×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

120Vac vs. 24Vdc control power in harsh environments
2

120Vac vs. 24Vdc control power in harsh environments

120Vac vs. 24Vdc control power in harsh environments

(OP)
thread237-164753: 120VAC vs 24VDC control
I am looking into the pros and cons of using 120Vac or 24Vdc with respect to industrial environments. I have had some problems with 24Vdc failing in places where the adjacent 120Vac has continued to work and would like to find some papers or studies that look into this. I have found a thread in which it is mentioned that GE had done test finding that about 100volts was needed to penetrant contact film, does anyone know where I can see these tests or any other ones like it? or does anyone just have any pros or cons they know of the dome piece?

RE: 120Vac vs. 24Vdc control power in harsh environments

A 24 VDC is more likely to get power drops if you have long cables and they aren't properly dimensioned.
It also burns relay tungs more then AC power, since DC do not have a sin wave that can terminate the arc in the airgap.
This isn't as bad for 24VDC as for 48VDC though since it often are very small currents applied.
Positive for 24VDC is that it is safer for people.
And there is more products to chose from, or maybe that is just me not knowing, since everything we build today is done 24VDC.

Best Regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

RE: 120Vac vs. 24Vdc control power in harsh environments

If you have electrical contacts (switches, relays) that are rated for 120 VAC then yes, films can be a problem for lower voltages. You might look at changing those devices to ones rated for 24 VDC. Low voltage contacts are made from different materials and the wiping action may also be greater.

RE: 120Vac vs. 24Vdc control power in harsh environments

That GE source is over 50 years old.
Genera Electric marketed a line of solid state logic devices and the contact film information was covered in the literature.
I worked with and taught the system in the early 70's.
A few years later Programmable Logic Controllers hit the market and Solid State Logic (GE, Square D, Allen Bradley and others) was obsolete overnight.
It may be difficult to find anything on the web. Obsolete documents 50+ years old are seldom digitized.
I did find a couple of re-prints of chapters from the original GE literature.
The source is given as;
"General Electric Company publication GET 3551"
You may find the specific information in the fine print, or use the information as a starting point for further searching.
Chapter 1
Chapter 3
Note: I am not up to date on current practice, however good quality industrial control devices such as push buttons, selector switches and some relays were designed so that the contacts closed with a small amount of wipe or sliding action in addition to the use of over 100 Volts to overcome contact film.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 120Vac vs. 24Vdc control power in harsh environments

Contact film. ponder
Is it like oxide on the contact surface or?
Do not know what it means.

/A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

RE: 120Vac vs. 24Vdc control power in harsh environments

Oxidation, dust, oil film; anything that prevents conduction, RedSnake

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 120Vac vs. 24Vdc control power in harsh environments

Okey thanks Bill.
I was quite sure that it was, what was meant, but one never knows. smile

/A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

RE: 120Vac vs. 24Vdc control power in harsh environments

Maybe it depends on your source of control power, but AC tends to fail during power dips, and outages. 24VDC may do the same.

For reliable control power, we use batteries, either 24 or 125 VDC. While the 24 VDC is limited to one control building, the 125VDC can be used all over a larger substation site.
But you do need to beware of voltage drops.

Yes it is a concern that surface fowling of contacts, which is why the higher voltage is better, but it is also needed to have contacts have some wiping action.

In industrial areas where dust is of concern, sealed, or dust resistant controls are needed.
I can tell you that gyp mining is dusty, which I doubt is what you are doing.

RE: 120Vac vs. 24Vdc control power in harsh environments

Most 24 VDC aggregats have built in kondensators that can keep the voltages levels for shorter power dips.

We have used 24VDC products for more the 25 years some longer and it is in a industrial environment, maybe not as dirty or dusty as some others, I would say normal.
Earlier we always hade air cooling via incoming ducts so the cabinets where over pressurized, no dust could come in.

Can't say we ever have hade any problems with contact film that have caused any problems.
Except between some I/O:s but I put it down to bad construction of the part.
Of course relays gets exchanged but northing lasts forever.

BR A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

RE: 120Vac vs. 24Vdc control power in harsh environments

(OP)
Thanks for the great responses guys, this is really quite helpful!

I was able to find some information in the links you sent waross, thanks!

