9
INM 7700 Rev 12
5 .3 .5
Bonding practice when hazardous-area equipment
cannot meet prescribed insulation standards
Where hazardous-area equipment is connected to earth (either
directly or indirectly) and/or it will not withstand a 500V insulation
test to ground or the alternative described in section 5.3.4,
eg, strain-gauge bridges with low-voltage insulation, pH and
conductivity sensors, bare and/or earthed thermocouples, and
some level detectors, the following considerations apply:
Safety considerations
a) The pipe, vessel, or body of the hazardous-area apparatus and/
or the adjacent metallic structure must be connected to the
barrier DIN-rail (see item d) by a bonding conductor of at least
4mm
2
cross-sectional area for lengths of 100m or less; or at
least 8mm
2
area for lengths between 100m and 200m.
b) Where bonding conductors are used, care must be taken to
avoid invading other intrinsically safe systems (those not using
bonding conductors) by elevation arising from currents which
may flow in the common earthing systems as a result of the
presence of the bonding conductor. Where this possibility
cannot be avoided, then barriers associated with bonded
systems should be mounted on a separate DIN-rail from those
on which other barriers are mounted. Additionally, the DIN-
rails themselves should also be earthed separately.
c) The hazardous-area equipment and/or adjacent metallic
structure bond connections must be secured against vibration
and corrosion. A terminal of the type used on ‘increased
safety’ (‘e’) equipment is the mandatory solution.
d)
The barrier DIN-rail connections must provide suitable
termination facilities for the bonding conductor and usual
‘earth return’ by being equipped with separate increased
safety (‘e’) terminals.
Operational requirements
e) Figure 18 shows the 0V rail of the safe-area equipment returned
to the barrier DIN-rail by a separate insulated conductor and
the structural earths of any safe-area equipment returned
separately to the neutral star-point. This reduces interference
problems but is not essential for safety reasons.
f) In general, the use of barriers in all measurement leads
reduces the possibility of earth circulating currents causing
measurement problems.
g) Resistance from the neutral star point to ‘terrestrial earth’ – via
the buried earth mat or rods – is determined by other
regulations and is not modified or determined by the intrinsic
safety requirements, which are concerned with the plant earth
only.
5 .4
Connections to dummy barriers
See figure 19. The MTL7799 dummy barriers provide safe
connection facilities for unused cable cores, cable screens and
0V returns. In addition, safe-area terminals 2 and 1 are connected
internally to provide a straight ‘feedthrough’ 24V dc supply
connection, enabling the units to be used as feed modules for
use with a power bus, useful in applications where the fault trip
mechanism of the MTL7798 is not needed.
6 MAINTENANCE
6 .1 General
Circuits in all MTL7700 range of barriers are encapsulated and so
cannot be repaired. However, provided they are correctly installed
and connected (as described in sections 4 and 5 of this manual)
and the circuits they protect are not themselves defective, barrier
faults are highly unlikely to occur. Therefore, servicing of barrier
installations consists mainly of routine inspection and earth testing
as described in this section.
More information about the maintenance of barrier installations is
given in BS EN 60079 -17.
6 .2
Routine inspection
At intervals not exceeding two years (more frequently for harsh
environments), make a visual check of the barrier installation.
Personnel undertaking these checks should comply with all
regulations relating to the safety of plant and personnel. Care must
be taken to prevent any inadvertent direct connection between
hazardous- and safe-area circuits and, at all times, the safety
precautions discussed in section 3 MUST be observed.
Figure 17:
Earthing and screening using module earth terminals or earth rail alternative
