Intrinsic Barriers

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fifty60

fifty60

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I am trying to get some traction with finding rules and regulations when it comes to intrinsically safe barriers. I have 2 types of sensors: Thermocouples and a 0-5V signal. I know thermocouples themselves do not have to be rated intrinsically safe, but does the 0-5V sensor have to be rated intrinsically safe? The power supply for the 0-5V sensor consumes >50mA, but I believe this is still high to use with an intrinsic barrier.

Does my 0-5V sensor have to be rated as intrinsically safe before I can use it with an ISB? Or does the ISB make the sensor intrinsically safe?
 
The rules for IS are in Article 504. The two most overlooked items are the requirements for a control drawing [504.10(A)] which will tell you where the barriers are to be located and that bonding requirements are the same as required in the underlying Class that the IS is installed in regardless of the wiring method used.[504.60]
 
fifty60 said:
Does my 0-5V sensor have to be rated as intrinsically safe before I can use it with an ISB? Or does the ISB make the sensor intrinsically safe?
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The barrier makes the circuit IS, at least the part downstream of the barrier.

The control drawing tells you what parts can be used with the barrier.
 
I get the following from Omega's website:

[FONT=Verdana, Geneva, sans-serif]Most of the apparatus that is mounted in the Hazardous area will have to be approved and certified for use in the Hazardous area with an approved barrier designed for use with that apparatus. Some simple devices like thermocouples, Rtds, LEDs and contacts can be used in the hazardous area without certification as long as it is wired in conjunction with an approved barrier.[/FONT]

[FONT=Verdana, Geneva, sans-serif]So if i have a 4-20mA transmitter, like a humidity sensor/transmitter, that transfmitter will have to be approved as well even if I use an ISB? The probe of the transmitter is inside the hazardous area.[/FONT]
 
fifty60 said:
I get the following from Omega's website:

Most of the apparatus that is mounted in the Hazardous area will have to be approved and certified for use in the Hazardous area with an approved barrier designed for use with that apparatus. Some simple devices like thermocouples, Rtds, LEDs and contacts can be used in the hazardous area without certification as long as it is wired in conjunction with an approved barrier.

So if i have a 4-20mA transmitter, like a humidity sensor/transmitter, that transfmitter will have to be approved as well even if I use an ISB? The probe of the transmitter is inside the hazardous area.
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Don't know who Omega is but, if that is an accurate quotation, they don't fully understand Section 504.20.

The "other" Bob (petersonra) noted the proper relationships. The real question is how is the transmitter powered and how does the associated control drawing identify proper barrier locations.
 
Would the 0-5V transmitter be considered an associated apparatus? I am trying to find in NEC 504 where it specifically says that my non-intrinsic sensor is ok to use with an ISB in an intrinsic circuit.
 
fifty60 said:
Would the 0-5V transmitter be considered an associated apparatus? I am trying to find in NEC 504 where it specifically says that my non-intrinsic sensor is ok to use with an ISB in an intrinsic circuit.
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Look at the definition of "simple apparatus". That tells you some of the things that you can potentially connect to an IS circuit without losing the IS system status.
Simple apparatus shall be permitted to be installed in any hazardous (classified) location in which the maximum surface temperature of the simple apparatus does not exceed the ignition temperature of the flammable gases or vapors, flammable liquids, combustible dusts, or ignitible fibers/flyings present.
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Whether the 0-5 transmitter would be OK would depend on whether it can operate properly with just the power available from the receiver or power supply when connected through the IS barrier.
 
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fifty60 said:
[FONT=Verdana, Geneva, sans-serif]So if i have a 4-20mA transmitter, like a humidity sensor/transmitter, that transfmitter will have to be approved as well even if I use an ISB? The probe of the transmitter is inside the hazardous area.[/FONT]
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The transmitter may or may not have the potential to be in an IS circuit.

Its also possible that the xmtr could be installed outside the hazardous area and wired thru an IS barrier to the probe.

You best bet for good information on this kind of thing is the manufacturer of the instrument.

I don't know if it is available for humidity sensors but many 2 wire xmtrs are available in XP versions for not a whole lot more than the regular version. Surprisingly, sometimes this option is less expensive on the whole than going the IS route.
 
I am still not 100% understanding NEC 506. 506.20 says that only equipment listed and marked as suitable for the location shall be permitted. This applies to zone 20,21, and 22. I am not seeing the exception that allows for a sensor, other than T/C, to not be labeled intrinsically safe.

I agree, however, that the ISB makes the sensor intrinsically safe by limiting the power available to the sensor. I am still not seeing where the NEC allows the sensor itself to be non-intrinsically safe, but it becomes intrinsically safe with an ISB.

Is it the definition in NEC 504.2 for Associated Apparatus that allows me to do this? Is the sensor, protected by a separate ISB, considered an Associated Apparatus? Does the ISB and the sensor combined constitute the Associated Apparatus?
 
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fifty60 said:
I am still not 100% understanding NEC 506. 506.20 says that only equipment listed and marked as suitable for the location shall be permitted. This applies to zone 20,21, and 22. I am not seeing the exception that allows for a sensor, other than T/C, to not be labeled intrinsically safe.

I agree, however, that the ISB makes the sensor intrinsically safe by limiting the power available to the sensor. I am still not seeing where the NEC allows the sensor itself to be non-intrinsically safe, but it becomes intrinsically safe with an ISB.

