120VAC vs 24VDC

dcam · Mar 6, 2020

24VDC is the way to go In my opinion when you way out the pros and cons. There are obviously places where higher voltages are necessary.

Just wanted to bring up a con of 24VDC: It loads down at times that is difficult to troubleshoot. I have had entire processes down with 50 workers put on break while I looked for what loaded down the 24VDC. It typically is a coil or a j box full of water....

Just real experience from my life......as I walk the world barefoot like KANE from KUNG-FU....

Saffa · Mar 6, 2020

dcam said:
Just real experience from my life......as I walk the world barefoot like KANE from KUNG-FU....

Better not be walking barefoot around those waterlogged JBs...

24V all the way. Just please, use glass fuses so that not EVERYTHING in the panel is just looped around off the nearest 24V terminal. Makes trouble shooting much easier because instead of getting to a panel and finding a 24VDC supply that is current limiting due to a short somewhere, you turn up and check a few fuses and after 30s have already divided your problem into a much smaller area to check.

For large plants I individually fuse every 24V powered device, every 4-20mA loop powered device, and every block of 8/16 DI or DO. The fuses with the LED indicator are awesome and totally worth the extra cost.

NetNathan · Mar 6, 2020

GLASS FUSES....YUK..something else to keep in the cabinet and how many times have I run across 10 and 15 amp glass fuses installed instead of 1 and 2 amp....
I spend the extra few dollars for CB.

padees · Mar 6, 2020

+1 on CB's.

JaxGTO · Mar 6, 2020

padees said:
+1 on CB's.

+1 on CB too. You can also now get power monitors for each DC circuit and bring them in as an analog input. Then just check the trend to see which circuit has spiked up in AMPs.

Saffa · Mar 6, 2020

I guess we all face different challenges, but I'd say my team of 6 only replace maybe 10 fuses a year. Each service vehicle has a box with several packets of 0.5A, 1A, 2A, 3A and 5A. Anything bigger than that is on a CB.

We mostly do remote telemetry applications where there's a pillar box or cabinet on the side of the road and space is a premium.

Of course if anyone has some slimline circuit breakers for DC that they can recommend, I'm all ears!

g.mccormick · Mar 6, 2020

Saffa said:
I guess we all face different challenges, but I'd say my team of 6 only replace maybe 10 fuses a year. Each service vehicle has a box with several packets of 0.5A, 1A, 2A, 3A and 5A. Anything bigger than that is on a CB.

We mostly do remote telemetry applications where there's a pillar box or cabinet on the side of the road and space is a premium.

Of course if anyone has some slimline circuit breakers for DC that they can recommend, I'm all ears!

https://www.automationdirect.com/ad...l_block_supplementary_protectors_(euro_series)

How bout now?

g.mccormick · Mar 6, 2020

One area where I have seen 120VAC and higher used extensively is in commercial and industrial refrigeration. Solenoids are all 120 or higher voltage and that is in evaporator cases that are subject to moisture, ice, etc. No 24VDC power at all, the AI power is all sourced from the DC created on the controllers. The controllers may have/be powered by 24VAC, but all output are relay outputs switching 120VAC or more. The panels are typically non-touchsafe 480VAC everywhere and don't even have a door interlocked disconnect. The doors just have 1/2 turn buckles on them. I guess since these are typically roof mounted and only serviced by "professionals" the need for safety is removed?

AustralIan · Mar 7, 2020

24V needs 5x the mm² for same watts.
For the same watts and same %voltage drop it needs 25x the mm². (As 120V)

If you have wet skin, your resistance is maybe 2300 Ohms (1000 per hand + 300 internal flesh) so grabbing 24V with one hand and 0V with the other is 10 mA through your heart. If you have a fresh cut on both hands you are left waiting patiently for an ambulance.

What I am trying to say is, don't let the lower voltage of 24V increase your complacency around electricity.

controlconcepts · Mar 7, 2020

JaxGTO said:
You can also now get power monitors for each DC circuit and bring them in as an analog input. Then just check the trend to see which circuit has spiked up in AMPs

I have been looking all over for something like this. Do you have a link or certain one's you use. My Google searches only came up with smart CB's and Power Link device's. I'm not using the correct sesrch terms.
Thanks

AustralIan said:
What I am trying to say is, don't let the lower voltage of 24V increase your complacency around electricity.

Very important point to bring up. And to add to that AC @ 60HZ you have break every 60 times a second as the field collapses and a better chance to have your muscles relax to pull away. DC is just that Direct Current no break in flow of electricty through your muscles. I have experienced this and heard many stories throughout the years of the difference of an AC shock compared to a DC shock and how you get "held up" worse on a DC circuit to an AC one.
One thing my high school trade school teacher always instilled on us is electricity is a just a contained fire that we work with and to always respect that.

JaxGTO · Mar 7, 2020

controlconcepts said:
I have been looking all over for something like this. Do you have a link or certain one's you use. My Google searches only came up with smart CB's and Power Link device's. I'm not using the correct sesrch terms.
Thanks

Sorry it was on a OEM machine and I didn't noticed who it was made by. Ill ask the tech back at the plant see if he can find it.

controlconcepts · Mar 8, 2020

Thanks JaxGTO, I appreciate it.

Aabeck · Mar 8, 2020

controlconcepts said:
And to add to that AC @ 60HZ you have break every 60 times a second as the field collapses and a better chance to have your muscles relax to pull away.

Yeah - NO!

60Hz actually has a zero point 120 times per second, and there is no way anyone is going to time a pull-off the 0.0002 seconds (at most) the voltage is 0.

If it's in contact with the palm side of your hand or fingers it will be puling the muscles IN tight for better contact.

BryanG · Mar 9, 2020

Rson - apologies again but I can't ignore contact from another of my species.

Aabeck, my low resistance Brother, or I think the politically correct term is 'ohmicly challenged', I thought I was the last of our species on this planet. Natural selection generally eliminates low resistance electricians.

Do you think we can call creating a low voltage, dare I say it, a 'super power'. I have already started a design for a rubber suit with an electric flash on the front, but a name that hasn't already been taken is tough to find, 'Electro Man'.

Never did figure out why I create a voltage, I wondered if it was differences in the metal of the meter probes that was causing a electrochemical reaction. You do realise that I am now going to have to grab a meter and see if it still happens, like I don't have enough to do today.

LadderLogic · Mar 9, 2020

These work well for us for 24VDC short protection:

https://www.wago.com/global/power-supply-modules/electronic-circuit-breaker-epsitron/p/787-1668

There is a 4-channel model as well.

120VAC vs 24VDC

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