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Old March 5th, 2020, 09:45 AM   #1
Rson
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120VAC vs 24VDC

The people I work with are not electrical people.

I run into this struggle almost every other week on why I choose to use 24VDC. I have some arguments listed below, if anyone has anything else to add or refute, I'd love to hear them!

120VAC is lethal. The danger does not just lie in the controls enclosure, but also to any instrument, valve, or other equipment that is running 120V.
24VDC has a near-zero chance of being lethal.

120VAC requires conduit to be run to any sensors, valves, etc up meet code.
24VDC doesn't require conduit to be run to valves, sensors, etc. Cables are acceptable. (Cables may still need to be routed in a tray)

120VAC requires PPE while working on any enclosure or wiring. (anything above 50VAC)
24VDC does not require any PPE

120VAC requires shielding for any analog signals and has a greater risk of inducing voltages on incoming/outgoing signals
24VDC does not induce any voltage on analog signals.

120VAC does have some advantages.
It tends to work better in very dirty areas, since the voltage can overcome resistance created by dirt.
When there is a short, it will burn up a wire or pop a fuse, so you can find problems quicker. When 24VDC shorts, usually the power supply gets pulled ‘low’ and the supply will be damaged. You don’t always find the short/issue at the point of failure.
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Old March 5th, 2020, 10:08 AM   #2
JesperMP
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Single phase AC is usually generated by a control-transformer.
That transformer is always a big heat generator inside the control cabinet.

A disadvantage with DC is that AC is more efficient for coils. DC contactors and solenoids are bigger and more expensive than AC.
In the olden days, you would some times see a variant where contactors and solenoids where supplied by 24VAC or 48VAC. This because DC was more expensive. Probably not so important today.

About 24VDC.
There should be enough circuit breakers in the DC distribution side, that pinpointing a short is easy. Also, even if a short is not interrupted by a circuit breaker, the DC PSU will not be damaged. Modern regulated DC PSUs are shortcircuit proof.
With 24VDC, you can realistically have every terminal that sends 24V out of the panel be knife terminals. In case of a short in the field, you open all the knife terminals that are supplied by the tripped circuit breaker, close the circuit breaker and close the knife terminals one by one. When the circuit breaker trips again you know where the short is. edit: We use knife terminals on outgoing DC for this reason.
Even more deluxe is every outgoing 24V via a fused terminal, but that is overkill IMO.
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Old March 5th, 2020, 10:13 AM   #3
Rson
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Love the knifeblade terminal idea!
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Old March 5th, 2020, 10:52 AM   #4
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Quote:
Originally Posted by Rson View Post
The people I work with are not electrical people.

I run into this struggle almost every other week on why I choose to use 24VDC. I have some arguments listed below, if anyone has anything else to add or refute, I'd love to hear them!
Sorry to tell you but you'll never change their mind/hearts. I've heard stories of guys testing 480vac with the back of their hand. Just quick swipe to see if its hot or not.

I'd say the best argument would be, how long do you want to be down?

24VDC I can find the problem faster and potentially fix it and bring other people into the process faster. 120VAC we have to gear up, or have a requirement the panel has to be dead before you can get in it. Ever try and troubleshoot a panel that has no power?
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Old March 5th, 2020, 10:57 AM   #5
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Articles from t'interweb thingy....

https://www.manufacturing.net/home/a...ty-reliability

https://control.com/forums/threads/why-24vdc.19624/

http://www.gohz.com/why-we-use-24v-dc

https://www.northwindts.com/24vdc-vs-120vac/
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Old March 5th, 2020, 11:27 AM   #6
OkiePC
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I used to list two excuses for using 120vac as control voltage:
1) distance
2) reliability

Number 2 is less relevant these with moden switching power supplies being nearly as trustworthy as a control transformer. There are still occasions where number 1 can be a valid excuse, but 24vac could be used when you need to send a signal for a great distance and still have safety.

