In shell instrument specs I found following lines

"Resulting from an increased use of DCS and PLC-based Systems in industry, the majority of field instrumentaton will be certified as suitable for inclusion in 1.5. circuitry. Hence, Ex 'd' protection may not be necessary for these applications."

I do not understand what they mean by inclusion in 1.5 circuitry , why don't we need Ex d protection? should we use intrinsic safe instruments every where with DCS/PLC? DO DCS/PLC complying what they say 1.5 circuitry have built in I.S barriers with I/O module so we need I.S instr. with them?

Please help me understand 1.5 circuitry and their use in DCS/PLC system

 

As a guess, "1.5" is a typographical error for I.S. or Intrinsically Safe.

 

Hi there,

I think Bruce is right it should be "I.S".
The above then means that since the instrumentation supplied will be IS certified instruments, you can do IS protection and does not need to do explosion proof protection of the loops. Explosion proof equipment is much more expensive.

To determine where you need to do IS protection, you first need to look at the Zone classification of your plant. Where is Zone 0 areas in the plant.

Only IS protection may be done in Zone 0 . IS protection may also be done in Zone 1 and 2 but Explosion proof (Ex d) and Increased safety protection (Ex e) may only be done in Zones 1 and 2.

Ex d protection combined with IS protection is normally done in Zone 0 but IS is minimum for Zone 0. You can combine IS with Ex d or Ex e for Zone 1 and 2 as well where needed. The important thing is to know what is the minimum protection required for a Zone.

So to recap on the minimum required protection for Zone 0,1 and 2.
Ex ia = 0, 1, 2
Ex d = 1, 2
Ex e = 1, 2

Zone 0 = Most hazardous environment where explosive gas will be present all the time or long periods of time.

Zone 1 = Areas where explosive gasses may be present for short period during normal operations.

Zone 2 = Areas where explosive gasses are unlikely to be present and if they are present will be present for short periods due to abnormal operations.