Simple Apparatus RTD in IECEx

[Cletus023]

I am a mechanical engineer designing electrical systems so bear with me. I have an installation where we are utilizing an RTD as an intrinsically safe device (no pothead, integral cable run to a JB and then to a safe room panel with a P&F Z954 barrier). Installation site is Australia. I know the application is acceptable for use in Zone 2 IIC T3 application, the difficulty I am having is in proving it to the client. At first they wanted entity parameters to perform loop calcs which I responded with they don't exist. My understanding is that the cable parameters go in to the loop calc/drawing along with the zener barrier parameters and the temp class of the RTD. There is a paragraph in IEC 60079.11 (para 5.7) that discusses basically assessing the max surface temp (through calcs or testing) to ensure the surface temp does not ignite the potential gas in the atmosphere. What I have not figured out is how to do that. Or is there another way to prove that it won't happen (i.e. megger testing the cable). Or do I need properties of the RTD to determine the surface temp? There is a formula for this in NEC, but no equivalent in IEC that I have found. Appreciate the help.

Andy

 

[Compositepro]

An RTD is not a heat generating device. It could not accurately measure temperature if it were. The Zener safety barrier limits the electrical energy transmitted to any down stream device to safe levels as long as there is no possible energy storage, like capacitors. It is only the safety barrier that needs certification.

 

[ScottyUK]

"An RTD is not a heat generating device."

It's a resistance. With current passing through it. How exactly is it not a heat dissipating device?



Cletus,

I don't have the standards to hand, not in work yet but will have a look when I get there.

 

[Michael2009]

RTD and Thermocouple probe assemblies and sensor are passive components or sources of generated energy is not more than 1.5V, 100mA and 25mW. therefore, it consider as simple apparatus. So, element do not need to be certified.

 

[ScottyUK]

Cletus,

My apologies, I forgot all about this thread. You might find the calculation examples in Section 9 of the attachment helpful.

 

[vichoo84]

HI

Basically IS is Intrinsic Safety, by applying this concept we are limiting the energy going to the field which are more hazardous. So to limit the energy a zener diode / barrier will be provided at the control system. This diode will limit the energy which is going to the field. Normally this IS instruments were used in ZONE 0 application. for Zone 2 normal exproof is sufficient.
Now what client ask you is whether the limited energy from the control system is sufficient enough for the instrument / transmitter to work.
So the min. transmitter parameters ( voltage, current, power, resistance etc) to be compared with the actual value coming from the system.
The system value shall be greater than the minimum value provided for the transmitter. with that confirmation we can assure that the transmitter will function.
Hope this helps.