Eaton MTL7700 Instruction Manual

Contents

12INM 7700 R ev 12128 2 2 Multimeter testsSee tables 4 to 10. The use of a digital multimeter for testing barriersis described in section 7. The tables assume that the multimeter isset to a suitable Ω range for the end-to-end resistance tests (exceptfor diode return channels) and for the continuity tests; and that themultimeter diode test function is used for the diode tests, channelisolation tests and for the end-to-end resistance tests for diodereturn channels.8 2 3 Constant-current testsFor these tests (last column of tables 4 to 10) a constant-currentgenerator capable of supplying 10μA, 20mA and 40mA currentsfrom a 30V source is needed. Ideally, a purpose-built currentgenerator should be used but, alternatively, a set-up based ona laboratory power supply and two multimeters (figure 23) willserve. The current is measured by one multimeter and trimmed byadjusting the output voltage of the power supply indicated on thesecond multimeter.When using a constant-current generator for testing MTL7700range of barriers, note the following points:a) The current must be limited to 50mA to avoid damaging thebarriers.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 reducethe need for connection changes.d) Using a high resistance in series with the barrier gives morestable results and makes it easier to set the required current.8 3 Tests for Active Barriers8 3 1 Tests for the MTL7706+Comprehensive testing requires specialised equipment, beyondthe scope of on-site tests. However, an effective test which willconfirm that the unit is operating correctly is shown in figure 24.Ammeter 1 measures the transmitter simulator current of between4 and 20mA flowing from terminal 4 and the safe-area load currentflowing to terminal 4 simultaneously. Since these two currents areequal and opposite, the resulting reading on the ammeter shouldbe virtually zero. Ammeter 2 verifies the presence of the 4–20mAtransmitter signal. Figure 22: Calibrating an RTD barrier circuit Figure 23: Constant-current test circuit using a power supplyand digital multimeters Figure 24: MTL7706+ test circuit8 3 2 Tests for the MTL7707+Since this unit incorporates a built-in protection circuit, it has tobe tested in a different manner to an ordinary shunt-diode barrier.Referring to figure 25, set the transmitter simulator to 4mA and to20mA, and check that the ammeter reads approximately the samevalue. Then, having set the simulator to 20mA, check that thevoltage between terminals 1 and 3 is less than 8.5V and betweenterminals 2 and 4 is less than 1.6V.Figure 25: MTL7707+ test circuit