Analog Input Isolation IF8

[Dale87]

Can someone please explain to me in simple terms, the best practice with multiple 4/20ma pressure/flow transducers on multiple different 24VDC power supplies, All wire back to a non isolated 5069 IF8 card. Do you have to use isolators or not. The common terminals "-" on the IF8 are all tied together internally. If all your power supply "-"'s land on these terminals, and your power supplies "-" weren't connected to earth ground, you wouldn't have any ground loops, and all your power supplies would be at the same "-" reference correct??? Just trying not to smoke a $1000 analog input card.

 

[Corsair]

My understanding of this situation is that you do not need isolators. I think power supply minus to earth ground connections are considered to be a code thing. If the supplies are in the same cabinet hook all their minus together and earth ground it at one point. If the supplies are in separate cabinets I'm not sure how to answer.

I'm a big fan of individually isolated points on analog input cards. It takes a few more wires to hook them up but there are a lot of advantages and even some potential safety implications. Allen Bradley seems to be weak in this area. They sometimes talk about 'differential' inputs which is not the same as true isolated inputs.

Schneider has a 16-point non-isolated module and an 8-point individually isolated module. My general strategy is to use the 8-point module and don't tell the customer that there is a choice. It's easier to beg forgiveness that seek permission, especially from an accounting type.

We had a situation a few years back where water got into a 120 volt instrument. It had a pair feeding into a non-isolated PLC input module. The resulting 120 vac noise trashed the data on all of the points on the module and resulted in the electrician getting bit from cables going to other devices. If it was an isolated input module the data would only have been trashed on the problem instrument and the electrician would not have been getting bit in a wet environment.

It seems to me that you never have to apologize for electrical isolation. And it beats suppression every time that it can be used - but that's another topic..

 

[TWS]

From the manual: Single-ended Wiring Method: Single-ended wiring compares one side of the signal input to the signal ground. This difference is used by the module to generate digital data for the controller.When using the single-ended wiring method, all input devices are tied to a common ground.The use of single-ended wiring and the common ground maximizes the number of usable channels on the module (eight channels for the 1756-IF8 and 1756-IF8K modules and 16 channels for the 1756-IF16 and 1756-IF16K modules).

The differential wiring method is recommended for applications that can have separate signal pairs or a common ground isn’t available. Differential wiring is recommended for environments where improved noise immunity is needed. In differential mode, the channels aren’t totally isolated from each other. If multiple differential input signals have different voltage common references, one channel could affect the reading of another channel. If this condition can’t be avoided, then wire these inputs on different modules or replace the non-isolated module with an isolated input module.

 

[Cheeseface]

If you can't connect the returns, use the 5069-IY4 module instead; it has per channel isolation.

 

[Bit_Bucket_07]

Most (not all, but most) transmitters these days have isolated outputs. I'd just tie all of the 24 VDC loop excitation power supply negatives together and ground them. If you encounter a non-isolated transmitter, it will be obvious, and you can install an isolation transmitter for connecting any such instrument.

 

[Dale87]

If you can't connect the returns, use the 5069-IY4 module instead; it has per channel isolation.

I guess I could use the IF8 for the 2 wire transmitters that I know originate from the panel power supply, and use the IY4 for the scale heads, and flow meters that have their own power supply built in, those are the ones I'm nervous about.

 

[Corsair]

Are you sure the iy4 is isolated?

 

[Dale87]

Are you sure the iy4 is isolated?

It actually says in the manual non isolated..... so Rockwell doesn't offer a isolated analog input in their 5000 series processors..... how can that be.....

 

[Corsair]

Entirely too many ab sales people thinking differential is the same thing as isolated because of the terminal screw count.

 

[Dale87]

Entirely too many ab sales people thinking differential is the same thing as isolated because of the terminal screw count.

That's the truth, well I guess I'm going to order a handful of isolators for the ones I'm suspicious of and go for it.

 

[Dale87]

Yea in control logix...... but not in the new 5000 series compact logix line...