Intrinsic Safety 1

[mgopalan]

I am trying to understand the process of designing intrinsically safe equipment for use in a Hazardous area...I need to place a Panel PC in the hazardous area and run data from it from the hazardous area to the safe area.
I already have the PC specd out and it is rated class 1 div 2 but where can I go to learn about safety barriers?

Specifically:
a) I have a 4-20 ma current loop sensor that needs to be powered and output measured..Does it need a barrier?
b) I have straight serial port data coming out of the PC to the safe area...Do I need a barrier for it?
c) And the biggest one of all...How do I send power to the PC itself? It need 24V at 6A!!

Any help?

MG

 

[RAMConsult]

Try this link for an easy explanation of what Intrinsic Safety is about:

http://www.ul.com/regulators/is_circuits.pdf

Now that you have a guide to the lingo, start calling the manufacturers of the equipment that you need and get their literature to ensure that everything is Class 1 Div 2 rated by a recognized authority like UL. For wiring methods use the NEC.

Safety Barriers are devices that control the open circuit voltage and short circuit current of the circuit such that there is never enough energy available to cause a condition that would ignite a flammable mixture.

All items located in a Class 1 Division 2 area must be approved for that use, including your transmitter and receiver. If the signal wires merely pass through the classified area and connect to devices in unclassified areas then barriers at both ends should be ok. Since you say that the PC is rated for this duty, the implication is that all it's terminal connections should be ok but you would have to ensure that the device at the other end has the proper barrier. As for the power supply, my guess is that the PC manufacturer has one specifically designed for this purpose, the use of an ordinary ps is definitely not a good idea.

 

[advidana]

a word to the wise: Hazardous area electrical systems are difficult system to work on for electrical people, much less mechical. Try getting some help from an electrical contractor that does hazardous work.

 

[EEIre]

Normally the PC is located in the non-hazardous area with an IS screen, keyboard and mouse located in the hazardous area.

For an example see:

http://www.extronics.com/Product.asp?productID=60

Regards

Sean

 

[mgopalan]

Thanks for the advice...

Some details..

I plan on using a Class 1 Div 2 Qualified PC from

http://www.laversab.com/2800.html

I need to monitor a Current loop pressure trandsucer and display this information to the screen in the hazardous area using this PC. The pressure transducer itself is Class 1 Div 1 All groups ABCD and is inrinsically safe..

http://www.druck.com/usa/products/pdsa085sept99.pdf

I plan to power the transducer from the safe area through a barrier and have my sense resistor, A/D etc in the safe area. But this pressure data needs to be sent to the PC in the hazardous area where all the math and DSp stuffis done...I plan to just send the data Ethernet or straight RS232 with approproate signal barriers so I think that side is covered.

However, I need to power the PC itself which can pull up to 90 Watts of power (18V @ up to 5 Amps). Obviosuly, the AC-DC switching power supply will be in the safe area. My question is how do I get power to the PC itself? I cannot seem to find any information on whether it is safe to do so without a barrier and if I need a barrier, I cannot find one that rated to 90 Watts.

Also am trying to understand the difference between hazardous area and Intrinsically safe specs...A Class 1 Div 2 certified product is not neccesarily intrinsically safe (right?). And even if it is certified intrinsically safe, it is only so in combination with the correct specified barrier...

Maybe I can get away with just the Class 1 Div 2 on the PC without having it be Intrinsically Safe...In this case, can I just send it power without any barriers?

As for the mechanical/electrical issue...No time like the present to learn. Seems all I do these days (with a masters in Mechanical Engr) is write firmware and DSP anyway...

 

[advidana]

Intrinsically safe equipment is primarily limited to process control instrumentation. ANSI/UL 913-1997 provides information on design, testing and energy requirements. Article 504 of NEC covers the installation and NFPA 493 covers standards for Instrinsic Safety.

Cooper Crouse- Hinds code digest touched on this subject (Hazardous area and C) also check on appleton NEC 2002 code review and a Book by EC&M books " HAZARDOUS LOCATIONS".

Check "intrinsic Barrier Application Guide for Standard Control Circuits" by mfg of Stahl product. Crouse Hinds "Instrinsic Safety" and Crouse Hinds "Spe504 Intrinsically safe Products Application Guide"



I would think that the PC would have to be outside the hazardous area. Check with the PC mfg if there equipment is permitted to be locate in hazardous area or is it permitted to be used with equipment in hazarous area.

 

[RAMConsult]

mgolopan, You are correct, IS and Class 1 Div 2 are not the same and need to be carefully interfaced.

I checked the data sheet on your PC, it has its own power supply that runs off of a 18-36VDC feed but this will definitely not be IS qualified. You need to supply that feed in a properly designed and sealed Class 1 Division 2 wiring system.

It is clear from your questions that it's time to engage the services of an electrical engineer who has done explosion proof work before. This is truly an area of engineering where "a little knowledge is a dangerous thing." You don't want this to end up as a case study in the Forensics Forum, do you?!

 

[BC2]

I belive you can also mount non-intrinsically safe components inside an I/S enclosure, inside of the hazerdous area. All cabling and conduit needs to be I/S. Enclosures inside hazerdous area will need to be purged and monitored with compressed air. I worked on a system where the plc was outside the hazerdous area and an I/S remote I/O network was used inside.

 

[TWW]

They sell class 1-div 1 power 120V outlets (Appleton N1 series). why not just plug your class-1 div-1 pc into the appropriately rated outlet? Of course you may have to install a new outlet - RGS conduit w/seals etc..

 

[merprorenegade]

Having designed intrinsically safe systems in the past, I would point you towards using an instrument engineering contractor or contact your local MTL instruments representative who should be able to assist you in specifying the safety barriers or isolators (try

http://www.mtl-inst.com).

For all instrumentation mounted in the hazardous area, will require to have a suitable isolation barrier. One area to watch out for if using IS systems instead of Explosion Proof is the requirement for a separate instrument earthing system.

 

[TWW]

Is the isolated grounding of IS systems/barriers a code issue?. A truely isolated grounding electrode , if not technically a code violation (according to Mike Holt it is not if bonded to equipment per NEC(96) 250-57), is generally considered a very bad, and potentially hazardous design practice. I am interested as I occationally have to design in areas that require IS devices installed. In these cases we still install the cabling in RGS with seals connected to the EP boxes that the IS sensors are connected.

 

[mgopalan]

OK, you guys have convinced me...Ill start talking to the boss about hiring a consultant...
Thanks for all the help
MG

 

[danrocxl5]

What about simplifying your system:

Since you want only to monitor a pressure transducer why not use a simple process indicator. It seem to me that you do not need a full PC for that purpose. A PC is excellent at making calculations, graphics, datalogging, etc. do you really need those functions to be executed inside the hasardeous area?

There is a lot of process indicator on the market and many of them are powered by the current loop itself. They would show simple current indication (4 to 20 mA) to calculated results like 143 kpa or 34 psi. So I am sure there is such hasardeous area rated indicators. And for me it seems that the price tag would be much lower on both buying, installing, maintaining your equipment. There is a good chance that your boss will appreciate it!!!

By the way: we tend to see PCs in every corner of our life but just think about maintaining or upgrading software in a PC and you might like me get your cold sensation in your neck.

I am sure that your suppliers have that kind of indicators available.

Good luck.
danrocxl5