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Dear Tariq,
First determine according the zone classification layout of your plant/asset
which zone the instrument has to be allocated.
Flameproof Exd is suitable for use in zone 1 & 2.
Intrinsically safe Exia is suitable for use in zone 0, 1 & 2.
Intrinsically safe Exib is suitable for use in zone 1 & 2.
As mentioned by Danny Van G. you should first determine whether your (passive
or active) Exd instrument is able to work with a barrier. The barriers task
is to achieve a decrease in the loop energy which will not cause a spark with
enough energy to ignite the hydrocarbon (for example in case of a failure
like short circuit). According the regulations by using a flameproof (Exd)
instrument you do not need to use a barrier. If you want to use a barrier
just check the instrument type (maybe dual certified Exd/Exi?) if it can work
with the lower energy levels at the secondary side of the barrier (field
side).
Please try to differ these two principles of safe
circuit design and first read the documentation before taking decisions.
Exd - IEC 60097-1
Exi - IEC 60079-11
Second is to review the plant/asset safe circuit design philosophy. Because
both ways (Exd/Exi) have advantages and disadvantages regarding
procurement/design/installation (space)/maintenance to consider.
Advantage for example: The instrinsically safe (Exia or b) circuit design
allows you to perform maintenance on a live (powered) loop since the energy
levels are low enough to create a safe situation.
Disadvantages for example: More design required with selection of correct
barrier as well additional cost for design. Enclosure space required for
accommodating the barrier. Proper grounding required in case of (zener)
barrier, not with a galvanic isolator!
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