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

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

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

(OP)
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.

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

Should work just fine.

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

Twin spring and gold-plated makes it even more reliable.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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

You need to assess the voltage available. If it's less than 12V you need gold contacts.

Also lots of control systems think an open means bad things, so even a brief open could trigger alarms. Check that.

Keith Cress
kcress - http://www.flaminsystems.com

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

Yes, I once had a small PLC where I maxed out on analog inputs. I needed to add 8 new well level inputs so I multiplexed the signals with a discrete relay output module and fed them into one analog, it worked great and would read all 8 in less than a second although I had it much slower than that.
If you are switching your analogs you need an arrangement to maintain the current through the unused analog otherwise it takes a second or two to power up and read the normal value. In the case I mentioned the inputs were 0.2 - 1 Volt so I just used a couple of diodes in series with each input.
If your input is 1-5 Volts you could short the inputs out with the relay contact and open the relay of interest, with a steering diode to send the signal to the analog input.
Hope this helps
Roy

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

The contacts in a mechanical relay or switch consist of two pieces of metal that touch each other so that electricity can flow between them. However, under certain conditions metal contacts can become coated in insulating sulphides and oxides due contamination from the air. This can prevent the contacts from conducting electricity when they are meant to.

When the contacts are controlling high power loads like light bulbs or motors this is not noticeable, because there is enough voltage and current in the circuit to blast away the thin insulating coating. In low power electronic circuits, however, say a digital limit switch input to an electronic controller or PLC, there may not be enough voltage and current to clean the contacts. The circuit will then fail to detect a contact closure, This has been cause of many mystery system failures. The consequences can be very serious indeed.

The solution is to ensure that the contacts' minimum voltage and current ratings match the voltage and current presented by the controller input. This means you don't select a nice rugged tungsten plated 20A switch but one designed specifically for low power operation, say bifurcated gold plate over silver.

David Stonier-Gibson
http://splatco.com

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

Hello bdn2004,

From your thread, what I got, you need to use one Analog Input for multiple reading.

Instead of relays,why you can't use Analog Multiplexer Integrated circuit, like ADG506A/507A... from Analog Devices..?

Only thing is need to play little with electronics. No worries on contact,tear/wear..etc

Regards

Shaji Narayanan

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

(OP)
I don't think I was clear in my original post...What I was wanting to do is wire the analog switching through dry contacts on limit switches in the field...not through a contactor or an interposing relay. It amounts to running about 50' of analog cabling. The replies here seem to indicate that it should go through an interposing relay with gold contacts. Is what I'm wanting to do bad design? It's the same signal and no power required..

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

(OP)
What is generating the noise? Are the limit switches...they just too clanky? Do they bounce or something? They don't seat well enough for this sensitive circuit?
There is 4-20mA analog cabling all over the place here, and it's described as a very hearty signal with or without the shield and on very long runs, close proximity to higher the whole bit and there is not complaints. Not argumentative, just want to know.

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

ShajiNarayanan (Petroleum)
I think that you should check the specs for those ADG506A/507A gates.
RON is 280 ohms typical and 600 ohms maximum. That, plus a typical receiver having 500 ohms, you are very close to, or above what a 4-20 mA transmitter can handle. There is also a restricted voltage handling capacity (typically 10 V) so a 24 V source will be way outside what the switches can handle.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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

bdn2004; Long term project huh?

Anywho, I can't begin to imagine why someone would want to run analog signals thru "limit switches" but if this really is what you want to do you still have to adhere to the gold contacts. You'd need to make sure whatever switches you use include that feature.

Keith Cress
kcress - http://www.flaminsystems.com

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

(OP)
Doing the detailed design on a fairly large but seemingly straight forward project by yourself is not that easy i,ve discovered. Deficiencies in the original design consume your time. I'm going to change this over the long holiday based on the comments in this forum. thx all. gold contacts it is.

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

They don't look right. Definitely load switches. Instead, I would use reed elements atuated by magnets on that red thing. And I would use two in parallel for best reliability.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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

(OP)
They might be load switches but they don't handle much load! They look to me to be rated .27A at 250vdc

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

The DC spec is an entirely different thing. It is about arcing and inductive loads. Has no bearing on dry switching.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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

(OP)
Skogsgurra,

0.27A at 250VDC is the maximum load that could be put to this switch. That's not right?

If you notice, there are eight (8) of these limits that change state as that cam turns. They are going to different circuits - including a 48VDC computer and a 125VDC hard-wired shutdown circuit. The hardwired circuit will hold closed a bunch of interposing relays when the switch is on. This is an inductive load so it can't add up to more than 0.27A at 250VDC or .54A at 125VDC? correct?

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

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

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

(OP)
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.

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

Right. That's the way to go.

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

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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

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

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

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

(OP)
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.

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

It certainly sounds like an interesting project.

Keith Cress
kcress - http://www.flaminsystems.com

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

(OP)
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)

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

Glad that you believed me bigsmile 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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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

(OP)
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.

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

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

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

Have a look at appendix 3 in the following report:
http://www.gke.org/rapporter/files/SpecialPaper%20...
All names and company identities have been changed to fictious names. The picture with the different contact types is good reading.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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

(OP)
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.

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

No probs then!

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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

(OP)
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?

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

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

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