12
INM 7700 Rev 12
12
8 .2 .2
Multimeter tests
See tables 4 to 10. The use of a digital multimeter for testing barriers
is described in section 7. The tables assume that the multimeter is
set to a suitable Ω range for the end-to-end resistance tests (except
for diode return channels) and for the continuity tests; and that the
multimeter diode test function is used for the diode tests, channel
isolation tests and for the end-to-end resistance tests for diode
return channels.
8 .2 .3
Constant-current tests
For these tests (last column of tables 4 to 10) a constant-current
generator capable of supplying 10µA, 20mA and 40mA currents
from a 30V source is needed. Ideally, a purpose-built current
generator should be used but, alternatively, a set-up based on
a laboratory power supply and two multimeters (figure 23) will
serve. The current is measured by one multimeter and trimmed by
adjusting the output voltage of the power supply indicated on the
second multimeter.
When using a constant-current generator for testing MTL7700
range of barriers, note the following points:
a) The current must be limited to 50mA to avoid damaging the
barriers.
b) The accuracy of the current is not critical and can vary by ±5%.
c) Test leads must be connected securely to the barrier terminals;
however, the changeover switch and current selector reduce
the need for connection changes.
d) Using a high resistance in series with the barrier gives more
stable results and makes it easier to set the required current.
8 .3
Tests for Active Barriers
8 .3 .1
Tests for the
Comprehensive testing requires specialised equipment, beyond
the scope of on-site tests. However, an effective test which will
confirm that the unit is operating correctly is shown in figure 24.
Ammeter 1 measures the transmitter simulator current of between
4 and 20mA flowing from terminal 4 and the safe-area load current
flowing to terminal 4 simultaneously. Since these two currents are
equal and opposite, the resulting reading on the ammeter should
be virtually zero. Ammeter 2 verifies the presence of the 4–20mA
transmitter signal.
Figure 22:
Calibrating an RTD barrier circuit
Figure 23:
Constant-current test circuit using a power supply
and digital multimeters
Figure 24:
test circuit
8 .3 .2
Tests for the
Since this unit incorporates a built-in protection circuit, it has to
be tested in a different manner to an ordinary shunt-diode barrier.
Referring to figure 25, set the transmitter simulator to 4mA and to
20mA, and check that the ammeter reads approximately the same
value. Then, having set the simulator to 20mA, check that the
voltage between terminals 1 and 3 is less than 8.5V and between
terminals 2 and 4 is less than 1.6V.
Figure 25:
test circuit
