Using A&B's Safety Relays for Conveyor E-stops

[majesus]

I got an application where a Toshiba MV VFD powers a 6000ft Overland conveyor. There is a E-stop pull cord on one side of the conveyor, so I have about ~12000ft of E-stop wiring to deal with. So my intention is going with a DC system.
(The Toshiba VFD has an internal incoming contactor for E-stop function.)

Looking at Rockwell Automation's Safety Relay MSR5T, it operates at 24Vdc, it says the Maximum Input Resistance is 200ohms.

http://www.ab.com/en/epub/catalogs/3377539/5866177/5985760/4444281/4444293/4444600/tab2.html

Trying to understand the meaning. Ignore all other factors and just deal with resistance in the circuit from only the cable run. Say I chose #10AWG, with a resistance of ~1.10ohm/1000ft. I can go about 181,000ft in wire length before I have an issue? That's 34 miles! What am I missing?

Cheers,
Majesus

 

[jraef]

Actually, it says the maximum input resistance is "200W", not 200ohms. I'm not clear on the input voltage but if we ASS-U-Me it is 24VDC, ohms = E²/P so that's 2.88ohms.

10ga wire is 1.018 ohms/1000ft, not 1.10, you apparently transposed a digit there. 200ohms / 1.018 = 69,000 feet. But you have to use the "round trip" tables because each input signal goes from the relay to the device and back. The round trip resistance for 10ga copper wire is 0.204 ohms per 100ft. So to get to 2.88ohms maximum, you now are only at approx. 1400ft. Then you need must add terminal resistance, i.e. the end terminations and junction boxes etc. and compensate for temperature, the above values are at 25C (77F), highest case temperature will increase the resistance.

There are other technologies that can allow a safety system to be monitored from 12,000ft, but you cannot directly wire an E-Stop button that far away. ASi relays can have repeaters, but I think the maximum distance is still 300m (may be less for ASi-Safe Circuits) so that still comes up short. ProfiSafe over ProfiNet (Ethernet) and/or fiber optics may be your only option.

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[jraef]

Note on the above, forget the "200ohms / 1.018 = 69,000 feet." part, that was a fragment left over from before I realized it said "200W", not 200ohms. I meant to change that to 2.88omhs / 1.018 = 2,829 ft.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[majesus]

Hmmm, 200W??? It clearly says 200 ohms. Here is another source, the pdf version:

http://www.farnell.com/datasheets/30121.pdf

No I didn't transpose a digit at all. For the resistance of #10 AWG it all depends on the temp, type of Copper, etc, etc, etc. Anixter's Electrical Characteristics on cables, Table 7.2– has published DC resistance of class B copper conductors, ohms per 1,000 feet, 75°C to be at approx 1.24 ohm/1000 feet. Okonite Company has DC resistance for Stranded Class B at 1.04/1000ft at 25C. I just picked 1.10ohm/1000ft just to give some numbers in my example.

Anyways, I'll give A&B a call on Monday for clearification. I just wanted to think about it over the weekend. #10AWG at roughly 1.10ohms/1000ft. That's about 181,000ft in wire length before I have an issue. Yup, there is a return path, that's why it's labeled wire length.

jraef, not trying to give you a hard time. I appreciate you posting the feedback.

Cheers,
Majesus

 

[jraef]

Clearly?

Inputs
Safety Inputs 1 N.C.
Encoders Supported T
Input Simultaneity Infinite
Input Resistance, Max. 200 W

But I agree, the PDF in your second link shows it as 200ohms. So I guess A-B is getting sloppy; I've noticed that on other things lately.

So maybe it would work, but other safety relays I've looked up state a 300m maximum distance, I doubt the A-B is that much different.

Notice by the way that it says 14AWG maximum stranded wire, you may have an issue there.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[majesus]

I took a Snap-shot of A&B website, it is in ohms. Something is wrong with your web browser.

ttp://files.engineering.com/getfile.aspx?folder=3e8f9678-eddc-469b-b117-a02b35f78372&file=Input_Resistance.JPG



Yeah I noticed the #14 AWG. I figured that's the size the wire that can fit in the terminals. Relay will be be in an I/O Panel anyways, connect field wiring to terminal strip, #14 AWG to Relay.

Anyways tomorrow is Monday, so I'll be calling A&B to inquire about this product. I'll let you know what they say.

Regards,
Majesus

 

[majesus]

Sorry, I cut and paste the snap shot link incorrectly. Here is is:

http://files.engineering.com/getfil...b-b117-a02b35f78372&file=Input_Resistance.JPG

 

[jraef]

Wow, interesting. I loaded the same URL into MSIE and it does show the ohm symbol, but if I use Firefox it shows a W! I wonder if that's a glitch from Mozilla in interpreting the ohm symbol, or from A-B who has two different sets of web pages to show you depending on the detected browser?

