Thermocouples are considered active devices (as they produce a voltage difference), where as RTDs are considered passive devices lacking energy storage. Both are deemed to be intrinsically safe apparatus only when used with suitable intrinsically safe barriers or galvanic isolators. The barriers or galvanic isolators provide the protection, they cannot be used by themselves without these energy limiting barriers or isolators in intrinsically safe circuits.

http://en.wikipedia.org/wiki/Thermocouple

http://en.wikipedia.org/wiki/Resistance_thermometer

http://en.wikipedia.org/wiki/Intrinsic_safety

]Yes.. without using IS Barrier / Galvanic Isolator , i cant say my loop is Intrinsically Safe . But cable vendor advised me to use non IS cable for RTD elemet. Any reason behind it ?

When you mention non intrinsically safe cable do you mean

Standard cable not protected by a barrier or isolator?

Standard cable with no built-in cable screen for EMI or mechanical protection?

Or standard cable not protected by a barrier or isolator AND not specifically coloured blue to designate that it is an intrinsically safe circuit?

Depending on what standard you are working to, if your circuit is NOT intrinsically safe then the wiring and/or cable sleeve cannot be blue (which visually designates an intrinsically safe circuit). Is this what your cable vendor was advising you?

Hi there,

A thermocouple is a source of generated energy. A RTD is a source of stored energy.

As you would know - a thermocouple will need the compensating type lead for it's type. i.e. J/K/S etc.

There is no such thing as IS and non IS cable.

The important thing when working with IS is to limit the energy to the loop. This energy is determined by the inductance, impedance and capacitance of the total loop (instrument, cable, barrier/isolator and loop length). This in turn will determine your cable size i.e. 0.5mm, 1.0mm or 1.5mm.

If you keep the above mentioned factors in mind and look at a few cable tables - you will notice that the only thing that is different from a cable used in IS loops and that used in non-IS loops is the color of the outer sheath. The physical parameters of the cable i.e. inductance, impedance and capacitance are all pretty much the same for most instrument cables of the same size - hence no IS or non-IS cable.

To get the values of the maximum inductance, impedance and capacitance allowed into the IS area, you would have to look at the standards that govern your country.

I am not sure what your vendor is trying to get at - perhaps you should get him to elaborate?

Regards,

Craig

Intrinsic safety is a method of preventing explosion that prevents the circuit from producing an explosive spark. It is about limiting the voltage and current in a circuit to the inside of an envelope within which an explosion-making spark cannot occur, and about limiting the inductive and capacitive properties of the circuit such that an explosion-making spark cannot occur.

Under that principle, two bare conductors could be used in the hazardous area circuitry provided the circuit parameters were inside the above two envelopes. Therefore, as correctly pointed out above, there is no such thing as intrinsically safe cable.

The correct selection of hazardous area equipment and the correct design of hazardous area circuits ideally requires formal training before embarking on the task. That the question has been raised here in the way it has suggests that the original poster's training is lacking. Should this be the case, then the application of an appropriate training course or the substitution of an appropriately-trained designer to carry out the circuit design is an urgent priority on safety grounds.

For thermocouple extension cables made of different than the TC joint alloys you must ensure that temperatures on the extension joints (all four connections preferably) are in the same temperature to have correct output. Think this is not very practical unless you're willing to use active temperature control on extension joints (!)