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

[Lawrence220]

thread237-164753
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?

 

[RedSnake]

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

 

[BrianE22]

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.

 

[waross]

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!

 

[RedSnake]

Contact film.
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

 

[waross]

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

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

 

[RedSnake]

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

/A

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

 

[cranky108]

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.

 

[RedSnake]

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

 

[Lawrence220]

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.

 

[RedSnake]

That is a bit worse than where I am.
Maybe you need to use Ex classed component isn't H[sup]2[/sup]S flammable also?

/A



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

 

[Lawrence220]

Yes [H][2]S is highly flammable and very toxic

 

[danw2]

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

 

[RedSnake]

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

 

[waross]

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, S0[sup]2[/sup] may have been of more concern than H[sup]2[/sup]S. Much higher levels of SO[sup]2[/sup] are tolerable compared to H[sup]2[/sup]S. 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!

 

[itsmoked]

Because 24Vdc requires power supplies it tends to not be affected by brief power glitches because the power supplies have large capacitors involved.

Here's the definitive text on relays:

Technical Explanation for General-purpose Relays

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

Why consider anything other than 125VDC?

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

 

[cranky108]

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

 

[davidbeach]

"Might work better" or simply be cheaper? ;-)

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. ;-)

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

 

[bacon4life]

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.