Is it the definition in NEC 504.2 for Associated Apparatus that allows me to do this? Is the sensor, protected by a separate ISB, considered an Associated Apparatus? Does the ISB and the sensor combined constitute the Associated Apparatus?
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is this a zone 20,21, or 22 application? I am not familiar at all with how the zone stuff works.

generally speaking, the sensor is never IS. The circuit is what is IS.

article 504 has a definitions section. Associated apparatus is the first definition.
 
Ever since ISA essentially took over NEC "Zones", I have my doubts that CMP14 has a 100% understanding of Article 506. It was basically written by ISA. In fact, ISA represents the US on all IEC hazardous location activities. They aced out UL, IEEE and NFPA.

That said, Section 506.8(C) specifically recognizes Article 504 for IS applications.
 
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Last edited by rbalex; Today at 09:19 AM. Reason: Added UL as an ISA victim
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I liked this editorial comment.

Personally, I wish the NFPA would never have accepted the idea of zones for hazardous areas. It just further confuses an already confused arena and really did not do much good. I suspect it has led to a lot of people installing inappropriate equipment.

It seems to me that it was oriented toward off shore platforms and could have been handled in some other way to allow zones to be used there. It is not even clear to me that off shore platforms are subject to the NEC anyway, being as they are off shore.
 
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Being new to the NEC requirements to IS, the zone classifications seem more straight forward to me. This equipment would be Zone 21. To me reading NEC 504, the transmitter itself is not an intrinsically safe apparatus, but the ISB is an associated apparatus.

The transmitter itself will not be in the hazardous zone, but the sensor to the transmitter will be in the hazardous zone. The sensor for the transmitter is not an intrinsically safe barrier. Powering the transmitter through an ISB makes the sensor and the transmitter output voltage intrinsically safe. The sensor then becomes an intrinsically safe circuit and can be located in the hazardous zone.
 
fifty60 said:
Being new to the NEC requirements to IS, the zone classifications seem more straight forward to me. This equipment would be Zone 21. To me reading NEC 504, the transmitter itself is not an intrinsically safe apparatus, but the ISB is an associated apparatus.

The transmitter itself will not be in the hazardous zone, but the sensor to the transmitter will be in the hazardous zone. The sensor for the transmitter is not an intrinsically safe barrier. Powering the transmitter through an ISB makes the sensor and the transmitter output voltage intrinsically safe. The sensor then becomes an intrinsically safe circuit and can be located in the hazardous zone.
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You need to stop thinking of the device as being IS and think that the circuit is IS. IS means that there is some means by which the energy that the circuit could generate is not sufficient to cause ignition. It is not just at the sensor or xmtr, but in the wiring that is protected by the IS barrier.

There is no guarantee that taking just any xmtr and powering it through just any IS barrier will make the downstream circuit IS. In fact, it is possible that there is no IS barrier that can make the circuit IS.

for instance, suppose the xmtr included a large capacitor. feeding power to the xmtr through an IS barrier would not prevent the storage of enough energy in the capacitor to prevent ignition if the energy was released.
 
fifty60 said:
Being new to the NEC requirements to IS, the zone classifications seem more straight forward to me. This equipment would be Zone 21. To me reading NEC 504, the transmitter itself is not an intrinsically safe apparatus, but the ISB is an associated apparatus.

The transmitter itself will not be in the hazardous zone, but the sensor to the transmitter will be in the hazardous zone. The sensor for the transmitter is not an intrinsically safe barrier. Powering the transmitter through an ISB makes the sensor and the transmitter output voltage intrinsically safe. The sensor then becomes an intrinsically safe circuit and can be located in the hazardous zone.
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From someone who has been doing electrical area classifications (both NEC and "classic" IEC based) over 45 years, considering NEC Zones being more straight forward is something of a surprise since NEC Zones are IEC Zones 'forced-fit" into NEC Divisions in the first place. Classifications by NEC Zones required a PE in 2005 and earlier. It was dropped because only two CMP14 members classified as "Users" (those that would actually do electrical area classifications) were PEs. BTW the non-PEs were well qualified.

Nevertheless, your basic analysis appears to be correct; however, you still seem to not have caught the control drawing requirement. (That is also an ISA burden) petersonra's comments are also well taken.
 
Rbalex, thanks for the help. I am trying to figure out the drawing requirement now.

I am concerned about Petersonra's comment about the transmitter may never be able to be part of an IS circuit, regardless of the ISB. How can I determine if the transmitter/sensor would not be able to be used in an IS circuit? Is there a particular sized capacitor on the PCB, or any other component that I should look for?

As I mentioned before, the transmitter itself is a printed circuit board and is located inside an enclosure outside of the hazardous area. The probe sensor from the transmitter is located inside the Zone 21 hazardous area. The transmitter and probe sensor are both powered through a single ISB. There are capacitors on the PCB, is the concern that they could discharge through the probe sensor? How real of a concern is this?
 
I agree, and that is why I am concerned. How do I know if my transmitter/sensor can be used in an IS circuit? Is the fact that it is not the transmitter but only the probe in the hazardous location sufficient? I would think that nearly every piece of equipment used with an ISB would have this risk.
 
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fifty60 said:
I agree, and that is why I am concerned. How do I know if my transmitter/sensor can be used in an IS circuit? Is the fact that it is not the transmitter but only the probe in the hazardous location sufficient? I would think that nearly every piece of equipment used with an ISB would have this risk.
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As long as the transmitter is outside the classified area and is on the other side of the IS barrier from the sensor and the control drawings are followed, I am going to give that a qualified yes. But I defer to rbalex for the right answer.
If the sensor and the transmitter are on the same side of the barrier, the transmitter could defeat the IS principle by storing energy that could then be released in the sensor or its wiring.
 
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