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Join Date: Oct 2009
Location: Michigan
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Thermocouple

01/05/2011 12:51 AM

T/C s are classified as simle divices.

It will not create / store sufficient energy to ignite mixture of volatile gases.

But still when it is installed in Hazardous area it not considered as intrinsically safe without a barrier to limit energy . why is it so ??

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#1

Re: Thermocouple

01/05/2011 2:54 AM

Maybe the sensing circuitry on the other end could energize the wires enough for a spark to occur.

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#2

Re: Thermocouple

01/05/2011 4:09 AM

The term intrinsic safety relates to a complete installation, not to an individual piece of equipment. It is a technique used to limit the available energy and power to a level where an explosion cannot occur. The term is the least understood among the techniques for preventing conflagrations in hazardous areas, according to a statistically insignificant poll of CR4 postings.

A thermocouple is simple apparatus in the context of intrinsic safety.

Intrinsic safety cannot be assured without some form of device in the circuit that limits the available energy stored and power dissipated in the hazardous area to a level where an explosion cannot occur.

So the answer to the question depends upon what the thermocouple is connected to and where that equipment is located. Please advise the forum.

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Guru

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#3

Re: Thermocouple

01/05/2011 9:26 AM

The barrier is designed on the principle that a human MISTAKE or equipment failure could energise any terminal on its SAFE side at 250V AC (with a considerable short circuit capacity). The barrier contains fuse(s), resistors, voltage limiting devices etc. which ensure that even that event could not allow a spark in the hazardous area which could ignite the specified flammable substance/air mixture. There is a condition that the panel in which the barrier is located, or a segregated (containing I.S. barriers) part of it, and circuits connected to barrier are powered only by an safety isolating transformer (250V or less output) which limits short circuit current to values which the fuse in the barrier can disconnect quickly enough.

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#4

Re: Thermocouple

01/17/2011 1:25 AM

Thermocouples are classified as IS device in general, If T/C is connected to the temperature transmitter in the classified area which is not Exi, then T/C will no be Is, since the transmitter is not Exi, so the entire T/C loop components like Transmitter, junction box to be exi, in additional to that a barrier is considered in this loop.

If T/C is connected by thermocouple extension cable, then the particular T/C loop can be considered as IS circuit.

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#5
In reply to #4

Re: Thermocouple

01/19/2011 6:25 AM

Disagree your last statement. Connection of T/C by thermocouple extension cable, does not make safe. Unless Connection of T/C to safe area is via barrier at boundary of safe area and connecting components comply with basic IS rules.

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Commentator

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#6
In reply to #4

Re: Thermocouple

08/22/2011 5:47 AM

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