I'm wondering why is that they set 24VDC as standard in industry in the first place? Plz do help me in this part as I'm doing my final project on PLC. However, the PLC is able to give control signal as 1 at the voltage as low as 6.6VDC. So, if that's the case, why they wanna set the standard as 24VDC? Thanks.

 

I'm not quite sure, but it is good to have several volts of margin between the input thresholds. I think IEC 61131 specifies this. Think about it, if you have noise in your system, you want your "high" and "low" signals to be able to tolerate voltage fluctuations of more than a couple of volts. So at 24V, if your switching threshold is between 11-13VDC, you can be guaranteed that a 24V signal with 5volts of noise will be alright.

Also, 12VDC is too low, and 48VDC is getting too high. )

24VDC is nice because its high engough voltage to break through minor oxides on switch and relay contacts that would normally require expensive gold terminals.

~Ken

 

this is an often opined about question.

it may have from car batteries. 2 car batteries in series is 24VDC. perhaps they wnated to be able to use battery backup.

 

Xiang, there is no standard, per se!

Often, especially in USA designed plants, the voltage split was dictated by the maintenance crafts. 24 Vdc was an instrument technician's
responsibility, while 120 Vac, was the electrician's domain.

However, if used in an Emergency Shutdown System (you can tell how old I am because now they're called SIS systems) available failure-rate data
shows that high-power, 120/220 Vac, actuation devices (mostly relays and solenoids) have failure-rates about 2½ times that of low-power, 24
Vdc, devices. Similarly, high-power, 115 Vdc, devices have failure-rates about double that of low-power, 24 Vdc, devices.

The above is failure-rate data referenced in my paper:

"Probabilistic Risk Assessment of Safety Systems!"

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Although I can tell you the history of 4-20mA - you've stumped me on the history of 24Vdc.

Honeywell was the bringing out transistorized electronics, but they had problems with the very early production components - the transistors wouldn't turn off all the way.

The way I heard it, an engineering guy told the marketing people that it didn't matter, having a "live zero" was in fact, an advantage.

But back to 24Vdc. Some military vehicles and diesel trucks used two 12 Vdc batteries, some aviation electronics were run at 24Vdc, but why industrial sensors and valves picked 24Vdc isn't clear to me. It's obvious that 24Vdc is below the shock threshold, and it works well for intrinsically safe devices, but why it gained such wide acceptance isn't clear to me.

Maybe some other old fart like me will tell us.

Bud

 

Many reasons. Low enough for safety and compatibility with solid state components, High enough to avoid dry switching and to perform actual work with the currents that are practical. And the exact voltage was quite probably chosen to be compatible with relay coils and available actuators. The telcos were heavily into relays so battery voltages, multiples of the 2 volts nominal of secondary lead-acid batteries were the obvious choices. For the same reason, panel lamps and tube filament voltages standardized on 6.3, 12.6, etc, nominally 6 and 12 Volts. The 6.6 volts you mention is part of it. That gives a 6 volt noise margin for a "0" and over twice that for a "1". Most specify something like 7-13 volts as the forbidden region for this reason. With your 6.6 volt turn on. there would have to be a lot of noise on a 24 volt signal to give you problems.

That's why. I don't think there is any one reason, it made sense to people doing automation at the time.

Regards
cww

 

I have the suspicion that it relates to aircraft and military power systems developed during the World War II era. As with the progression from 6 volt to 12 volt automobile power systems, 24 volt systems were probably a similar progression relating to the virtues of power (volts x Amps) versus weight of the batteries and the influence on conductor size/weight. Not to mention the price and availability of copper during the war.

Tremendous advances in "mobile" electrical power and control systems were made during the 1930's & 40's.

Current aircraft and aerospace DC power systems still use 28 volt systems derived from 12 fully charged storage battery cells with 24 volts being the "full discharge" endpoint.

I work with the Space Shuttle Main Engine and it has, among others, a hefty 28 volt power system.

