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Join Date: Aug 2008
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Cable Length and Sensor Type

02/12/2009 10:27 AM

Dear All,

This is with reference to deciding the effectiveness of output signal from a sensor to the PLC situated far away from the sensor.

Our situation is like this :

We have project at jetty where the client can not provide PLC very near to the sensors on the equipments supplied by us. Usually, we do not do automation , but only supply the sensors along with junction boxes on the sensors. Now, client is asking us to decide upon the maximum distance where they can keep their PLC panel.

If some body has a reply and wish to be paid by us, then also can reply to this.

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Guru
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#1

Re: The lenght of cable and type for sensors

02/12/2009 10:42 AM

There should be added more details for your inquire for sensor's signal specs: mA, mV, V, or whatever else. I'm personally using thumb rule don't exceed some hundreds meters between signal source and PLC unless there are using repeaters (signal normalizers or/and signal conditioners).

Cable are to be shielded. Hope someone here will provide more details for that as I'm not aware of western national standards for wiring and cables' nomenclature.

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Guru

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#2

Re: The length of cable and type for sensors

02/12/2009 11:47 AM

Carambas right, you really need to provide more info.

What level signal? How far between the sensor and PLC? What kind of electrical noise environment are you in? How many signals? What is the bandwidth of the signals? How big a problem is it if the sensor signal gets corrupted?

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#6
In reply to #2

Re: The length of cable and type for sensors

02/13/2009 11:06 AM

Ok.

The sensor is giving 4 to 20 ma.

The sensor out put is digital.

It is cabled and not working on radio frequencie.

The electrical disturbances are less as this is installed at a lonely sea shore.

It is not much important if signal is delayed. But, the delay should not be more than 1 sec.

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Guru
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#7
In reply to #6

Re: The length of cable and type for sensors

02/13/2009 11:31 AM

Okay, it had been discussed here on cr4 .

Vulcan has given complete answer for. Last year I'd finished and are maintaining now project where 4-20 mA signals from sensors had been gathering on PLC inputs from up to 400 m distance with no problems in extremely hazardous conditions.

Delays are mostly caused the DAC/ADC converting performances than distance between PLC and sensor(s).

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Guru
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#3

Re: Cable Length and Sensor Type

02/12/2009 11:54 PM

If your sensor is giving current output and if cable is of sufficient gauge to give minimum reistance, it should be possible to transmit the signal to long distance.

Voltage output will be a problem.

Additional data such as type of output and distance between sensor and the PLC will help in deciding as others have already said.

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Guru
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#4

Re: Cable Length and Sensor Type

02/13/2009 7:26 AM

What are the specs of your sensor?

What are the specs of your PLC input?

How far is far away?

Your PLC input has voltage specs for low or OFF signals and high or ON signals. For example: Voltages below 3 volts are considered as LOW or OFF. Voltages above 15 volts are considered as HIGH or ON.

Your sensor should also have specs that say how much current or voltage it puts out when ON or OFF.

With these information, you should be able to calculate how long your wire can be if you obtain its Ω/meter. All you need is Ohm's Law. The bigger the wire, the longer the distance.

I've strung sensors on cables up to 200 meters with no problem.

regards,

Vulcan

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Active Contributor

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#5

Re: Cable Length and Sensor Type

02/13/2009 10:46 AM

Have you considered a battery/solar operated data collection system on-site? Housing a data collection PLC close to the sensors is often a better solution than long lead lengths.

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#8

Re: Cable Length and Sensor Type

02/16/2009 1:24 AM

Hi all,

From the top of my head - if you are using a 4-20mA signal at 24VDC. And your cable

is at least 1.0mm² and your cable has the following ratings 18.24 pF/m and 0.0608 uH/m and 0.0195 Ω/m - You will be able to run a cable with the length of up to 6304 meters. This is if you are working in an intrinsic safe are.

If you are not working in an intrinsic safe area your cable run can be a maximum of 8683 meters. In instrumentation - cable length is very rarely a problem if you use the correct cable. Make sure your cable has at least the above mentioned specs. It should also be overall and individually screened. Have a google search for Dekabon - it is expensive but well worth it.

For more information on cable calculations - goto aberdare cables.

Your basic formula would be:

Max Cable voltdrop = Voltage - Transmitter Voltdrop - Card Voltdrop

Max cable resistance = Max cable voltdrop / Max Current

Max Cable Distance = Max Cable resistance / Cable resistance(per meter) x 2

You need to multiply by two as you have two wires for the signal - + and -.

Regards,

Craig

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Anonymous Poster (1); caramba (2); craigza (1); gsuhas (1); leanhard (1); stevem (1); Vulcan (1)

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