Eaton MTL7700 Instruction Manual

Contents

11INM 7700 R ev 127 2 Power supply checkCheck that the power supply to an individual barrier circuit (or toan MTL7798 power feed module or an MTL7799 dummy barriersourcing the power bus ) is functional and that the voltage acrossthe supply with respect to earth is correct. For example, in Figure 20the presence of 24V on terminal 1 and 12V on terminal 2 when theswitch in the hazardous area is closed confirms the serviceabilityof almost the complete circuit.7 3 Barrier resistance testTo test the resistance of a barrier on site, refer to figure 20 andcarry out the following procedure:a) Unplug hazardous-area cables from terminals 3 and 4.b) Unplug safe-area cables from terminals 1 and 2.WARNING: Take care when handling safe-area cables; therelay contacts in figure 20 for example could be carryingmains voltagec) Measure the end-to-end resistance of the barrier byconnecting a digital multimeter (set to a suitable Ω range)between terminals 1 and 3. The reading should typically beslightly less (eg 1–3% lower) than the maximum end-to-endresistance listed in tables 4 to 10 for the appropriate barriertype or approximately 10–20% higher than the figure specifiedin the safety description.d) Check the functioning of the diode-return channel (MTL7787+,7787P+, only) by selecting the diode test function on themultimeter and connecting it between terminals 4 (+ve) and 2(–ve). This measures the forward voltage drop of the Schottkydiodes in the chain: the reading should be less than 0.9V.Repeat the test with the connections reversed (ie, terminals4 (–ve) and 2 (+ve) for an expected reading of ∞ for the reversevoltage drop.e) Tests c) and d) confirm the continuity of both barrier channels.If either channel is open-circuit it is most likely that the fusehas blown. The safe-area circuit should be investigated in anattempt to discover the cause of the fault.Note: if the barrier is removed, make sure the safe-area andhazardous-area cables disconnected during operations a) andb) are connected to an earth-rail, a dummy barrier or insulatedcompletely.c) If it is not possible to disconnect wiring to the barrier for stepsa) and b), carry out the tests given in table 4.8 EQUIPMENT AND BARRIER TESTS8 1 Thermocouple and RTD tests8 1 1 Thermocouple circuit testingThermocouple test and calibration equipment is rarely certifiedintrinsically safe and therefore requires special authorisation beforeit can be used for testing or calibrating thermocouple circuits inhazardous areas. To overcome this problem, the thermocouplecircuits can be protected by using an MTL7760ac barrier asshown in figure 21. This permits the thermocouple output to bemeasured without needing special authorisation to use uncertifiedthermocouple test equipment.As it is seldom possible to measure accurately the temperatures ofthermocouples located in hazardous areas, a safe way of adjustingcalibration tables to compensate for plant temperatures is needed.This can be done by disconnecting the compensating cables fromthe thermocouple, shorting them together, and measuring thetemperature of the shorting point.8 1 2 Resistance temperature detector (RTD) circuittestingResistance temperature detector (RTD) circuits can be testedby disconnecting the measuring leads from the RTD head inthe hazardous area and connecting them to a resistance box.Sometimes it is more convenient to connect the resistance box inthe safe area, for instance at point ‘xx’ in figure 22. For the lattermethod, however, the RTD must be shorted out or an allowancemade for its temperature. The effect of a negative temperaturechange can be simulated by connecting the resistance box into themeasurement lead at point ‘yy’. The advantage of connecting theresistance box at the RTD head is that any leakage can also bedetermined by connecting the resistance box at point ‘yy’.8 2 Barrier tests8 2 1 GeneralMTL7700 range of barriers do not need to be subjected to routinetesting if they are in normal use. Generally, the inspection testsdescribed in section 6 are more than adequate. However, if theperformance of barriers is in any way suspect, then carry out thedetailed tests described in this section. Barriers which pass thesetests satisfactorily are highly unlikely to represent an unacceptablelevel of risk or cause a circuit malfunction.There are two types of test (as shown by tables 4 to 10); a simpletest using a digital multimeter to test barriers on site and a morecomprehensive bench test using a constant current source toestablish the breakdown characteristics.Although certification requirements concentrate on pulse-current tests, in many ways the leakage current tests describedin this section are a more satisfactory method of testing suspectdiodes. If a complete functional check of a barrier is needed, thenthe multimeter and constant current tests described in sections8.2.2 and 8.2.3 should be made although, for most purposes, themultimeter tests should suffice.ΩΩ Figure 20: Switch-status transfer circuit using an MTL7787 barrier Figure 21: Calibrating a thermocouple barrier circuit