Can you run 0-20mA signals through dry contacts? 2

[bdn2004]

We are running 0-20mA signal from two different places into one PLC analog input. The signal source will be determined by the position of two mechanically operated switches equipped with contacts that change state with the position of the switch. Only one switch will be open or closed at any given time. The easiest way to do it seems to be to go through a set of contacts with the 0-20mA signal on each switch.

Is there a reason you wouldn't do this? Could the contacts degrade the 4-20mA signal? There may be an easier way but I'm trying to work with what we have not buy new stuff or have the PLC re-programmed. Thanks.

 

[Skogsgurra]

Yes, fine and dandy. But it has nothing with the ability to carry low level signals (dry circuit) to do. They are still power contacts and, as such, not suited to switch low level signals.

I do not think that we should do all the electrical classes here. You simply have to believe me.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[bdn2004]

It would be a hassle to mount another switch onto this at this point. We have 24VDC, 48VDC and 125VDC available in our control panel. I'll get a gold plated low level relay and which is energized through one of these limit swithes.

We aren't paying the $5000-a-day commissioning guy yet. I was just trying to get installed this as inexpensive way as we can. In the end I suspect all this will come out and will be done via two direct 4-20mA signals that are switched with software. It's a flaw in the design. We want to make that their problem, not ours.

 

[Skogsgurra]

Right. That's the way to go.

Hmm... 5000 dollars per day. I think that I should work more in the US!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

A very big chunk of the $5000 may be going for liability insurance Gunnar.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bdn2004]

Do I sense some sarcasm in this above post? Maybe I deserve that...but it's safe and it's simple, and it will WORK, that's the bottom line.
There must be a clean break between (2) independent hardwired control systems, (2) independent hardwired metering sytems and the (2) independent power systems with no chance of backfeed. And then toggle back to the other system when required. That's the project.

"You simply have to believe me." I appreciate the bluntness of this statement.

And $5000 a day is not an exaggeration. That's what it averages for the cost of one week per our Vendor. We have to make best use of their time. They will have to check out the whole system, this being a very small part, and to check the major parts of the system it must be ON and running. And these analog inputs, as with all the others must be there before it can come ON. Oh well I'm probably ranting to myself at this point....thx again.

 

[itsmoked]

It certainly sounds like an interesting project.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[bdn2004]

For the record....attached is a schematic of what I'm trying to do. I find I look at this stuff a year from now sometimes.



And below are the specifications of the analog current sources. There are two of these feeding into the input.

4-20 mA Signal Specifications:
Full-Scale Measuring Range ±5kA to ±100kA
Full-Scale Protection Range ±5kA to ±200kA
Signal Output Calibration Accuracy ±0.75% full-scale
Signal Output Type (configurable) ±20mA, ±1 Volt, or ±10V Full-Scale
Signal Output Maximum Burden 10 volts
Maximum Load Resistance (Current Loop Output) 500Ω44
Minimum Load Resistance (Voltage Output) 100kΩ
Linearity Error ±0.1% of full-scale
Repeatability Error Limits ±0.1% of full-scale
Temperature Sensitivity (Note 1.) ±0.02%/°C
Response Time (td) ≤ 50 μs
di/dt Accurately Followed 500 A/μs
Frequency Response (refer to Figure 3-1)

 

[Skogsgurra]

Glad that you believed me and went the relay route. With twin and gold plated contacts, that schematic is OK. There may or may not be a problem with fault indication in the intermediate position when both circuits are open. That can usually be solved by using a latching relay that switches quickly between the two signal sources. The switching shall be fast enough so that the zero current detector (if there is one) doesn't react.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[bdn2004]

And after looking at the extra stuff they sent with the control cabinet I find these ABB CR-M024DC3LD spare control relays, that looks like their purpose is what I describe. I'm not sure about these specifications if they will do the job. Attached is the spec sheet (5 sheets) for these relays. The last couple sheets it indicates the minimum the minimum current is 5mA for the type we have on hand.
5mA isn't 4mA. So...

Gold contact ones looks to meet the spec, and since it's got the same base we can go ahead and wire it. I looked online and some of these gold contact relays are like $400 each. Yikes. It would be nice if the spares would do it. I gotta make sure this will work before we buy these things. And I'm not telling anyone there is gold in them!

Any other suggestions or if there is one you typically use in a similar situation would be appreciated.

 

[Skogsgurra]

There's a huge cost difference between gold flash (cheap), gold plated (not so cheap, but still not expensive) and gold contacts (horribly expensive). What relay base do you have? Round 11 pole or square?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

Have a look at appendix 3 in the following report:

http://www.gke.org/rapporter/files/SpecialPaper Mill report may 2013.pdf

All names and company identities have been changed to fictious names. The picture with the different contact types is good reading.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[bdn2004]

When either of these power switches are opened or closed, there will be zero current in either line. No switching is ever done hot.
Both sides will be dead. These are hand cranked switches requiring planned outages to operate.

 

[Skogsgurra]

No probs then!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[bdn2004]

1SVR405651R2100

They look like square bases to me.

 

[bdn2004]

CR-M3LS base to be exact. Square

These meters don't control anything. The unit we purchased comes with it's own meter that does that function. They provide the required inputs to their master system and a reliable comparison to the meters onboard the unit.

I think what I'm going to do is wire these to our spares on hand, see if it works, explain the situation to the Vendor and project people and see if they want to invest the $$ for the gold contact ones. It's hard to believe the spares will conduct for 5mA and not 4mA?

 

[Skogsgurra]

There's no magic limit. It just gets less and less reliable when you get below the spec. If the application isn't critical (as I think you just said), then you don't have to bother at all.

"Much ado about nothing" (William Shakespeare, English poet and playwright, 16th century)

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.