IS Circuits 1

[jcfoley]

As you gentlemen have been so much help in the past, I wonder if you would mind verifying my assessment regarding an IS device installation.

Basically, a 4-20 mA transmitter is connected to a zener barrier and then to a PLC. The transmitter has no hazardous ratings whatsoever. An IS circuit is protected not only by a power limiting barrier but also by preventing a device (or even the wiring) from storing enough energy to create a spark. If the device has never been investigated or approved for this function (via the entity parameters) then the complete circuit is not an IS circuit. Right?

Now considering that I have a Class I, Div 2 location, would a non-hazardous transmitter used with a barrier be considered non-incendive?

Thanks

 

[itsmoked]

The barrier DOES NOT prevent a device from being incendive because it may have caps or inductors that are too large in it.

But maybe this is what you were saying.. Wasn't clear to me.

We often put devices that have no "IS rating" on barriers in hazardous locations. But before we did we would get a letter or confirmation or examine the device to asses it's suitability. On the other hand, we were a company that made IS devices and had little fear of error.

Everyone does this. For instance you won't find a thermocouple or RTD that is IS rated! They are by default.

Someone here will chip in with the "book" hopefully.

 

[resqcapt19]

I don't see any requirement that an electronic device used in a Class I, Division 2 location be listed as IS or non-incendive. See the exception to 501.105(B)(2) in the 2005 NEC. (501.3(B)(2) in the 2002 NEC)
Don

 

[Rheinhardt]

Hi Guys,

I was reading your above posts and just have a general question.

When approving your IS-Loops with an explosion prevention authority. If there is no information regarding the mutual capacitance and nominal inductance in the suppliers documentation, what figures do you use for the intrinsically safe maximum cable lenght calculation.

Quote : itsmoked
"We often put devices that have no "IS rating" on barriers in hazardous locations. But before we did we would get a letter or confirmation or examine the device to asses it's suitability. On the other hand, we were a company that made IS devices and had little fear of error."

What happens if there is an explosion on the plant, they trace back that there is no IS-Rating on the spesified device in the field and no calculation was done??

Wouldn't that cause havoc??

I have been to plants where there was zone classification on equipment but no barriers, IS-loops nothing.

The system was still working but i asked the client what the result would be if an explosion did occur.

What is your take on this???

Thanks in advance

 

[jcfoley]

This is exactly the sort of thing that I am trying to address. My company supplies compression equipment for natural gas. Everything that we do is CL I, Div 2 and many of the "this is how we have always done it" methods are not even close to what is require by code.

It is sad to say that many people do not understand hazardous locations at all. We have someone in our company who decided that non-rated transmitters were acceptbable in one unit simply because another application used one. This is truly the blind leading the blind.

As for handling a non-rated instrument with a barrier and making your own calcs, it is very risky. In the event of an acccident were the instrumentation had no protection, ratings or barriers, the legal proceedings would be extreme. If I were the PE sitting on the defence's side, I would be unmerciful as we are paid to protect the public.

I believe that the IS testing parameters were written under UL guidance. UL and other NRTLs exist to limit liability to the end user (or designer). As I am not an electrical expert, I will not use any device that is not approved by an NRTL or is proscribed by NEC or other industry code. This is for the company's protection, the end user's protection as well as my professional career protection. It all boils down to protecting the public.

 

[Rheinhardt]

jcfoley,

This issue is one that i would adress at your company and even take it to a higher level (internally) if neccesary ASAP, as you say we are here to protect our employees, public and ourselves.

I would certainly document proceedings whiles tackling the issue, although many will shunt you off as this will be a costly exercise. In the end you raised the issue and you must ensure that if and when something goes wrong you cannot be kept liable, this issue can go as far into court as a drunken driver killing a pedestrian.

We recently had an explosion at a petroleum refinery, needless to say i know of a couple of engineers which hav had to put their careers on hold pending investigations,trials ect.

As for Intrinsic safety i feel this issue alone must have it own forum group, we cannot exclamate the importance of this enough, its crazy that Ex classified instrumentation is so expensive and i find the cost related issue being traded for safety. Alot of companies get off with safety malpractise, but you never know when something gonna wrong on your plant.


Regards

--Off all the things i've lost , i miss my mind the most--

 

[Rheinhardt]

jcfoley,


Here is the link i was looking for

http://www.iaei.org/magazine/02_f/babiarz.htm

Explains most of what you need to know

Regards

Rheinhardt

--Off all the things i've lost , i miss my mind the most--

 

[resqcapt19]

We have someone in our company who decided that non-rated transmitters were acceptbable ...

The NEC, in many cases, does not require an instrument that is listed or rated for use in classified areas for this application.
Don

 

[itsmoked]

Hi Rheinhardt;

We had a document that had all the curves for each environment and the ignition energies. The curves showed voltage verse capacitance and inductance verses current.

If your system stayed below the curves, for example if you kept voltage below five volts you could have infinite capacitance in a device, etc.

It becomes pretty easy to say this device that has 0.47uF in it and will be excited thru a barrier with 5V will BE safe.

This was what we did.

But alas other than remembering the book was some ISA publication I have been unable to find it again.


BTW that link is a nice one, thanks!

 

[Rheinhardt]

Hi resqcapt,

you are absolutely right, for a Class 1 Div 2 zone classification you need no certification excepting the exclusions as you said.

I has to do some homework and found this very very helpfull link regarding classification and certifications.

It shows a comparison between US and Canada, we here in africa use the zone classifications.

http://www.hubbell-canada.com/whitePapers/wiring/codesSWP.asp

A Zone 2 (Class1 ,Div2) is stated as a zone where under normal conditions the explosive material is not present, only under abnormal conditions (pipe break) ect.

Thanks again.

jcfoley, maybe it would be a good idea to get the NEC,IEC documentation.

Kind Regards

Rheinhardt

--Off all the things i've lost , i miss my mind the most--