Introduction Engineering Manual
16
Voltage Drop Example 1
The first example in Figure 6 shows two devices connected to the control panel. The first is 20m
from the panel and has 120mA being taken from its auxiliary supply to power detectors. The
second is 20m from the first and also has 120mA being taken from its auxiliary supply to power
detectors. The example shows that the voltage levels at both nodes is acceptable. The
voltage at each device is as follows:
1. The total current being drawn at the first device = 180mA + 180mA = 360mA.
2. Voltage drop to the first device = 360mA @ 20m = 1.15V (see table).
3. Voltage at first device = control panel voltage - voltage drop = 13.60 -1.15 = 12.45V.
4. The total current being drawn at the second device = 180mA.
5. Voltage drop to the second device = 180mA @ 20m = 0.58 (see table).
6. Voltage at second node = first device voltage - voltage drop = 12.45 - 0.58 = 11.87V.
Voltage Drop Example 2
The second example in Figure 6 shows what happens when a third device is connected 20
metres from the second.
The voltage at each device is as follows:
1. The total current being drawn at the first device = 180mA + 180mA +180mA = 540mA.
2. Voltage drop to the first device = 540mA @ 20m = 1.73V (see table).
3. Voltage at first device = control panel voltage - voltage drop = 13.60 -1.73 = 11.87V.
4. The total current being drawn at the second device = 180mA + 180mA = 360mA.
5. Voltage drop to the second device = 360mA @ 20m = 1.15V (see table).
6. Voltage at second device = first device voltage - voltage drop = 11.87 - 1.15 = 10.77V.
7. The total current being drawn at the third device = 180mA.
8. Voltage drop to the third device = 180mA @ 20m = 0.58V (see table).
9. Voltage at third device = second device voltage - voltage drop = 10.77 - 0.58 =
10.19V.
The second example shows that when the third device is added to the network the current
drawn at each is increased and therefore the voltage drop is also increased. The voltage level
at the third device is now below the 10.5V threshold and will cause problems.
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