And for crank108, you are right i am not in the gypsum mining industry. Working at a pulp & paper mill and the main concern is with H2S tarnishing and not with dust or oil film.

RE: 120Vac vs. 24Vdc control power in harsh environments

That is a bit worse than where I am.
Maybe you need to use Ex classed component isn't H2S flammable also?

/A ponytails2 wink

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

RE: 120Vac vs. 24Vdc control power in harsh environments

(OP)
Yes [H][2]S is highly flammable and very toxic upsidedown

RE: 120Vac vs. 24Vdc control power in harsh environments

I've always followed these debates about 120VAC vs 24DC control signals, and just yesterday ran across a 2005 statistic for 24Vdc vs 120Vac useage yesterday in an EC&M article. Interesting because I had no idea what the split was. According to a Phoenix Contact power supply product manager, in 2005 80% of I/O in automation applications were supplied with 24Vdc. 120Vac accounted for 15%.

Link

RE: 120Vac vs. 24Vdc control power in harsh environments

It is understandable everything gets smaller with 24 VDC.
The space needed for control funktions have gone from large to almost nothing at all.
One I/O card when I started was like a 40 cm high 30 cm deep and 5 cm thick, now it is maybe 6 cm high 4 cm deep and 1,5 cm thick.
Storing / shipping / and in cabinets all they way 24VDC takes less place.

It's harder to come down to does sizes with 120 VAC.

And it is also a question of power consumption, it is cheaper in the long run.

BR A

RE: 120Vac vs. 24Vdc control power in harsh environments

It depends on the type of control.
When the old GE document was published, over 50 years ago, the control circuits of most interest and the most common were field mounted push buttons, selector switches, limit switches and similar devices.
In some industries, oil film was an issue.
In saw mills, fine dust was an issue.
In pulp mills, S02 may have been of more concern than H2S. Much higher levels of SO2 are tolerable compared to H2S. And 50 years ago, safety legislation was both lax and under-enforced.
Pick your industry, pick your contaminant.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 120Vac vs. 24Vdc control power in harsh environments

Why consider anything other than 125VDC?

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: 120Vac vs. 24Vdc control power in harsh environments

For substations I agree 125VDC is best, but industrial, I can see other things might work better.

RE: 120Vac vs. 24Vdc control power in harsh environments

"Might work better" or simply be cheaper? winky smile

There's probably a reason that utilities standardized on a few common approaches. Even if there's still a lot of variation around the edges. Event the number of 125VDC systems remains up for debate, but 125VDC itself doesn't seem to be. Unless you ask the commies, then you get some weird answer about grounded 48VDC. winky smile

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: 120Vac vs. 24Vdc control power in harsh environments

Quote:

Why consider anything other than 125VDC?
-125 VCD requires 5x battery cells than 24 VDC and 2.4x more than 48 VDC. This may require a larger battery area and additional time to do routine cell maintenance.
-Some equipment is not available for purchase an ungrounded 125 VDC configuration.
-Ungrounded battery systems require actively monitoring for ground faults. For facilities with poor maintenance practices, tripping a 24 VDC system for the first ground fault may be better than let a 125 VDC ungrounded system sit grounded for an extended duration.


RE: 120Vac vs. 24Vdc control power in harsh environments

One major is battery power versus grid power.
For a substation the ability to maintain protection and switching during a fault condition or outage is very important.
Battery power provides that ability. Batteries require dependable care and maintenance.
On the other hand, in an industrial plant where a power outage brings all motors to a halt, loss of control power is a non issue.
A companion issue is choice of voltage.
Davidbeach is much more qualifies than I to comment of voltage selection for a substation.
For an industrial plant, voltage selection is a compromise between a number of things.
Cost, safety, immunity from interference, contact film issues, etc.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 120Vac vs. 24Vdc control power in harsh environments

Even in substations 120VAC is used in limited control fashion, such as transformer controls for cooling and LTC applications. If there is no power, then there is no need for cooling.
Except that the station power often comes from a different source than the transformer high voltage.
Many times a short loss of power may not be much of an issue.

The choice of controls power is a thoughtful process.

However sloppy maintenance should not be a factor.

RE: 120Vac vs. 24Vdc control power in harsh environments

In EU almost all control circuits today is built 24VDC for industri for the reasons I explained before, low power consumption, less space, less material, less environmental impacts, low freight cost, less manufacturing cost, etc. All other voltages products that can be bought is more or less for maintaining old systems or grid AC for home use where transformers aren't so common.

