4-20 ma hookup

Some months ago I applied for a job and was asked "On a 4-20 ma
loop which wire do you disconnect first and why?". I have been involved with electronics all my life and with 4-20 ma loops the last
14 years and I have never run across info that specfied which wire was best for connection or disconnection. I'd like to know. Thanks

For the safety of the transmitting device, I always disconnnect the power going to the transponder from the power supply, as this immediately isolates the rest of the circuit.

I was told to do this to save the transponder in case a disconnected wire grounds out & fries the device.

Not really sure if this is the right answer but it seems logical & safe.

My first response to that question is that power should be turned OFF then you can disconnect either wire and it should not matter. My second statement would have been that it may be ok for an individual to work with low voltage/current dc while its powered BUT what happens if you accidentally short out the connection. Lastly if they wanted to know what wire I would disconnect at the device with the dc power ON....it would be the common/ground because there would be less likelihood of a short that could harm the power supply or other devices.

rsdoran,

Keeping in mind, this is probably being troubleshot (troubleshooted?) while the line/process is running - disconnecting the common/ground would kill ALL the analog PLC inputs, at least on one card.

If I were troubleshooting &/or replacing a bad transponder, disconnecting the power side would isolate the one circuit without affecting the other analog signals (as long as the power wire didn't ground out & blow the fuse for all analog power - but everyone needs some fun in their workday.)


Maybe it's regional, but in the Detroit area, very few industrial plants will let their machine be turned off for controls updates or repairs unless it wont start. I get calls almost every day asking for service, and the first stipulation is the machine or line can not be stopped. Thanks to ford & JIT.

We were talking about one device....ie if you disconnect the common wire AT that device then other device(s) SHOULD NOT be affected. This situation could depend on ................. I was talking in general.
1. First, power should be removed
2. If troubleshooting with power applied then attempt to prevent harm to people and/or machines....remove the common first, if removed AT the device it will not have an effect on other devices.

Depends on WHERE/HOW the power is connected if other devices are affected by making a disconnection of either common or power.

Its kind of a mute point where you are....to replace a device BOTH wires have to be removed at some point....power off it doesnt matter...power on, the positive can be shorted to ground and take out the power supply.

Just my way of thinking....dont make it right or wrong.gb

4-20ma loop

I work with tons of 4-20 circuits and never heard of such a question. The first thing you do before working on any loop is make sure your control is in manual. I always as a rule disconnect the 24v+ first as I do not like having it hooked to any type of device without the common being connected. But I have never heard of any problems either way. If I were going to hire someone, this question would be at bottom of the list.

The red

Hi
Like most of the replies i have worked with many 4-20ma loops and i have never heared this questions.
My suggestion is you should remove the "red ' wire first if you remove the "Blue" wire first it will explode.
I am just kidding but i recommend that you should off the powe before working if it's not easy remove the common first this will break the loop as well .
BR

I have never had an issue with disconnecting a 4-20 device before, but did in re-connecting, ALWAYS ground it first, IE return, connecting hot first then grounding will let the smoke out, and it is very difficult puttting the smoke back in!!!!

First, I whole-heartedly agree with both the "you should turn it off first, so it doesn't matter" AND with the "what a stupid question" comments that have been made.

That having been said, I'd think you should disconnect the power line first (then tape it off; I always treat every wire as if it were alive and about to be shorted to ground through me). My reasoning is that at the source, voltage will try to rise to get the current necessary for the process. Assuming an infinite source, the voltage will go to infinity in its attempt to get 4 mA through an open circuit. The ground terminal would then be removed second, and we can rest assured that the ground is at 0V potential.

I've worked on some older 4 - 20 mA stuff that was sourced off of +160V and +136V giving you the 24 Volts normally associated with these types of circuits. I'm not sure what was going on behind the scenes of this source, but at my equipment, I got 24V differential, but those voltages to ground.

It may have been a grounding issue, and it may have been intentional, I just don't know. I do know that I had to set up a differential input to get it to work.

Regards...

I hope my password works this time.

Geeezzz... "What a maroon." (the spelling is correct, Doc!)

Everybody knows from UXB training... whenever you are trying to disarm anything, you ALWAYS cut the RED wire! (even if they are all red.)

Originally quoted by Terry Woods
Everybody knows from UXB training... whenever you are trying to disarm anything, you ALWAYS cut the RED wire! (even if they are all red.)

Click to expand...

Unless, that is, there is a blue wire running together with a green wire AND a small glass vial containing a silvery liquid. In that case, cut the yellow wire.

I think I would answer the question with this question,,,,
"Is this a loop powered device?" and see if the interviewer knew why he asked the question to begin with. I agree with the others here, it's not a very good intrviewer question.
If nothing else it would have sent the interviewer to the next question real quick because without knowing what that means, how would he expect to understand any answer I would give him.

Answer a silly question with another question. Beauty.

I recall a Monty Python skit where a troll asked questions of people wanting to pass. If they said "I don't know" something bad happened and the last guy pulled a "Bitmore" on him. Something about the maximum airspeed of a swallow. Was that a European Swallow or an African Swallow? asked the sly fellow. "I don't know" says the troll. No more troll!!