RTD failure

J

Thread Starter

Joe

Hi,

Please help regarding an RTD reading spikes when a pump turns ON. The RTD PT100 is transmitted to a Huber, and when a motor 2 meters distance to the RTD sensor starts, spikes comes up.
Tried so far:

- double check the RTD termination
- changed cable into shielded wire

After changing into a shielded cable, it gives lesser spikes reading. Previously, from 25C to 400C, but after changed of cable, it goes from 25C to 35C.

Any suggestions in this regard.

I appreciate it so much:)

Joe
 
B
Make sure the cable is grounded only at one end - best at the monitor end.

You may want to reroute the shielded RTD cable well away from any high voltage cables to limit crosstalk.

If it still gives a spike try slipping the shielded RTD cable into a flex conduit and ground the conduit aluminum only at the same point as the RTD shield is grounded.

Double check that all shields are well insulated at the RTD end - even a small short will defeat the purpose of the shield.

Bob
 
you omitted valuable considerations

1. ungrounded rtd should be used

2. the shield pair should be ungrounded at the sensor

3. the shield cable should be run in its own conduit and/or section of the cable tray, i.e. segregated from any power leads.

4. the shield should be grounded only at the transducer connection.
 
Joe,

If the shielded cable is run in the same conduit with the motor leads, there's not too much more that can be done--unless there is some kind of "filtering" on the Huber that can be used to kind of smooth spikes.

In-rush current can be high, and is likely to cause problems if the low-level signal (RTD, in this case) are run in the same conduit or same cable tray for any appreciable distance. Sometimes, depending on the in-rush current level even a couple of meters can cause problems such as you're describing.

It's acceptable to make the terminations (low-level and high-level) in the junction box on the side of the motor, but it's not a good idea to run the cables in the same conduit or cable tray or in parallel and in close proximity to each other for even a couple of meters or more (again, depending on the in-rush current, and even the running current, if high enough).

Lastly, a common mistake is that people ground both ends of the low-level shielded cable's drain wire. Only one end of the low-level cable's shield drain wire is to be terminated, and the convention is that it is to be terminated where the power for the cable originates--which is at the Huber or the cabinet where the Huber is located (i.e., not in the motor junction box).

Hope this helps!
 
Joe,

One more very important clarification. I don't think your RTD is failing if the feedback from the RTD "seems" to be spiking when the motor is started.

The "spike" is caused by induced voltage in the low-level signal cables/wires. The likely cause of the induced voltage is the in-rush current (which only flows for a brief period of time while the motor is starting).

So, if the output of the RTD is stable and produces a believable value at all other times, then the RTD hasn't failed--it's the induced voltage caused by the in-rush current during starting that's causing the momentary spike in the reading from the RTD. Actually, the RTD resistance isn't changing, it's the result of the induced voltage that's causing the reading from the entire circuit (RTD plus interconnecting wiring) to spike.
 
Thank you so much guys for your response.

I also believe that it wasn't an RTD issue (but just to let others know on what we have tried), basically we have two other RTDs into a second tank system, one mounted at the bottom of the tank and its upper portion, and no spikes at all.

Here's something interesting, we actually have two systems. Both has its own pump and RTDs. First system which has the spike has one RTD mounted at the bottom, and its pump located 2 meters away as mentioned(RTD goes to the Huber). The second system (which works well) has pump located 2 meters away from the bottom mounted RTD (the second RTD is on top of the tank) used the old cable and gave us no issue at all (RTD however trasmits signal directly to an analog RTD type I/O). The tanks and its pumps are 5 meters away.

Before changing the cable on the first system, spikes also occur when the pump starts on the second system, and after cable changed, it still occurs but negligible with respect to the requirement from xx.25C to xx.75C. This is why I'm wondering if Huber itself is also causing the issue due to its mechanical things inside.

But surely we'll follow your suggestions and I'll get back ;)

I appreciate and thanks again.

Joe :)
 
S

solution_provider

1. Make sure the equipment that the cable is connected is properly grounded. Use an earth ground wherever possible and check the connection between the ground point and the equipment. Eliminating noise depends on a low resistance path to ground.

2. Ground the cable at one end. This eliminates the potential for noise inducing ground loops.

3. Use a cable suited to the application - foil, braid, foil/braid
 
perhaps not device failure but operational or perhaps a failure of the installation design. of course at the work order level, failure is a bit less well defined.
 
Joe... can you provide addtional detail on the shielded cable installation:

1) Which end of the shield is grounded?

2) Are you sure about the integrity of the "Ground" path?

3) Do you know the duration of transient?

2) What is the nominal-current or Hp of the motor believed to be causing the disturbance?

3) Are solenoids denergized at the same time the motor is started?

Regards,
Phil Corso
 
I thank so much for all of you guys. Problem solved - I connected only one end to the ground.

Regards,
Joe
 
Joe,
if there is any power cable dressed within 300mm gap of the RTD cable, it shuld be shifted 300mm awy from it.

check the resistance value that's shown by the RTD once you turn on the drive.

troubleshoot it starting from the instrument.
1) remove the termination, turn on the drive- check the resistance

2) check the megger value

3 remove the termination from the PLC/DCS termination point, check the value

just try this out and you can finally conclude where exactly the problem is
 
you omitted valuable considerations

1. ungrounded rtd should be used

2. the shield pair should be ungrounded at the sensor

3. the shield cable should be run in its own conduit and/or section of the cable tray, i.e. segregated from any power leads.

4. the shield should be grounded only at the transducer connection.
Dear d-

Thank you for your valuable reply,

I know it's been a while for the thread, but for better confirmation from my side,

I always read that it's best to ground the shield from the Receiver Side, which is an Analog PLC Module or DCS Card or PID but in your kind answer you stated it's better to ground the shield from the Transducer side,

Please i kindly need further technical clarification, please
 
I thank so much for all of you guys. Problem solved - I connected only one end to the ground.

Regards,
Joe
Dear Joe,

Thank you so much for sharing your technical malfunction for better knowledge,

I know it's sometime but i am facing similar issue here,

Could you please advice from which end did you ground the RTD, from the Analog I/O or Transducer side, and also, please do not mind my ignorance, what did you ground ? the shield or the ground wire ( Ground wire i mean the 0V wire ).

I highly appreciate your valuable reply,
 
The shield/screen gets grounded, typically at the analog input end. An RTD should have at least 3 wires, the 3rd wire being lead wire compensation. None of the RTD wires should ever be grounded.

Have you noted that the previous posts were 8-9 years ago?
 
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