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Topic Title: Can anyone share a 3-wire instrument loop sheet?
Topic Summary: 3 wire gas sensor
Created On: 08 August 2012 09:03 AM
Status: Read Only
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 08 August 2012 09:03 AM
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gadge

Posts: 2
Joined: 30 December 2009

I've never worked with 3 wire instruments before so this is a new one to me.

I'm installing 3-wire gas sensor's in a haz area that will go direct into our siemens S7 PLC (iSP haz area I/O).

Apparently the loop power is not enough so i have to supply the sensor with external power and use a 4-wire AI card abd set the sensor as source and not sink powered.

Has anyone got a loop of how this should be wired?

Thanks
 08 August 2012 02:26 PM
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JohnLindop

Posts: 15
Joined: 30 October 2004

The ones i have used have:
3 wires or connections
+v (Brown wire) positive from power supply.
sig (Black wire) signal wired into input of PLC I/O.
0v (Blue wire) 0v from power supply.
Not sure about the wiring of a IS module from Siemens but a would guess that the 4 th terminal should be tied to either 0v or signal.
Best of luck.

John
 15 August 2012 04:01 AM
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alamatec

Posts: 73
Joined: 04 January 2007

Originally posted by: gadge

I've never worked with 3 wire instruments before so this is a new one to me.



I'm installing 3-wire gas sensor's in a haz area that will go direct into our siemens S7 PLC (iSP haz area I/O).



Apparently the loop power is not enough so i have to supply the sensor with external power and use a 4-wire AI card abd set the sensor as source and not sink powered.



Has anyone got a loop of how this should be wired?



Thanks

2 of the wires provide power to the instrument +24V DC & 0V DC. The third wire provides the current drive to the card 4-20mA. So you are correct that the instrument is the source. Make sure that both the instrument and the I/O card use the same power supply.
 28 August 2012 09:22 AM
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chartassuk

Posts: 57
Joined: 25 July 2008

Hello gadge,

Additional criteria to be satisfied with the proposed loop is that Intrinsic Safety (or the agreed concept of explosion protection) is achieved.
 14 October 2012 08:28 AM
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gadge

Posts: 2
Joined: 30 December 2009

Thanks for replies. Didnt get notification for some reason, anyway..

I set the gas sensor up on the bench, with a 24vdc supply going to the sensor across + and 0V, and for the signal out i doubled up on the 0V and 4-20mA +. I was getting the correct mA reading on the multimeter.

Good point about using the same PSU, i was going to use a seperate one, only because i dont think there are any spare ways off the PSU supplying the ET200 iSP PLC.
 17 October 2012 02:51 PM
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MAXMIRA

Posts: 79
Joined: 25 January 2011

Just a note to say beware that you do not infringe the I.S. integrity of the device. I would carefully read the atex cert of the gas sensor and also explore the possibility of using a galvanic device driver.
 17 December 2012 07:10 AM
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RockWood

Posts: 3
Joined: 14 December 2012

3-wire connections means It's +24 , 0 Vdc & signal (+)
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