10
INM 7700 Rev 12
Check that:
a) Barriers are of the types and polarities specified in the safety
documentation.
b) The barriers are attached securely and correctly to the DIN-rail
to make sure the earth connection is safe.
c) There are no apparent signs of damage or corrosion to the
barriers, the ETL7000 earth terminals, the plant earth
connections, and, if fitted, any earth rail assemblies.
d) All hazardous-area and safe-area cable connections are made
correctly and the terminals properly tightened.
e) Interconnecting cables are of the type and rating specified
by the safety documentation and that they are not frayed or
otherwise damaged.
f) All earth returns and cable screens from the hazardous area
are connected to earth either through a barrier, a dummy
barrier or an earth rail.
g) Visually examine the earth conductors and make sure they are
not damaged in any way and that their terminations are secure
and free from corrosion.
h) Using a low-voltage low-current test meter (ie, a meter with an
output not exceeding 3V and 50mA), measure the resistance
between the DIN-rail and the neutral star-point of the supply
and make sure it does not exceed 1Ω. Record the reading and
compare it with readings taken during previous inspections.
A consistent reading repeated over a long period of time is a
reassuring sign indicating a sound earth return which is likely
to remain so. If two earth conductors are used as described
in section 5.3.2, the loop resistance should be measured as
described in that section and the reading should not be greater
than 2Ω.
WARNING: do not try to carry out a high-current earth
resistance test unless it is confirmed by the authority in
charge of the plant that the plant is gas-free .
7 FAULT-FINDING
7 .1 Introduction
Most barrier-protected systems are relatively simple and their
operation is easy to check. However, when fault finding is
necessary, it must only be undertaken after checking with plant
personnel responsible for safety that it is safe to proceed.
The fault-finding procedures described in this section call for the
use of a digital multimeter – this being the most common type used.
However, other types can be used provided their characteristics
when measuring silicon diodes are known.
Many digital multimeters include a diode test function which is
useful when diode chains are included in the test. These usually
operate by passing 1mA through the diode and measuring the
voltage across it. When measuring more than two diode drops in
series, note that the full-scale range of some multimeters is only
2V on the diode test range. With three or more diode drops it is
therefore possible that the meter may indicate over-range. In the
test tables (tables 4 to 10), section 8, any drop of more than 4V is
indicated as infinity (∞).
Zener diodes and ordinary silicon diodes have a typical forward
voltage drop of approximately 0.6V/diode. Diode return paths with
Schottky diodes have a typical voltage drop of <0.3V for each diode
in the chain, eg, , 7787P+.
Figure 20 depicts a typical switch-status transfer circuit protected
by an barrier which illustrates some of the fault-finding
techniques discussed in this section. To determine the serviceability
of MTL7700 range of barriers, follow the steps described in sections
7.2 to 7.3.
Hazardous-area
equipment
incapable of
withstanding
insulation test
Safe-area
equipment
0V
Local
distribution
transformer
1
Ω
maximum
Safe area
Hazardous area
(Zone 2)
or safe area
Hazardous area
Zone 0, 1 or 2
Bonding conductor
<100m: 4mm
2
minimum
100 200m (maximum): 8mm
2
minimum
Enclosure
DIN-rail
Barriers
Figure 18
: Bonding practice where hazardous-area equipment cannot meet required standards of insulation from earth
Figure 19:
MTL7799 dummy barrier connections
