PAL-AT Operating Manual
45
5.4 Zener Barrier Panel
A Zener Barrier Panel (ZBP) is required when sensor cables are installed in hazardous locations to limit
the energy passing onto the cable. It is mounted adjacent to the PAL-AT panel. A Zener Barrier Assembly
(ZBA) is installed in the ZBP and provides a protective circuit between PAL-AT and the ZBP fuse terminals
for the sensing string. Up to four ZBAs can be installed in each ZBP.
When the ZBP is used, the jumper cable at the beginning of the sensing string is connected to the terminals
in the ZBP as shown in figure 5-1. There is an internal fuse in each ZBA. If the fuse fails, the ZBA must
be replaced.
Figure 5-1
Zener Barrier Panel Wiring
5.4.1 Zener Barrier Assembly Tests
If the internal fuse in the ZBA fails, PAL-AT displays the message "NO END FOUND" or "FAULT IN FIRST
50 FEET". The ZBA internal fuse can be checked by measuring the resistance through the ZBA with an
ohmmeter. Make sure the Impedance Selection jumper is in position D (see figure 1.1).
Method 1 – ZBA connected to PAL-AT, PAL-AT power off.
Open the fuse holder terminal to isolate the ZBA from the sensing string. The normal resistance
from the ZBA center conductor to the ZBA shield, measured at the fuse holder terminal, is 60-65
ohms.
Method 2 – ZBA plug disconnected from PAL-AT
The normal resistance from the center conductor at the fuse holder terminal to the center
conductor at the end of the 5 ft [1.5 m] cable to the PAL-AT panel is 20-25 ohms.
If the reading is outside the normal range using either test method, replace the ZBA.
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