Give me a break.
A pair may mean 2 conductors, if a shield is added it may mean there are three conductors (a triad). The definition of a triad or a pair (whether three or two conductors or more) can be subjective.
You can even get into three core coaxial cables (also triads) thats what I think you were originally interested in.
A coaxial triad has better noise rejection than a twisted pair.
Triad cable will have 3 conductor cores & it is used as signal cable for 3 wire transmitters. You can have multiple triad cable for example 8Tx1.5mm2 means 8 triad (8x3=24 Coductor cores) where as 1.5mm2 is coductor cross section area.
Where as Pair cable as name suggests the conductor core will be in pair (2 Cores); here also you can have multipair cables (e.g.:6Px1.5mm2).
Triads are usually twisted, shielded three wire (three conductor) cables. The twisting and shielding help to reduce the possiblity of voltages and currents being induced on the wires of the triad. (As with twisted, shielded pair instrumentation cables, the shield drain wire should be terminated at only one end of the cable. It doesn't matter which end, but by convention (not standard, just typical practice and convention) the shield drain wire is to be terminated at the "source" of the power being carried by the cable.)
Some devices that operate at low voltages/currents require three wires for proper operation. Examples of uses of the three wires might be that one wire would be a common or neutral wire, a second wire might be +24VDC, and the third wire might be the signal coming from (or going to) the device. The implication here is that the power supply and the signal share the same common or neutral, so a fourth wire is unnecessary.
Some older pressure- or temperature or level transmitters used triad cables. Some hazardous gas sensors require triad cables. Some vibration and proximity sensors require triad cables. RTDs usually require triad cables.
Hope this helps!
The most common use for triad cable in instrumentation is for RTDs, from the sensor to transmitter.
Sometimes used for 3 wire transmitters but these are not so common.
Yes and as said Roy, you can find triad for RTD in 3-wire-configuration (cable between temperature element - TE - and temperature transmitter - TT - when you need to install this TT far away from TE).
After, you can find quad (four wires) for RTD in 4-wire -configuration (or sometimes 2 pairs) for the same purpose to reduce error of distance and keep all the wires of the same TE in one shielded cable. We use this configuration in nuclear plant because we can not install transmitter near TE (RTD 4 wires) in some systems and we need good precision.
I use both twisted pair and triad shielded cable because I use some 2 wire 4-20ma transmitters, and also use RTD's as well as 3-wire transmitters.
I am looking at standardizing on 1 cable to take to the field for all situations. Does anyone know of any drawbacks to using Shielded Triad cable when only 2 of the wires are being used? Should I just clip the 3rd wire, or tie it in with the shield at one end?
Basically what I am asking is if the 3rd wire is not connected - does it have the potential to pick up noise?
While I can understand the need for standardization, I find your solution quite excessive. First, your connections that use 2 wires only will be connected at 50% higher price. If you use copper cables, those 50% will be quite a lot. Then, all your cables will be thicker than necessary resulting in bigger cable glands (extra cost), possibly bigger junction boxes (again extra cost), extra space taken in control panels where usually all these cables are collected (more difficult to handle, clean, maintain, troubleshoot).
On the other hand, using only 2 wires from a triad should make no difference if you make sure the unused one is properly fixed and isolated.
So, if you just want to have some spare cable around just in case, having only 1 type, twisted and shielded triads, is not a terrible idea. Overusing it... well... in my opinion is a terrible idea.
It is quite ok to use just two of the wires for a two wire instrument. Don't cut the other off, sometimes it comes in handy for testing.
In multi pair wires between a DCS and field junction box it's also common to use paired cable for RTDs.
Example: if you had 2 RTDs in an 8 pair junction box you can use a 1-1/2 pairs per RTD or just one common wire for many RTDs.
The important thing is all the wires to an RTD have exactly the same resistance.
>What is difference between Triad and Pair cable?
A Triad has three conductors twisted, and a pair has only two conductors twisted. As an example see this link to Armored Triad Cable which further delves into your question: https://1xtechnologies.com/armored-triad-cable/