Hi,

I would like to ask if we can connect Flame proof device EEX d (faure herman ultrasonic transmitter) to an intrinsically safe system(junction box and barrier).if yes could you please improve that by any standard or document.

kbadawi69 [at] yahoo.com

 

You cannot connect a device which is certified as Ex d only on to an IS circuit. Any components connected to an IS circuit downstream of the barrier or isolator must be certified as IS.

However, if you are referring to a device certified as Ex d [ia] or Ex d [ib], the device itself can supply an energy-limited output from a signal port, as well as operate with a flameproof main powered circuit. The [ ] indicate that the device also contains auxiliary components.

You need to carefully read and understand the certification documents to check these points - that is your first port of call for any information. Then check applicable standards - for the IEC the main reference for IS is IEC 60079/11.

 

I think the answer would be yes, assuming your power supply is 24VDC loop powered. Flame proof EEx d enclosure means that the enclosure can withstand any internal explosion inside the enclosure itself.

Intrinsically Safe Ex ia or Ex ib means that the whole circuitry energy would not be able to ignite an explosive gas mixture. So looking at it, I think it is actually a double protection already to have an equipment with EExd enclosure and an Ex ia or Ex ib system.

No improvement required.
Hope that makes sense. Tq.

 

THE ADVICE GIVEN BY KALAN IS INCORRECT and if followed could lead to a fire or explosion.

An intrinsic safety system relies on total control of the electrical energy in the field circuit exposed to the flammable material. This requires that the voltage and current in the circuit are both known, and also that capacitance and inductance of the circuit are known. Unless you have this information for the equipment in the Ex d enclosure, you cannot be sure that the total energy stored in the circuit is limited to a safe level.

Barriers and isolators are certified as safe with a specified maximum circuit capacitance and inductance, and part of the design is to ensure that the safe limits are not exceeded. Google for "IS entity calculations" for further information.

Bruce.

 

Dear TALKAN,

Your opinion if you can improve it will help me a lot so please try to figure to any document or standard.

 

Yesterday, I had to sign yet another multipage agreement to get a contractor's badge for access to an industrial plant. the jist of which says that I can't sue them for their faults, whatever they are.

Part and parcel of the access ritual is the mandatory safety training class where the instructor astounded most of his audience when he bragged that his plant was almost as safe BP's plant nearby. I didn't know the guy sitting next to me but we instantly bonded with a mutual look of, "Uh-oh, what did we get ourselves into now?"

It's threads like this that make me shudder. I can't help but wonder which of the Class 1, Div 1 areas I have to go into had someone take a short cut like ajh is so desperately seeking on a public forum to justify wiring an EXP circuit into an I/S box because it's convenient.

Dude, these hazardous area electrical requirements are NOT 'recommended practice', it's code for a reason. People get killed when a circuit arcs or sparks in a hazardous area.

Mr. Durdle is absolutely correct. What you want to do is dangerous and not allowed by code.

 

Dear Bruce, thanks for pointing that out. You are right, and Jim too!

Anyway, Mr ajb, you may want to go through this website to get some more info on this IS subject matter: www.ronan.com/artofis.pdf

My earlier reply may applicable if the flowmeter is used to measure say cooling water flow at non-classified Cooling Water Tower area. It is an option to be considered instead of throwing away the ultrasonic flowmeter already purchased! Well, the code and standard is there but I think sometime we need to exercise our engineering call and common sense depending on the situation.

Cheers,
kalan

 

Dear Bruce,

Thanks for your help and also thanks for kalan and Jim

 

Let me play devils advocate here.. I can directly wire the exd device in the Haz area taking into account the voltage/current limitations to make the cct safe.

So, if my barrier supplies a similar voltage/current then what's the difference to direct connect as above.

 

Yes, you CAN quite easily design a circuit that will limit voltage and current - I think that's what you're suggesting.

The question is - can you if necessary prove that your design will perform as required in normal service, and in expected and some unexpected fault conditions (that last one's a real gotcha) and with two independent faults in the circuit That's what you get when you buy a certified barrier - and it's the suppliers and certifiers who will have to answer the questions from the lawyers and the media.

 

I have a similar application and i want to check before i go ahead with the installation.

A new 3-wire Exd gas sensor. Powered via 24vdc from the safe area, no barriers involved.

And also a instrument cable (4-20mA)from the sensor direct to our Exi AI IO.

With the sensor being Exd, do i need to goto EXi IO? Plus the instrument cable i was going to use is un-armoured.

or do i need to send the power through a barrier, but then the sensor needs 5w of power to operate.

Any advice?

 

If the sensor is Ex d with no other designation, then the associated circuits will not be IS regardless of whether the safe area equipment is connected via a barrier.

The system is safe to use provided the installation complies with the requirements for Ex d. Connecting it to an Ex i-protected input is not going to add any protection, and may get complicated later on in the life of the plant. There are also possible issues with segregation from other Ex i circuits connected to the same module. If there are no non-IS modules available, then connect the gas detector to one on its own and plaster the installation with labels indicating that the field system is not actually IS.

 

A related question for Bruce

I am verifying a vendor package right now. As per the spec the vendor was only required to provide instrument adhering to EXd II C. So intrinsic safety was not required.

However as per his panel design he is using Intrinsic safety barriers for these instrument which are rated EExd [ia] IIC.

