Hi Control.com experts,
I have question regarding best wiring practice of thermocouple to PLC input channel. currently what I have to be integrated in my system are:
- Thermocouple type K (NiCr/Ni)
- Allen-Bradley Controllogix PLC with Thermocouple input module ( 1756-IT16)
- terminal block for type K thermocouple (Phoenix-Contact 3100062)
- Themocouple cable (Lapp 152045 )
My question is:
Do we need to use ferrules/sleeves that made from same thermocouple
materials as well?
Thanks in advance
I am unaware of wiring crimp ferrules made of thermocouple alloys. That's not to say that such a product isn't out there, but I haven't seen it. I suspect that the market is not large enough to warrant producing them.
If you were to use conventional ferrules, designed for copper wire, the Thermocouple law of intermediate metals comes into play, which says that as long as the terminal connections are isothermal (at the same temperature) then there is no effect on the T-C signal and hence no measurement error. However, there is an error EMF created when there is temperature differential between the thermocouple legs at the connection points.
Why would you pay extra for thermocouple-alloy terminal blocks, which avoids the effects of the law of intermediate metals, and then introduce potential error by using ferrules which invokes the law of intermediate metals?
If you use conventional, standard (for copper wire) terminal blocks, then use standard crimp ferrules and live with the potential for non-isothermal conditions/error.
If you use 'special' thermocouple-alloy terminal blocks, do not use standard (non-thermocouple alloy) crimp ferrules, connect the extension wire directly.
If you locate thermocouple alloy crimp ferrules, please let me know the source.
You do not need ferrules or sleeves made of thermocouple material. You also don't need T/C material terminal blocks as long as where the termination is made, both legs of the T/C circuit are at the same temperature, which in most cases, they are, since they are typically adjacent to each other.
What "ferrules/sleeves" are you referring in reference to wiring?
William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP
"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson
Any information is provided on a Caveat Emptor basis.
Sorry if my word was confusing you, I was referring "ferrules/sleeves" to a tinned copper sleeve, which is placed onto the end of the wire where it is crimped.
Thank you for your suggestion Bill.
With the equipment you list, there is no need for ferrules. Some product spec' sheets may provide information for using such, but it misleads one to believe they are required. They are not. All connecting means are suitable for stripped wire, as all terminals are pressure-plate screw terminals.
If there are any intermediate junctions between panel i/o terminal blocks and the T/C itself use a junction box with the same i/o terminal blocks (or their functional equivalent) or use standard T/C connectors (two-pole spade plug and jack w/color-coded body). A junction box for the latter is not required but can be used to protect from inadvertent disconnection, corrosive atmosphere, or physical damage.
Found some disturbing info last week, RA will be removing most of the ControlLogix analog input/output cards from sale in 2017, and the replacement cards are NOT pin compatible. Using a new card will require re-wiring and schematic/print changes. A partial list is below:
Suggest you search the Silver Series web page from RA
You may want to stock pile a number of spares or consider an alternate supply source (PLC Center or the like).
"Do we need to use ferrules/sleeves that made from same
thermocouple materials as well?"
If anyone ever does need spade lugs that are made of thermocouple material they are available from Omega Engineering. The part numbers for Type K is SLCH-20 for chromel and SLAL-20 for Alumel. They are intended for a screw type terminal lug made of thermocouple material not a compression plate type terminal as they wouldn't be needed. I ran into this once on a Westinghouse turbine and kept it in the back of my mind in case I needed it in the future. I could've rigged it to work without them but it was more professional looking to use them.