I am not trying to defend your customer, just playing devil's advocate.
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Last edited by OkiePC; March 5th, 2020 at 11:48 AM.
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Old March 5th, 2020, 12:00 PM   #7
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I will always choose 24VDC control system over 120VAC control system. Safety first. You can troubleshoot a live 24VDC panel, but not a 120VAC panel, without suiting up.


in a previous life, I spent a lot of time working for an integrator. Everything was 24VDC controls. Then we started doing a lot of work for the water industry here in Florida. For whatever reason, all the controls were 120VAC. I was never able to get an explanation from anyone as to why. Ever try to work on a control valve in a remote pump station or wellfield, standing on wet ground, that used 120VAC for limit swithes? Maybe it is the water treatment industry standard, or maybe it is Florida. Just never made sense to me.
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Old March 5th, 2020, 12:57 PM   #8
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20 years ago it was normal to see control cabinets operating at 110-120 VAC but now it is considered obsolete or dangerous, in addition the modern 24VDC power supplies with three-phase input makes it easier.

No need of a big transformer and its heat.
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Old March 5th, 2020, 01:15 PM   #9
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One customer I work with has 120VAC on switches/pilot devices, not a huge fan (24VDC for analog instruments). This is a brand new plant as well, designed and built in 2017. Pretty small site, so distance isn't a factor. I assume it's just the way the engineering firm that designed it has done it for decades and they can churn out designs quickly if they just stick to it.
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Old March 5th, 2020, 01:44 PM   #10
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I worked on a water well building last week that used 460vac for control, and even had it running to another pump panel through a light switch mounted inside. The light switch is mounted nicely in a metal box and labeled so that anyone working on it knows that there is 460vac FROM ANOTHER PANEL that needs to be isolated.

I really don't like knowing that the selector switch I am about to flip has 460vac just a 1/2" away from my bare hands inside the box.

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Old March 5th, 2020, 01:45 PM   #11
Rson
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Quote:
Originally Posted by mbartoli View Post
I will always choose 24VDC control system over 120VAC control system. Safety first. You can troubleshoot a live 24VDC panel, but not a 120VAC panel, without suiting up.


in a previous life, I spent a lot of time working for an integrator. Everything was 24VDC controls. Then we started doing a lot of work for the water industry here in Florida. For whatever reason, all the controls were 120VAC. I was never able to get an explanation from anyone as to why. Ever try to work on a control valve in a remote pump station or wellfield, standing on wet ground, that used 120VAC for limit swithes? Maybe it is the water treatment industry standard, or maybe it is Florida. Just never made sense to me.
This is probably it for me. We do some water treatment site controls, and when our sales guys look at other's equipment it is all 120VAC for some reason. That is where most of the pushback is for me. I tell them 120VAC is mostly a dinosaur except for certain applications and 24VDC is the standard, but all they see is 120V everywhere!
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Old March 5th, 2020, 03:32 PM   #12
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Quote:
Originally Posted by Rson View Post
I run into this struggle almost every other week on why I choose to use 24VDC.

120VAC is lethal.
That should be enough.
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Last edited by Timbert; March 5th, 2020 at 03:35 PM.
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Old March 5th, 2020, 04:55 PM   #13
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You usually only see 120v control voltage on really old machines from the ark or anything American.
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Old March 5th, 2020, 08:39 PM   #14
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I used to be more pro-120V but am now more in favor of 24V. I haven't had many problems with 24V, and the improved safety is a big bonus. I've seen fires start from loose 120V connections in a prox!



Quote:
Originally Posted by OkiePC View Post
I used to list two excuses for using 120vac as control voltage:
1) distance
2) reliability

Number 2 is less relevant these with moden switching power supplies being nearly as trustworthy as a control transformer. There are still occasions where number 1 can be a valid excuse, but 24vac could be used when you need to send a signal for a great distance and still have safety.

I am not trying to defend your customer, just playing devil's advocate.

Distance (particularly if there's a lot of remote loads like contactors and relays, not just sensors).



On reliability I think more of 120V's capability to punch through dirty or poor connections than the reliability of the PSU or control transformer. I've rarely, if ever, come across a failed 24V power supply, even after sins like connecting 480V to the output (downstream of the circuit breaker, which tripped, and most of the 24V devices were damaged).


Three phase 480V-24V power supplies make sense in a panel with 480V motors. Though I've seen interesting designs. 480V-24V power supplies in panels strictly with low current 24V loads (a panelview or an I/O rack), when it would make more sense to run 24V from the main upstream panel, or maybe a 120V-24V power supply in the panel if it's too far away.
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Old March 5th, 2020, 10:51 PM   #15
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I use 24vac control in areas with very high humidity. Eliminates the electrolysis that happens with DC connections. That might be a reason for some 120VAC control circuits.
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