4 -20 ma analog output problem

Dear friends,
Presently I am working in a water treatment plant where T&C is going on, we are using VFD to control motors while commissioning we ordered one technician to measure the 4-20 ma output from PLC to VFD, instead of measuring in series he connected multimeter parallely to the wires in the terminal box that means literally i think he short circuited the plc output 4-20ma, we had 100 ma fuse also in the circuit. In this scenario i assume that fuse should have blown biut nothing happened to the fuse or plc card but VFD input(4-20ma) started malfunctioning i couldnt understand what really happened i am really new to the profession can someone please explain me?

What type of PLC are you working with?

100mA is actually quite a large fuse for a 4-20mA circuit. I usually use a 32mA fuse.

What voltage are you feeding your loop? Check the voltage between the +and - of your PLC analog output. Check the voltage between the + and - of your VFD analog input.

thank u for the
response sir , we r using Siemens s7 plc, and altivar 61 vfd , i don't know about the voltage will check tmrw but can u give me a general idea what happens when we short circuit the 4-20 ma (not a loop powered one) wires by wrong usage of multimeter. thanks in advance .

Most PLC outputs are sources and current limited. I'm guessing your tech actually shorted the +24 VDC to the VFD input, but not through the Multi-meter (which has it's own internal fusing).

I often check voltage across (in parallel) two terminals of a 4-20 loop with no problems, as long as I've remembered to set the multi-meter to measure voltage. If I forget and have the meter set to measure mA then I blow the fuse in the meter, but I don't damage the circuit. I have done this so often that the standard field kit contains a package of fuses for the meter!

Just like any circuit, there are limitations on how much current you can run through them. Most mA inputs are actually voltage inputs with a resister across them. Those resistors have a wattage rating. Long story short if you short the power feeding them so that the voltage applied to them is too high, you will burn up this resistor.

Tom beat me to it, but I suspect the same thing he does.

thats what I thought, nothing should happen if we connect the multimeter in ma mode parallel to plc output , because as u said current from plc is limited it will get dropped across multimeter and vfd will receive even less ampere. am I right?

Not quite. I'm not very familiar with S7 I/O but many outputs have a minimum resistance of the loop. It is possible that putting the multimeter in parallel could have overloaded the analog output. I would think that the S7 would have some protection from this but I'm not familiar enough with it.

If the multimeter is in Voltage no problem, you can see variations, as hopefully in the VFD there is a resistor of 250 ohms.
so you will see from 1 to 5 volts, handy and easy without taking loose any connections.
In mA mode the system is shorted but as the PLC gives a mA signal no worry, but the VFD wil not work on this setup.
best is to make the mA in connected over a fuse, take out the fuse and everything will stay on working without interuption.

Tom Jenkins said:

Most PLC outputs are sources and current limited. I'm guessing your tech actually shorted the +24 VDC to the VFD input, but not through the Multi-meter (which has it's own internal fusing).

I often check voltage across (in parallel) two terminals of a 4-20 loop with no problems, as long as I've remembered to set the multi-meter to measure voltage. If I forget and have the meter set to measure mA then I blow the fuse in the meter, but I don't damage the circuit. I have done this so often that the standard field kit contains a package of fuses for the meter!

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Tom
Thank you - I feel much better now. I bought a five pack of fuses. Now that I have that I have not blown a meter fuse - YET.

It is going to be so nice when I overcome this human stuff, become perfect and stop making stupid mistakes.

Dan Bentler

leitmotif said:

Tom

It is going to be so nice when I overcome this human stuff, become perfect and stop making stupid mistakes.

Dan Bentler

Click to expand...

When you get that worked out, let me know how you did it!

Tom Jenkins said:

When you get that worked out, let me know how you did it!

Click to expand...

Bowing and scraping is inappropriate at this stage. When I become perfect then it may be appropriate. You will be first to show how to do perfection.

Just made another attempt at perfection - screwed it up. Well that is progress - know what not to do next time.

Dan Bentler