Control.com - Nerds in Control

We have a requirement to clarify if we still need to use a barrier in spite of choosing an RTD with a EExd certification. Can anybody provide a thorough technical justification in either case? Also, is a third party verification required when the quantity of RTDs in use is quite high (~90)? Where can I find more technical information? I am only aware that RTDs do not cause serious trouble but would like to back it up with some technical document.

If the RTD has Ex d certification, you have two choices.

You can use it in a system wired to Ex d rules without a barrier. This means being careful in selecting cables, cable glands, etc. However, if using this approach you do not need a barrier.

You can use it without certification as IS in an IS loop if it meets the requirements for "simple apparatus". In this case, it must be installed in a circuit protected with a barrier and does not need to meet the Ex d wiring requirements. However, you must make it clear in the documentation that you are not using the Ex d protection, and this should also be made clear at the apparatus - otherwise you run the risk of someone treating the RTD as Ex d which could invalidate the Ex i protection.

Bruce

We install many instruments EExd in Class 1 Div 2 without barriers. For Class 1 Div 1
CEC 18-100 (b) (ii)

You will still need to protect against explosive vapours migrating back from the field to your control system.

Regards
Roy

As I understand it, the IS barrier exists to block high current/voltages from entering the low voltage circuit from the unsafe zone such as the control room or electrical center. You still need a barrier.

Dick Caro
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The RTD can be classed as simple apparatus and therefore the device does not need IS certification. The device should be used with a barrier/galvanic isolator if the loop is going to be installed as an IS loop with BS5308 cable installed between the switch room and the field mounted transmitter. The designer should self-certify the loop like he should do with all IS systems he designs. The fact that the system has EExd construction is not really a factor in the IS loop build. It just means you have an extra heavy duty enclosure for your IS loop measuring device. The same principle applies to EExd push buttons, pressure switches, etc.

Bryce

Dear,

Compromises always end up in industrial disaster!!!

Secondly you get excellent field-control room isolation in case anything goes wrong in field.

RTD are connected to an opamp in PLC or DCS input card so ignoring IS in hazardous location is not advisable.

Also check your insurance policy!!!

The problem with IS barriers is they can introduce errors into your measurement (active barriers anyway). Some DCS systems like DeltaV have intrinsic safe IO modules.

It's alright to wire without barriers if you use the appropriate wiring methods, explosion proof seals etc. You don't need to use those big cast boxes any more providing your equipment is non-sparking and not too hot. Many of our clients in petrochemical won't accept IS systems. Most PLC systems are approved for class 1 Div 2 without a purged cabinet.

The rules covering hazardous locations have changed significantly in the last few years. They are also very hard to interpret.

Regards
Roy

I hate to disagree with Roy Matson, but he seems to be talking about
explosion-proof equipment, or nonincendive wiring, not IS.

The original question was "do I need IS barriers for my RTD's?" RTD's themselves are non-sparking passive devices, usually located in thermowells or as windings in motors. If the plant area is subject to explosive gas mixtures or vapors (Div 0 or 1; Zone 0, 1, or 2), then the RTD itself is generally regarded as safe, but it must be connected to some type of bridge circuit or op-amp to be read. If
that electronics is located in the safe area such as an instrumentation room or control room, AND you have chosen to use IS for fire protection, AND the connecting wires lead from a hazardous area to a safe area, then an IS barrier is required to prevent surge voltages that MAY occur in the reading/converting electronics from reaching the hazardous are via the RTD extension wires. IS barriers are generally passive devices using zener diodes and a current limiting resistor. They impose no bias to properly calibrated RTD circuits.

If the plant area is Div. 2 or Zone 2, you may choose to use Explosion-Proof or nonincendive equipment and not use Intrinsic Safety, in which case you do not need a barrier. That is between your company and the insurance company that provides fire hazard protection coverage for your plant.

Dick Caro
===========================================
Richard H. Caro, Certified Automation Professional, CEO, CMC Associates,
2 Beth Circle, Acton, MA 01720
Tel: +1.978.635.9449 Mobile: +.978.764.4728
Fax: +1.978.246.1270
E-mail: RCaro@CMC.us
Web: http://www.CMC.us

To answer Dick Caro:
The original post asked if there was an alternative to purchasing ~90 IS barriers. I pointed out a couple of alternatives. Thermocouples are another interesting problem because they only generate mV at low current. Do you need a barrier for those? Some say yes, others No. It seems to me that the electrical codes should be a lot clearer on the subject instead of leaving it open to interpretation.

I agree, Zenner type barriers shouldn't effect the accuracy but I have had to use active barriers on occasion because the zenner type would not work with the particular I/O module referenced to +24 not ground.

Roy

Roy,

It's not the energy generated by the T/Cs (or RTDs) that matters - it is the energy that can be fed to the field if a fault occurs, connecting say the power supply to signal element wiring. The barrier or isolator keeps the maximum voltage the element can pass to a safe level. So I reckon you do need a barrier on any wiring from the safe area to the hazardous area, unless another method of protection is used.

Circuit separation between power circuits and any wiring on the field side of the barriers or isolators is of course essential.

Bruce

Bruce,

The original post asked if IS was required. Since he has ~ 90 thermocouples, IS barriers would be a considerable added cost. I agree if you go the IS route you need barriers. I tried to point out there are other methods that may or may not be applicable "Methods of Protection":
Ex p Pressurized (might be possible with conduit)
Ex d Flameproof - Quite often used
Ex e Increased Safety - This is achievable
Ex nA Non Sparking - Thermocouples fit this
Non-incendive - cannot cause a spark when the wires are separated.

As I said before, the electrical codes give very poor guidance.

Roy

Why does the issue of EEx d and EEx i installations always cause confusion?

I assume you mean that your RTD has been purchased already installed in a thermowell with a housing that has been certified to EExd. If so, then the answer is NO you do not need any barriers or isolators, BUT you do need to ensure that the Glands are suitable and certified to EEx d also they are correctly fitted. This applies also if the RTD has a Resistance to Current transmitter fitted at the Head as the certification applies to the assembly.

The principle of the type of protection is very different. EEx i (intrinsically safe) functions by limiting the energy of Sparks and Surface Temperature. Refer to IEC 60079-11. EEx d (flameproof) functions by the housing containing the explosion. Refer to IEC 60079-1.

The other consideration is the increase in maintenance costs involved with EEx d, as they need to be subjected to regular inspection to ensure ongoing compliance.

Hope this helps.

Sandy