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Nonincendive transmitters 5

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rauval

rauval

Mechanical
Jun 2, 2009
7
I am working on a design in a Class 1 Div 2 hazardous location. Have 10 Yokogawa pressure transmitters, 24 VDC supplied, loop powered, wired to an AB Contrologix PLC. The transmitter model I selected is rated as nonincendive (NI)for Class 1 Div 2 and intrinsically safe (IS)for Class 1 Div 1. Customer wants me to use an explosionproof transmitter, arguing that my loop diagrams do not show any NI or IS wiring, which is true. How do I show NI wiring in a loop diagram? How do I argue against the explosion proof request?
How do I wire the transmitters in order to conform to the nonincendive rating? According to NEC 501.4(B)(3), I can use wiring methods allowable in non-hazardous locations.
 
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If I was your customer I would be close to being a former customer if I thought the design was being done by someone unfamiliar with the subject. If you aren't 100% certain that you know what you are doing with hazardous area design then I really suggest leaving it to someone who does. There's no shame in admitting that something is beyond your knowledge. Is there anyone in your organisation with expertise on the subject who you can ask for help?

Here's s starting point: have you got a correctly specified IS barrier in the loop? Check out MTL's web site for some useful information, also Pepperl & Fuchs is worth a look.


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Under the NEC standard electronic transmitters are often used in a Class I, Division 2 area citing the exception to 501.105(B)(2) (2008 code).
501.105(B)(2) Resistors and Similar Equipment. Resistors, resistance devices, thermionic tubes, rectifiers, and similar equipment that are used in or in connection with meters, instruments, and relays shall comply with 501.105(A).
Exception: General-purpose-type enclosures shall be permitted if such equipment is without make-and-break or sliding contacts [other than as provided in 501.105(B)(1)] and if the maximum operating temperature of any exposed surface will not exceed 80 percent of the ignition temperature in degrees Celsius of the gas or vapor involved or has been tested and found incapable of igniting the gas or vapor. This exception shall not apply to thermionic tubes.
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It would be a rare combination of equipment and products where the normal operating temperature of a transmitter would exceed 80% of the auto ignition temperature of the flammable product.
 
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Although I buy few Yokogawa transmitters, I suspect that the transmitter is likely suitable for an explosion-proof installation with rigid conduit, etc. The electrical installation materials are among the differences between an explosion-proof installation and non incindive. Contact your Yokogawa peddler. I too am with your customer on this.
 
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People get Div 1 methods of protection messed up with Div 2 methods of protection all the time.

A device which is rated Class 1 Div 2 NONINCENDIVE by an approved testing laboratory is suitable for use in a Class 1 Div 2 area classification. It does not require the use of an XP enclosure and poured seal, or zener barrier and IS wiring (intrinsic safety methods for Div 1 areas.

Some clients have standards which require that seals be installed and poured on all devices with an explosion-proof housing, REGARDLESS of whether or not the device is ALSO rated Div 2 nonincendive. The idea is that some devices are rendered suitable for Div 2 areas SOLELY by means of explosion proofing (ie. the devices themselves are NOT rated as non-incendive for Div 2). Pouring the seal removes the risk. And it's a nice, simple rule that their electricians can understand: if it's in an XP box, you pour a seal- always.

By the same logic, some clients require that any device which is rendered IS by the use of a barrier, be installed with a barrier and IS wiring regardless of whether or not the device is ALSO rated as nonincendive for Div 2 (ie. WITHOUT a barrier).

You didn't mention which code you are designing under, but these "belt and suspenders" client standards go above and beyond the requirements of NEC (NFPA 70. Customers are of course free to specify anything they want which goes beyond code- as long as they pay for it.

 
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moltenmetal,
Thank you for your tip. I am designing by NEC (NFPA 70), but I also found an FM approval standard 3611, which specifies calculations that have to be run in order to approve the entire system (transmitter+wiring) as nonincendive. I am going through it right now.
 
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I am disappointed in the number of posts that jump on rauval for his question. If you review his post and you understand C1,D2 non-incendive design, you will notice that his design basis is solid. His only problem is a client that does not understand non-incendive design. His post here looks to me to be due diligence. Designing to the NEC, NI wiring doesnt need to be specified since GP wiring to NI devices is specifically permitted. Does anyone indicate GP wiring on their non-hazardous locations?

Russell White, P.E.
Automation Technologies, Inc.

Automation Help
 
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In addition to the NEC, address ISA S5.4 and ISA 12.12.01. Among the minimum content Instrument Loop Diagrams require information regarding the energy sources. It is important that the transmitter be rated non incindive but that is not the end of the story. Doesn't a Non Incindive design require some device must assure that the energy is limited? Isn't some barrier like device required for nonincendive field wiring circuits to limit the energy levels. An intrinsic safety barrier would be such a device.

Does the loop drawing show an energy limited analog input module selected for the AB Contrologix PLC? Does such I/O exist? If I touch a tool across the transmitter leads the energy level must be below the level that could ignite a source. Non incindive is like intrinsic safety less the redundancy etc.

Wiring per se, a.k.a. the conductors are neither hazardous nor non hazardous. The energy must be limited; or contained.

How does rauval show NI wiring; or wire the transmitter? Show the energy limiter.
How does he argue against the explosion proof requist? I would not argue but agree to comply. However demonstrate that the loop is compliant. The NI transmitter itself does not comply.

If all of this is wrong then I don't understand NI wiring requirements. I don't use NI wiring devices. I prefer explosion proof practices. The transmitter is likely available compliant with IS, NI and explosion proof practices.

Regardless of the system assure that nothing within the design would cause a disaster.
 
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NI does not require a barrier. The field device gets the rating and it indicates primarily that there are no arc producing components. There is more but that is the major part of it. Not special wiring is required either.

Russell White, P.E.
Automation Technologies, Inc.

Automation Help
 
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How can you run any kind of power supply wire into a hazardous area without that wire having been through a barrier? (Not talking explosion proof.)

Just because the device is NI doesn't mean the wires running to it can't be compromised and deliver ignition energy.

What am I missing here? Is this because it's Div2 as compared to Div1?

Keith Cress
kcress -
 
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Yes division 2 is much less stringent than division 1. If you look at many of the PLC's and operator interfaces today they are rated for Class 1, Div 2. It used to be that you had C1,D2 approved fronts (enclosure had to be purged) for your operator interfaces, but now the whole device is rated for the area. How is this justified for a hazardous area? The assumption with division 2 is that the vapors are not normally present.

If you are carful with your component selection, you can have a non purged I/O or control panel in a c1,D2 area. Usually relays are the hardest component to get compliant, but there are relays with hermetically sealed contacts.

Russell

Russell White, P.E.
Automation Technologies, Inc.

Automation Help
 
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I've learned somewhting there - US requirements for Division 2 areas appear less onerous than for the vaguely similar ATEX Zone 2 in terms of barriers. My mistake - and thanks Russell.



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You can put a Division 2 rated PLC in a NEMA 4 enclosure in a Division 2 area. However you need warning labels to perform gas checks or remove power (explosion proof disconnect) before servicing, not disconnect any terminations etc. The NEC even has a means to put certain fuses in a Division 2 area with the same requirement to remove power or verify to be gas free.
 
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