Bases)
+ 10 x 0.1 (Isolator Bases)
= 225 which is less than the maximum allowable of 250.
Cable length is well under the limits specified in
Table 3-3.
(iii) Calculate IB Load
IB Units for each section = 20 * 1.2 (814PH) + 1 * 2.5 (814SB)
= 26.5 which is less than 100.
As all parameters are within the specified limits, the design is satisfactory.