4 to 20mA Security Specs

4-20mA loops are open- and short-circuit proof. In the posted instance above, errors in the wiring caused a loss to some PLC inputs. On correcting the faults, the loops will behave normally.

Hi PWSlack,

Not so, unfortunately, as the shorted sensor wires caused burnt circuits in the detector. I know the fault was caused by the short because all was working ok but with some sensors connected to the wrong inputs.

Do you actually have a reference to a standard spec. for this? While I believe you that a 4 to 20mA loop should be short-circuit proof without a reference it is just here-say.

regards

Chas

Then something else caused the fault: at 20mA from a 24V supply the fault power is 24V x 20mA = 480mW!

There is a big bad ugly wiring error somewhere that cannot be seen at this distance. "Seek and ye shall find".

Chas,

By definition, a 4 to 20 mA circuit is impedance limited and consequently it should never provide more than 20 mA even in a short circuit. Of course it would also be rather impossible to provide 4 mA to an open circuit since it can not be open and flowing at 4 mA by definition.

Perhaps the supply for said circuit is oversized. Or the current limiting resistors were also shorted out or removed when you "made some simple wiring changes". It pays to make sure such sins were not committed! Else, bad things could happen!!! (i.e. punishment for your sins!)

If you search the internet for 4-20 mA control circuits you will find a lot about how they work or at least should work. Remember the output signal is a reflection but usually independent of the input signal. The current limiting resistors will be chosen for the magnitude of the voltage output. So you could achieve 20 mAmps at 10 volts or at 100 volts depending on the current limiting resistor. Without the current limiting resistor in place, you could easily drive external voltages back into your sensors.

Look at the circuit and relocate those resistors (with the power off) if you need to in order to prevent that kind of damage.

Hi Joe,

I "know" they are impedance limited, (more correctly, have always found them to be)and assumed them to be so in this case. Hence my carelessness and the consequent damage. I am dealing with a unit under warranty and so I cannot open the unit or modify any of the circuits.

I have found out subsequently that there is a hard 24V rail with outputs to the sensors. The return goes directly into a custom PLC display and alarm system.

My problem is that I consider myself responsible to pay the repairs to this unit unless I can show that it does not meet the standards for a 4~20mA loop system. (at least it was my own muck up and not one of my crew). Incidentally I have asked for the bill from the manufacturers and have yet to recieve one. Perhaps they have realised that they have left out the limiting resistors from their unit?

Can anyone quote chapter and verse for this spec? there must be a dyed in the wool instrument tech here somewhere who has his old college books wih the "regs" for a loop.

Although I have, untill now, always found 4~20mA loops to be limited I have usually found them to give >20mA in s/c which is logical as the limiter is chosen for wire and circuit safety but has to allow for the max circuit length and for the fact that an active loop powered unit cannot work with 0V drop across it. A current >20mA also indicates a fault in the same way that <4mA does and is sometimes used to trigger a failure alarm.

regards

Chas

I "know" they are impedance limited, (more correctly, have always found them to be)and assumed them to be so in this case. Hence my carelessness and the consequent damage

It seems your faulty wireing is the cause, so acknowledge your responsibility, you should have checked the components specs (and wiring diagram)

<....responsible to pay the repairs to this unit unless I can show that it does not meet the standards for a 4~20mA loop system....>

So it's a blame thing! Ah! CR4 is Engineering, not legal!

There is no specification or standard as such.

It is up to you to be competent and test, isolate, and test again.

to PWSlack,

No, I am not looking for a legal out. I have accepted my responsibility and will pay for the repair. I should not have assumed that this would behave as the vast majority of loop circuits. However, for my own peace of mind, I would like to know whether my assumption was reasonable or not. You said in another post a few days ago that you only post statements, not opinions. While it would do for me to say " well, if PWSlack said it then it must be so" I don't think it would hold much sway with the manufacturers in Germany!

There is no wiring error. The two input wires were briefly shorted during the reshuffle, nothing more. the current flowing is not 20mA but 24V/R (input to neg). It is not my bad wiring, but my bad wiring practice that caused the problem.

To Epke, Ther is no documentation for the PLC concerned. It didn't even have a manufacturer's label. Fortunately the main vessel's diagrams referred to it as a block with the manufacturer's name which we found by web-search for the contact details. It is often impractical or impossible to refer to the manufacturer for such details.

To Guest, I agree i should have removed the power but you say there is no standard. Are you sure? Are you an instrument tech by background?

regards

Chas

Since you have verified that the + terminal is a 24V rail: If the + side can find its way to power supply common (not the - input), that will cause fault current. Many old analog boards had a fuse on the + supply for this purpose. I can't say that this was a standard, but a very good design practice nonetheless.

Would that PSC have been on a digital I/O terminal? Who knows. Having done cause investigations around I&C for many years, the absolute truth can be elusive, as memory fades with age, fear, and confusion.

Chas, it may be worth "peeking" in the box to see if there's a fuse you can replace, good luck.

From the standpoint of the loop power supply integral to the receiving instrument, such as yours, what happens between the + and - terminals is open/short tolerant.

Sounds like tuition paid at the school of hard knocks. Good luck.

Hi mkruger21,

The diagrams show the loop coming from the plc but when we eventually physically located the unit we found that the pos side is actually hard 24V, no fuses, limiting resistors, nothing. If a sensor goes short then it will cause the processor unit to fail which I think is unacceptable.

regards

Chas

The input impedance for a 4-20 mA input is often small and easily damaged. If 24 Vdc was put across the current input it is likely the shunt resistors were damaged. Some manufacturers have a polyswitch in series with the input to protect the shunt resistors from "mis-connection".

There is no specification... each current input device typically comes with a data sheet or manual that has the individual specifications... they vary but the one constant for current inputs is the low... and vulnerable... shunt resistance.

It all depends on what was shorted to what. Even grounding a loop to a higher voltage ground point could back-feed enough voltage to do damage. It is very difficult to know for sure what happened without being there.