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[flexoprinting]

Hi Jeff

I use Linux Mint-1.0 With Firefox version 3.6.12 and see the omega symbol as well. Could be a font selection problem.

 

[fab1961]

Such a long distance may cause other problems, you may pick up noise along the way (I experienced a case like this) - I would install one more or two more units and use the safety contact of one as E-stop for the next and so on.

Hope this may help you.

 

[jraef]

Hmmm... I must have typed this on another PC and never submitted it.

We use Sick and Siemens safety relays, and on occasion we use Omron / STi. In all of those, they state a distance limit of the input field device wiring. Sick is 100M, Siemens is 300M, Omron is 1000M. So obviously it's all over the map, I'll be curious to see what A-B says. We use them in machines so distance is never an issue for us, but inquiring minds need to know.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[DRWeig]

I just gotta pitch in on the W versus [Ω] deal.

Nine times out of ten, when I submit a document filled with [Ω] symbols to the graphic / marketing folks to turn it into something glossy (like a catalog page), it comes back with W characters where the [Ω] symbol should have been.

Some fonts have only one [Ω] symbol, some have a couple -- and the W shows up most often when translating between PC and Mac. The symbol I typed in my document is an [Ω] on my PC and its associated printers, but it's a W when the font is changed by the artist.

It's a proofreading battle, let me tell ya!

It's very interesting to hear it can swap between browsers.

Good on ya,

Goober Dave

 

[majesus]

Interesting how this thread became two topics: ? versus W and relay distance.

In regards to the safety relay discussion, I called and confirmed with A&B, they said as long as the input resistance is less than the specification you are ok.

I ended up going with A&B’s MSR200 safety relay, it is a module system. Depending on your application, you can add more modules for the input and output requirements. The MSR200 relay is located in the I/O panel at the conveyor’s drive house. The base module can have two E-stop inputs, so to minimize circuit distance, I split the e-stop into a circuit covering the conveyor’s head, which has 5 e-stops and a total wire length of 4000ft. The tail circuit contains 14 more e-stops and a wire distance of 8000 ft. The safety relay is based on a 24VDC platform. The relay actually sends a 24Vdc pulse signal, through the E-stop switches and it monitors the returning waveform. If the pulse train differs then there is a system fault and the relay outputs open which release the motor's power contactor.

The E-stop system requires annunciation, and the E-stop switches are grouped in pairs and each pair has a dedicate 120Vac input into the DCS which also located at the Drive house. To minimize noise issues, I wired all the e-stops back to the input card using 24Vdc. I specified a solid-state interposing relay with a 10mA coil draw. The output is wired to 120Vac and the DCS card. The solid state relay has a leakage current of ~500uA and poses no problems to the DCS.

I used #12 AWG for this application. The #12AWG has a resistance of ~1.88ohm/1000 ft (source: Anixter DC and AC Resistance of Class B Copper
Conductors @ 60C). Thus based on this number, my input resistance from the conductor run of 4000ft is ~7 ohms. The MSR200 has a maximum input requirement of 900ohms.

For the voltage drops in the annunciation system, the solid state relay requires 9mA @ 24Vdc, I bundled the return path of longest run. Assuming a worst care scenario of 14 relays I have about 126 mA of current running 4000ft. (It’s not really the case, the E-stops are not all 4000ft away, I am just simplifying the calculations.) The calculate voltage drop is about 1.6Vdc. The relay requires 80% of nominal voltage.

I used 10C-#12 Teck for my runs to cover the E-stop hardwire and annunciation circuitry. I only needed 5C-#12 for the head,, but we bought bulk order cables and used 10C as the common.

 

[majesus]

Sorry I made a typo mistake, it's not the return path that is bundled, it is the 24VDC+ wire feeding each announciation group. The return path are seperated as that's the pull-cord status signal.

 

[bentov]

Continuing the digression, I argued with Siemens a while back over warranty date code on a soft starter. I got somebody on the phone, made them navigate to the year/letter code chart on their site supporting my case. They're looking at the same url, telling me I'm wrong. I notice the characters on my screen are a bit crowded (though clearly legible, no other problems viewing the site), ask what browser they're using (IE8) - mine was still IE6, been putting off the "upgrade". When I installed IE8, the character crowding went away, an entire column of chart data executed a one line vertical shift, and my warranty went away.

!@%$#%@ browsers!