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Old December 12th, 2017, 04:07 AM   #1
Charbel
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230VAC vs 24VDC

Dear,

what is the advantage of using 230VAC over 24VDC for discrete signals (digital input and outputs)

It is well understood that 24VDC is to be used for safety purposes

however, the client is asking for 230VAC to be used for DI/DO since it can handle better bad environmental conditions (It is used for field instruments far from the PLC in a large sewage treatment facility), please advise.

bad environmental conditions in this area (Kuwait) can be heavy sandstorm, high temperature (which I doubt it will affect)

thank you,

charbel
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Old December 12th, 2017, 04:48 AM   #2
Saffa
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I can't think of any reason? Many modern PLC systems don't even have 230V inputs anymore?

If your cables are properly screened, terminated and run seperately from other 230/400V services then you will have no issue.

Heat has no effect.
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Old December 12th, 2017, 04:59 AM   #3
keithkyll
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With very long runs, voltage drop can be significant. Let's say the run is so long that we have a voltage drop of 10 volts. At 24 volts, this is a serious problem.
At 230 VAC, it's negligible. I just checked AD Productivity 2000 PLC modules. You can get 120V and 120-240V I/O.
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Old December 12th, 2017, 05:02 AM   #4
Dirt
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When I worked at a nuclear power plant, we had similar designs at two reactors. There was lots of reed relays for detecting position and volume flow (very specialized equipment)

One plant ran 24VDC (with a modernized control and safety system) and the one I worked at ran 110VDC center ground system. The 24VDC had a tendency to miss closed contacts due to oxidization.

The 110VDC center grounded system only needed one side, 55VDC basically to actually detect and had higher percentage of detection.

But this was a very old system and like nothing that'd be used in modern equipment. 24VDC is sufficient in most places, situations and SIL classes. As long as cable runs aren't too long, eg. voltage drop.
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Old December 12th, 2017, 05:09 AM   #5
geniusintraining
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I agree with the voltage drop over long runs

And just a WAG is it cheaper to run a 240 volt over 24? From the power consumption point but if you're pulling in a large coil would it be cheaper for 240? I don't know but maybe a reason

I would also think the 240 would be heavy-duty vs the 24, I work on a system that had a bunch of 240v outputs for larger motor starters
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Old December 12th, 2017, 05:21 AM   #6
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Keith I understand what you're saying but i don't think that applies here.

Purely hypothetical, but a micrologix 1400 digital input turn on voltage is 10V. Nominal draw per input is 5mA.

Using a 0.5mm2 instrument cable with a 36.8ohm/km resistance, I can still run 38km of cable before i have an issue....

Not that you would of course... I'm not rolling out that drum!!!



Ine
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Old December 12th, 2017, 05:52 AM   #7
keithkyll
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Yes, in a perfect world with no corrosion.
Some customers have silos that are 100 feet tall. Full sensors are motorized paddles with dry contacts. The silos will be wired by electricians. They use bare copper, TFFN.
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Old December 12th, 2017, 05:54 AM   #8
pal
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Another point - with ac , you could have up to 10 ms signal delay from waveform . If timing is not critical then no problem .

Paul
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Old December 12th, 2017, 06:24 AM   #9
Saffa
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As it's a WWTP, nothing is likely to need 10ms resolution so safe on that account

Specifying (and checking!) Cable type is important, as is glanding, to ensure minimum corrosion.

I've been working in water and waste for a decade and i have yet to see any 230V input cards. A few 24AC ones but DC has been the norm.
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Old December 12th, 2017, 07:02 AM   #10
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Just from a common sense point of view you wouldn't want 240 vac anywhere near field equipment.

The amount of poorly maintained plants I see with prox wires exposed because of chaffing springs to mind.

It still works so nobody bothers with it.
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Old December 12th, 2017, 07:39 AM   #11
Dirt
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Quote:
Originally Posted by saffa View Post
as it's a wwtp, nothing is likely to need 10ms resolution so safe on that account

specifying (and checking!) cable type is important, as is glanding, to ensure minimum corrosion.

I've been working in water and waste for a decade and i have yet to see any 230v input cards. A few 24ac ones but dc has been the norm.
Quote:
Originally Posted by janner_10 View Post
just from a common sense point of view you wouldn't want 240 vac anywhere near field equipment.

The amount of poorly maintained plants i see with prox wires exposed because of chaffing springs to mind.

It still works so nobody bothers with it.
+1
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Old December 12th, 2017, 09:01 AM   #12
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120 VAC IO is typically used in harsh industrial environments because the higher voltage can be detected through an oxidized, high resistance contact closure that would appear open to a 24 VDC input. I've encountered very few systems employing 230 VAC IO, and I've never built a new system using that control voltage. I suspect that 230 VAC IO has been used primarily to interface to existing 230 VAC hardwired control systems when upgrading to PLC controls.
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Old December 12th, 2017, 02:03 PM   #13
Alfons Gessl
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24V DC is the standard for instrumentation in general for longer distances I use ET stations via ProfiBus or ProfiNet.
Only for return signals from Motor Isolator I use 230V AC, reason if the return signal wire gets in contact with power wires from the motor it will bow my entire 24V DC system. A 230V AC feedback can handle that. In the panel I reduce the 230V AC to 24V DC vie relay for the PLC input cards.
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Old December 12th, 2017, 09:12 PM   #14
mk42
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Quote:
Originally Posted by Bit_Bucket_07 View Post
120 VAC IO is typically used in harsh industrial environments because the higher voltage can be detected through an oxidized, high resistance contact closure that would appear open to a 24 VDC input. I've encountered very few systems employing 230 VAC IO, and I've never built a new system using that control voltage. I suspect that 230 VAC IO has been used primarily to interface to existing 230 VAC hardwired control systems when upgrading to PLC controls.
My guess is that it just the European standard AC, compared to our 110/120 over here. As an example, I don't think siemens has any 120 only modules. Any digital AC stuff is 110-240
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Old December 13th, 2017, 04:58 AM   #15
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How about running the long signals 220V for corrosion resistance and in the panel have a bank of 24V interposing relays going to the inputs & powered off the outputs?
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