Eaton MTL7700 Instruction Manual

Contents

9INM 7700 R ev 125 3 5 Bonding practice when hazardous-area equipmentcannot meet prescribed insulation standardsWhere hazardous-area equipment is connected to earth (eitherdirectly or indirectly) and/or it will not withstand a 500V insulationtest to ground or the alternative described in section 5.3.4,eg, strain-gauge bridges with low-voltage insulation, pH andconductivity sensors, bare and/or earthed thermocouples, andsome level detectors, the following considerations apply:Safety considerationsa) The pipe, vessel, or body of the hazardous-area apparatus and/or the adjacent metallic structure must be connected to thebarrier DIN-rail (see item d) by a bonding conductor of at least4mm2 cross-sectional area for lengths of 100m or less; or atleast 8mm2 area for lengths between 100m and 200m.b) Where bonding conductors are used, care must be taken toavoid invading other intrinsically safe systems (those not usingbonding conductors) by elevation arising from currents whichmay flow in the common earthing systems as a result of thepresence of the bonding conductor. Where this possibilitycannot be avoided, then barriers associated with bondedsystems should be mounted on a separate DIN-rail from thoseon which other barriers are mounted. Additionally, the DIN-rails themselves should also be earthed separately.c) The hazardous-area equipment and/or adjacent metallicstructure bond connections must be secured against vibrationand corrosion. A terminal of the type used on ‘increasedsafety’ (‘e’) equipment is the mandatory solution.d) The barrier DIN-rail connections must provide suitabletermination facilities for the bonding conductor and usual‘earth return’ by being equipped with separate increasedsafety (‘e’) terminals.Operational requirementse) Figure 18 shows the 0V rail of the safe-area equipment returnedto the barrier DIN-rail by a separate insulated conductor andthe structural earths of any safe-area equipment returnedseparately to the neutral star-point. This reduces interferenceproblems but is not essential for safety reasons.f) In general, the use of barriers in all measurement leadsreduces the possibility of earth circulating currents causingmeasurement problems.g) Resistance from the neutral star point to ‘terrestrial earth’ – viathe buried earth mat or rods – is determined by otherregulations and is not modified or determined by the intrinsicsafety requirements, which are concerned with the plant earthonly.5 4 Connections to dummy barriersSee figure 19. The MTL7799 dummy barriers provide safeconnection facilities for unused cable cores, cable screens and0V returns. In addition, safe-area terminals 2 and 1 are connectedinternally to provide a straight ‘feedthrough’ 24V dc supplyconnection, enabling the units to be used as feed modules foruse with a power bus, useful in applications where the fault tripmechanism of the MTL7798 is not needed.6 MAINTENANCE6 1 GeneralCircuits in all MTL7700 range of barriers are encapsulated and socannot be repaired. However, provided they are correctly installedand connected (as described in sections 4 and 5 of this manual)and the circuits they protect are not themselves defective, barrierfaults are highly unlikely to occur. Therefore, servicing of barrierinstallations consists mainly of routine inspection and earth testingas described in this section.More information about the maintenance of barrier installations isgiven in BS EN 60079 -17.6 2 Routine inspectionAt intervals not exceeding two years (more frequently for harshenvironments), make a visual check of the barrier installation.Personnel undertaking these checks should comply with allregulations relating to the safety of plant and personnel. Care mustbe taken to prevent any inadvertent direct connection betweenhazardous- and safe-area circuits and, at all times, the safetyprecautions discussed in section 3 MUST be observed. Figure 17: Earthing and screening using module earth terminals or earth rail alternative