Problem with Siemen SM431 analog module

[Pandiani]

Hello guys,

Today I have experienced very strange problem with SM431 module 16 inputs x13 bit. On this card, there are 12 4-20 mA signals coming from Pt100/4-20mA transmitters

After 10 years of working majority of measurements went out of range. We have measured mA from the field and concluded that module has failed. We have replaced the module and everything worked for 3-4 hours and and happened similar but this time all measurements are in measuring range but showing significantly less than expected.

So we measured again and noticed some strange. We disconnected the filed and connect to the terminal strips in the cabinet 4-20mA Fluke simulator. We have checked all wired inputs. The results are the following:
Ch1 : 4mA -> 32767, 12mA -> 2600, 20 mA ->14400;
Ch2: 4mA ->32767, 12 mA->2800, 20 mA ->15000;
Ch3: 4mA ->32767, 12 mA->3000, 20 mA ->15600;
...
Ch10: 4mA ->32767, 12 mA->3200, 20 mA ->15800;
...

Therefore, one would expect that 20 mA = 27648, and 12 mA would be around 13000.

It seems that another module failed. We have checked the cables for 3 or 4 input signals from the field (from the sensors), but they seem OK, no short circuits to the ground or anything like that .

What could be the possible cause that card failed in such manner that all inputs have range disturbance, but in very similar manner?

First analog module has all inputs to 32767 and it was fairly easy to notice that module failed. But the second module is real mystery.
It is highly unlikely that two modules fail without some disturbance coming from the field. But to fail in such way to disturb the range but to show some nonlinear change is very strange.

Has anyone else had similar experience?


Regards,

Pandiani

 

[KalleOlsen]

Yes.

One or more of the channels are wiping the system voltage out of reach for the internal electronics.

Check if the M-ANA pin is connected to 0V, and evt. also connect the - inputs of the channels.

Kalle

 

[Pandiani]

Hello Kalle,

thank you for your answer. Attached please find the "wiring diagram for this card". Is M-ana pin actually nr. 48 or 26? Can you explain this part?

Like I said, when replacing the card, I have just plug out the front connector so I didn't change the wiring.

From your answer I conclude that you think that card is not defective?


Thank you

P.S: Of course, there is no need to note that when replacing the card, we observed position of side jumpers and all were set to the same positions like previous card. Since all connected sensors are 4-20 mA loop powered, side jumpers were set to "D"

 

[danw]

It's always possible that a replacement AI card is marginal and fails sooner than expected, but it's an outside chance.

I suspect that something has changed over time and that the card is subject to excessive common mode voltage.

Here's the specs for the SM431-0HH0 AI 16 x 13 bit card:
Max. permissible common mode voltage between the channels or the reference potentials of the connected Sensors and central ground point: 2 VDC/AC;

2 volts is not a lot of common mode tolerance.

I'd check for a faulty field transmitter by disconnecting the wiring and checking for continuity to ground or a low resistance to ground. Field transmitters are typically isolated from ground, so a short or low resistance to ground indicates a problem.

I'd also check for DC or AC voltage on the transmitter wiring at the AI card. When disconnected from the AI card, neither field transmitter wire should have any voltage with respect to the analog ground. Any voltage present with the wiring disconnected from the AI card is common mode.

If you are in a rainy climate, look for water in junction boxes or water in the transmitter housings.

With a 16 channel card, there's lots of devices and it only takes one fault to ground to ruin things.

 

[Pandiani]

Thank you danw,

since inputs of this card are not electrically isolated, I know that it takes only one faulty transmitter wiring to cause the problem.
Perhaps, what was happened is the following:
Wiring problem in the field occurred that caused card to failed. When another card was placed, it worked for about 3 hours with the problem which caused new problem on the card.

We'll check all wiring from field transmitters both voltage and ohm toward ground when card is disconnected.