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Thermocouple Installation


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#1 Kathy321

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Posted 01 January 2014 - 11:11 PM

I ;am installing a new thermocouple in my kiln. There is a bare ground wire  from the kiln controller to the thermocouple along with the red and the yellow clad wires. Can anyone tell me what to do with the ground wire? 

Thanks,

Kathy

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#2 Mark C.

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Posted 01 January 2014 - 11:31 PM

You could wrap it around one of those square drive screws which could be loosened 1st than retighten the screw down on wire.

I have never seen a three wire thermo wire.

Mark


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#3 bny

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Posted 02 January 2014 - 12:17 AM

Examine the controller end of the cable. If the third wire is connected at the controller end, you do not need to do anything at the thermocouple end. If it is not connected at the controller, as the previous reply noted, put a wrap under the head of one of the cable clamp screws. If this is difficult, I would not worry about connecting it.

This wire is what is referred to as a drain wire. It provides easy to terminate contact to the foil shield of the cable. This in turn is intended to reduce electrical noise coming into the thermocouple circuit. Usual practice is to terminate cable shields to ground at one end of the cable only.

 

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Why one end only?  What we want the shield to do, is to be at the ground potential of one end or the other of the circuit.  The idea is for external signals incident against the shield, to dissipate mostly into ground, and not make their way into the wires inside the shield.  Operation of the main power relays to the kiln elements as part of thermostatic control cycling, can be expected to generate significant electrical noise, especially due to sparking at the relay contacts.

 

It seems tempting to ground both ends of the shield.  The problem here comes when "ground" (whatever you tie the shield drain wire to) is not at the same potential at both ends.  This can result in currents flowing through the shield (a so-called ground loop), which can either negate the effectiveness of the shield, or cause a hazard.



#4 Kathy321

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Posted 02 January 2014 - 02:19 AM

Thank you all for the very instructional and useful answers.

Mark C. it took me a few minutes to figure out you meant the Robertson head screws;-) I remember now that you don't have them south of the border - too bad as they are the best!






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