IS barriers with motor RTD's

[flandes]

We have a customer requiring IS barriers for machine instrumentation in a haz area. Most instruments are fine but we are discussing embedded RTD's in a large (1000HP), haz rated, induction motor.
Is it not true IS barriers require physical distances from non-IS devices when wired? This certainly true in a terminal box. If RTD's are embedded in the stator of a 10KV motor, are IS barriers the proper solution.
I'm thinking sealed conduit and the like, would be better.
Any comments?

Thanks!

 

[itsmoked]

Generally you need sealed conduit anyway. This is to keep the hazardous atmosphere on the 'other side of the wall'.

You need barriers so if the machine reading the RTDs spazzes out and injects power to the RTD it can't shove more energy than allowed into the hazardous area.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

I imagine the area of concern is the external wiring. The RTDs themselves will be inside the explossion proof motor.
yours

 

[flandes]

Thanks for your input.
My concern is more with the RTD's being nestled in with the high voltage windings of the motor. Worst case, a failure could put high voltage on the RTD leads.
These lead back to the IS wiring in the control enclosure for other instruments.
IS installation required 2" air space between IS wiring and non-protected wiring (ie motor windings).
In my mind, IS protection should not be applied to motor wind RTDs. Does this logic seem correct?

 

[54494060]

The RTD's are "part of the motor" which is explosion proof. If a failure to a/the winding should take place at a position occupied by the RTD it would be in the Hazardous area as waross pointed out. (Motor, Equipment Room, etc.) The RTD conductors are also located inside the Haz area and have to be sealed in the raceway that is exiting the area. Depending on the Class and Division, there should not be any arc switching device in that area, which the RTD's are not a switching device. Therefore "Intrinsically Safe" Article 504 NEC should not apply to your application unless I'm not seeing the total picture (installation). Corrections are appreciated.

12fish
As Iron sharpens Iron so does a man sharpen the countenance of his friends.

 

[sitl]

to flandes:

IS barrier cannot protect you from the fault on high voltage which then skips onto the RTD wiring. Its only purpose is to protect RTD and its wiring from excessive energy surge from the main control panel and as a result rising the temperature of RTD or its wires above the explosion temperature.
You can avoid IS barrier if you are able to put RTD wiring in sealed conduit (and by sealed I mean sealed on both ends in certified way). Sealing should be according to the class and division, but you should check for local regulations. If you do not use the sealed conduit, I would certainly use IS barrier or otherwise prove by calculations that there is no way that those wires from control panel to the RTD can be overheated above the temperature on which surrounding gas might explode.

 

[electricpete]

what does IS stand for?

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[bklauba]

IS = Instrinsically Safe

BK

 

[electricpete]

Thx!

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