Eaton MTL7700 Instruction Manual

Contents

10INM 7700 R ev 12Check that:a) Barriers are of the types and polarities specified in the safetydocumentation.b) The barriers are attached securely and correctly to the DIN-railto make sure the earth connection is safe.c) There are no apparent signs of damage or corrosion to thebarriers, the ETL7000 earth terminals, the plant earthconnections, and, if fitted, any earth rail assemblies.d) All hazardous-area and safe-area cable connections are madecorrectly and the terminals properly tightened.e) Interconnecting cables are of the type and rating specifiedby the safety documentation and that they are not frayed orotherwise damaged.f) All earth returns and cable screens from the hazardous areaare connected to earth either through a barrier, a dummybarrier or an earth rail.g) Visually examine the earth conductors and make sure they arenot damaged in any way and that their terminations are secureand free from corrosion.h) Using a low-voltage low-current test meter (ie, a meter with anoutput not exceeding 3V and 50mA), measure the resistancebetween the DIN-rail and the neutral star-point of the supplyand make sure it does not exceed 1Ω. Record the reading andcompare it with readings taken during previous inspections.A consistent reading repeated over a long period of time is areassuring sign indicating a sound earth return which is likelyto remain so. If two earth conductors are used as describedin section 5.3.2, the loop resistance should be measured asdescribed in that section and the reading should not be greaterthan 2Ω.WARNING: do not try to carry out a high-current earthresistance test unless it is confirmed by the authority incharge of the plant that the plant is gas-free7 FAULT-FINDING7 1 IntroductionMost barrier-protected systems are relatively simple and theiroperation is easy to check. However, when fault finding isnecessary, it must only be undertaken after checking with plantpersonnel responsible for safety that it is safe to proceed.The fault-finding procedures described in this section call for theuse of a digital multimeter – this being the most common type used.However, other types can be used provided their characteristicswhen measuring silicon diodes are known.Many digital multimeters include a diode test function which isuseful when diode chains are included in the test. These usuallyoperate by passing 1mA through the diode and measuring thevoltage across it. When measuring more than two diode drops inseries, note that the full-scale range of some multimeters is only2V on the diode test range. With three or more diode drops it istherefore possible that the meter may indicate over-range. In thetest tables (tables 4 to 10), section 8, any drop of more than 4V isindicated as infinity (∞).Zener diodes and ordinary silicon diodes have a typical forwardvoltage drop of approximately 0.6V/diode. Diode return paths withSchottky diodes have a typical voltage drop of <0.3V for each diodein the chain, eg, MTL7787+, 7787P+.Figure 20 depicts a typical switch-status transfer circuit protectedby an MTL7787+ barrier which illustrates some of the fault-findingtechniques discussed in this section. To determine the serviceabilityof MTL7700 range of barriers, follow the steps described in sections7.2 to 7.3.Hazardous-areaequipmentincapable ofwithstandinginsulation testSafe-areaequipment0VLocaldistributiontransformer1Ω maximumSafe areaHazardous area(Zone 2)or safe areaHazardous areaZone 0, 1 or 2Bonding conductor<100m: 4mm2 minimum100 200m (maximum): 8mm 2 minimumEnclosureDIN-rail BarriersFigure 18: Bonding practice where hazardous-area equipment cannot meet required standards of insulation from earth Figure 19: MTL7799 dummy barrier connections