The PLC panel is also to be installed in hazardous area next to the SKID. The panel is also EXD IIC certified.

Should I remove the Intrinsic Barrier, as i believe the vendor has not correctly designed the IS loop, and it will be very difficult to rearrange the panel to maintain the required segregation

Or i could leave the IS barrier as it is and specify with a mark that the loop is not certified for IS.

 

Hi Rashid,

As I interpret your situation, the instruments which are rated Ex d [ia] IIC will have a body which is inherently flameproof (the Ex d part). This also has a separate set of terminals which provide an energy-limited output rated as Ex ia. The designation does not indicate whether there is a separate terminal compartment for the Ex i signals, or if the Ex ia terminals are protected by flameproofing.

If the Ex i terminations are not protected by the Ex d enclosure, you have to wire them according to Ex i rules, which means that the receiving end (the PLC) must also have energy limiting fitted to prevent excessive voltage or current being fed to the circuit from the PLC. If you are connecting directly from one flameproof enclosure to another, with nothing else (such as a local meter) connected to the cable, the IS protection is not required.

You need to check in detail with the certificate of compliance for the field instruments, and see exactly how it is being protected. Also check with the manufacturer to see what their recommendation is for your case. (A local supplier may not have the expertise to give a definitive ruling on this installation.) It would also pay to cross-check with the system designer, and get them to provide a detailed explanation of how the system is supposed to work. If you have to satisfy a local enforcement body, also find out how they will interpret the rules.

If all else fails, you need to use your own judgement. Ask yourself what could happen if a short-circuit should occur at the IS terminals of the instrument, and figure out what if anything is stopping the effects from causing an explosion in the external atmosphere. It's probably a good idea to do this anyway regardless of what information you get from other sources.

Hope this helps,

Bruce.

 

Field instruments are now largely available with dual certification i.e. Ex(d) and Ex(i) . This is however not always the case. You should therefore always check the certificates of the instruments involved as supplied by the vendors and compare these with the nameplate on the instrument. Normally an intrinsically safe loop has to be approved / certified as intrinsically safe by some authority. If you were to use an Ex(d) instrument in an Ex(i) loop then you would not get approval for the loop as Ex(i)from such an authority. The danger in using barriers with an Ex(d) instrument is that maintenance personnel assume that the loop is intrinsically safe. Should an explosion subsequently occur from such a loop then legally you will have problems.

 

One further clarification Bruce

The system designer is using isolating switch relays for all input instruments which are only Exd rated.e.g Temparature elements (Pt 100 and RTD in motor enclosure), Position sensors, Level transmitter etc.

Should i remove the Isolation switch relays or there is no harm in keeping them. Because i know considering the space in the panel, i will not be able to effectively segregate the wiring.

Supposing the Ex Ia terminals are protected by the flameproof enclosure, do i have to use the IS barrier.

Appreciate your quick response

regards
Rashid IQbal

 

Hi Rashid,

The situation you are describing seems quite complex - and is getting well away from a simple system that can be dealt with over this forum.

You need to:
- get hold of some definitive documents (specifically the applicable IEC or other standards that apply in your part of the world) and satisfy yourself that what is being done is reasonable (or otherwise)

- or find someone local who can give independent and expert advice after reviewing the drawings and other documents for the installation.

I appreciate that either of these approaches will take time and the second will also cost money. Unfortunately in many cases the requirements to deal with explosive atmospheres are often poorly recognised by those in charge of a project, in particular the need for people who are competent in all aspects of the work (from design through installation and maintenance to maintenance).

I can't really help you much further without a lot more information, such as detailed drawings, equipment manuals and certification documents, etc.

 

Bruce i found your earlier suggestions very useful.

In another example we have Local control panels with Allen Bradley Remote IOs. The hazard area classification of this panel is Dust Ignition Proof.

The vendor intends to wire a Flow transmitter which is intrinsically safe. Therefore he wants to install a isolation barrier in the local panel. Does this barrier itself need to be rated for Dust Ignition proof.

Is this loop design consistent with Intrinsic safety.

 

Hi Rashid,

Since the isolator will be located inside the dust-excluding enclosure, it will not be exposed to dust (provided of course all the rules relating to dust-exclusion are followed). So there is no need for further protection of the isolator.

Once again, a point to note is that the IS wiring inside the panel must be segregated from other non-IS services. There are also restrictions on the type of IS equipment that can be installed in a dust zone as well - but if the flow sensor is rated as Ex i GD this is covered.

It doesn't specifically say so in any of the standards that I'm familiar with, but even if the flow transmitter is located in a gas/vapour hazard area, any IS circuits traversing dust zones must be safe for the dust application as well.

 

> THE ADVICE GIVEN BY KALAN IS INCORRECT and if followed could lead to a fire or explosion.

Bruce you are correct, however, one has to remember that a "simple device" such as a switch or thermocouple, RTD, etc. does not require certification. Therefore, should the RTD connection head be EExd (no transmitter within), then, provided that all circuitry not classed as I.S. is out of the hazardous area, e.g. on the safe" side, and suitable cabling, glands, etc. are used it should be OK. It is a minefield (npi) so one should get familiar with the literature (this last to the enquirer not you Bruce as I´m sure you are already clued up).