For Ex classed environments, and now I am making an assumption, it's easier to comply with the regulations with 24 VDC since the energi needed is less, easier to "contain".

We have used UPS for shorter start up times for PLC:s or PC:s but my experience from the past is that they stand for more of the stop time then the eventual time you save at a power drop.
Now this is for old UPS systems, maybe the new ones are better.

But as it is sade they need maintaining and if that does not work it's often better to be without.

/A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

RE: 120Vac vs. 24Vdc control power in harsh environments

Does the industrial process rely on simple plug in style relays? Sometimes the 120 V coils will drop out during a voltage sag the motors can ride through. Battery backed up DC coils will not be affected by supply sags.

RE: 120Vac vs. 24Vdc control power in harsh environments

On the subject of coils, AC coils have a high inrush current compared to holding and not all can tolerate rapid cycling without overheating. DC coils can cycle continuously.

RE: 120Vac vs. 24Vdc control power in harsh environments

125 Vdc hurts, but it works!

RE: 120Vac vs. 24Vdc control power in harsh environments

Quote (davidbeach (Electrical))

Why consider anything other than 125VDC?

Quote (waross (Electrical))

One major is battery power versus grid power.
For a substation the ability to maintain protection and switching during a fault condition or outage is very important.
Battery power provides that ability. Batteries require dependable care and maintenance.
On the other hand, in an industrial plant where a power outage brings all motors to a halt, loss of control power is a non issue.
A companion issue is choice of voltage.
Davidbeach is much more qualifies than I to comment of voltage selection for a substation.
For an industrial plant, voltage selection is a compromise between a number of things.
Cost, safety, immunity from interference, contact film issues, etc.

Indeed, choice of voltage can also be governed by factors such as power required for the equipment to operate and availability.
All of my work in Power Generation has been done with 24V control voltages, but I do LV projects where all the operating coils for breakers are small enough to be able to use 24V, and all of the other control equipment is standardised on 24V DC, so its a sensible choice.
In substations, the power required for breaker operating coils (as far as I'm aware, anyway) is often higher and justifies the use of a higher voltage, but still with the backup requirement. In that context, I'd agree with davidbeach.

To me though, use of a UPS as compared to a battery supply to keep the control power up is an unnecessary complication, but that can also be governed by the control equipment and whether it has a DC supply option.

Positive grounded 48V DC is another one, although that appears to be limited to a specific industry.

EDMS Australia

RE: 120Vac vs. 24Vdc control power in harsh environments

Quote:

On the subject of coils, AC coils have a high inrush current compared to holding and not all can tolerate rapid cycling without overheating. DC coils can cycle continuously.
A correct but misleading statement.
AC inrush depends on the air gap or armature travel. As the relay operates and the air gap closes, the inductive reactance increases, dropping the current.
DC on the other hand needs a similar level of current to magnetize the air gap, but when the air gap closes the current doesn't dropas it does with an AC coil.
The inrush of larger DC contactors is so great that economizer circuits must be used to prevent rapid burnout of the DC coil.
For a given relay, the minimum RMS AC current required to pull in won't be that much different than the minimum DC current required to pull in.
The minimum AC current may even be less due to the peak currents, but that depends on a lot of mechanical factors such as the inertia of the relay armature and the distance it moves at the peak current.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 120Vac vs. 24Vdc control power in harsh environments

Or, said another way:
AC coils draw a large current to pull in. As the armature closes the air gap, there is a reduction of current.
DC coils draw a large current to pull in. As the armature closes the air gap,there is no reduction of current.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 120Vac vs. 24Vdc control power in harsh environments

Great question! Great answers!

Fact is we are stuck with 24V AC/DC control systems. It has to do with circuit boards and board level circuitry components. Everything is getting smaller and cost savings continue to drive all trends. Ideally 120 Volt control circuitry works best across most control panels and systems because of its hold in capability, ability to work in dusty environments, and its aversion to droop from fluxuating control power, transformers or otherwise. I've seen control circuits on MCC in paper mills spec out for 480 volt control just because of the hold in requirement. (Mike 480 volt Glen - you know who you are)

Best regards,

TF

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close