Next point of curiosity: Why do aircraft & aerospace power systems utilize 400 Hz AC power?
Hint: Transformer core size / weight as relates to frequency.


David McConnell
Rocketdyne Propulsion & Power
Stennis Space Center, MS

 

There's an interesting article at http://aztec.asu.edu/dakota/c53.html on
the history of 12 VDC vs 24 VDC electrical systems on the C-53 aircraft.

Another article on marine electrical systems at
http://www.dcmt.bc.ca/stories-v2.cfm?stories.ID=9

What seems apparent is that the "standardization" on 24 VDC is the result of technical and financial evolution rather than decision by some expert panel.

Carl Ramer, Engineer
Space Gateway Support
Kennedy Space Center, Florida
[email protected]
(v)321-867-1812
(f)321-867-1495

The opinions expressed above are personal and not the position of any company or government agency.

 

Yes, everything in a power supply gets smaller and more efficient as the frequency increases, to a point. Iron losses get higher after that. Large (electrolytic) capacitors were discouraged for reliability and temperature range reasons. Also 3 phase has been preferred to further decrease the filtering needed and to maximize motor efficiency. The alternators are smaller and more reliable as well. I've still got a couple 3ph 400Hz gyros around someplace. 400 Hz would be good for the power grid as well if it didn't cause other, (radiative and dielectric) losses as well.

Regards
cww

 

Well, first of all, 'they' is us. There is no 'they' per se. These things evolve out of common practice as 'defacto' standards. So, where did it come from? In my opinion, besides the fact that we need a low voltage signal there is the aspect that 24 is a 'base 12' number. Base 12 arithmetic is neat because it is divisible by 1,2,3,4,6, and 12 (much like the current US tool size standard which will never be completely replaced by metric). I don't know, but it would seem to me to have been a factor some where along the line.

Robert Trask, PE
Los Angeles, CA USA
[email protected]

 

thanx for the information, however, about the IEC standard, I know that IEC 61131 is about PLC but did it mention something about PLC rating? Where actually can I get a copy of that?

 

IEC standards can be purchased online (with a credit card) at http://www.iec.ch - or several other companies that sell standards.

Search here:
http://domino.iec.ch/webstore/webstore.nsf/$$search?openform

You need IEC 61131-2 (latest rev is of 2003-02-19) title "Programmable controllers - Part 2: Equipment requirements and tests". Price: 274 Swiss Francs

Meir

 

As a new Ham back in the 50s I haunted surplus stores looking for usable gear to convert. I was sometimes overjoyed when I saw an ad for something that ran on 110 VAC . . . until I saw 400-cycle. Also, the cars I drove had not yet switched over to 12 VDC, so the 14- and 28-volt DC generators, etc. were of no use to me. After a while I stopped going to the surplus stores (J J Glass, Downtown Los Angeles!), and started buying (or building) things that run on 6-volts or 110 VAC - 60 cycles. But, it was fun crawling through "junk" that had once possibly seen action somewhere. The stuff that I did manage to convert had a high degree of failure rate, which made me wonder how anyone ever had reliable communication during the Big War?

 

*The stuff that I did manage to convert had a high degree of failure rate, which made me wonder how anyone ever had reliable communication during the Big War?*

Two way radio communication was notoriously unreliable at that point in time other than between base stations serviced by well trained technicians.

Even during the Viet Nam era it was often iffy.

--
Bob
http://ilbob.blogspot.com/

 

seems that there were electronic 48 vdc systems in the 60's with vacuum tube controllers, but 24vdc quickly became common place.

 

Dear Friend,

24VDC is a standard. there are some basic reasons for using 24VDC in industries.

1. safety: 24DC will not generate sparks. but if we go for a higher voltage, it may generate spark in case of a short circuit/loose contact. so 24 VDC can even use in Zone 1, Ex'd areas & instruments.

2. Eliminate high current in the system: All Electronic devices in industry have a minimum power consumption. it can be provide by using low voltage & high current / low current & high voltage. It is not a safe practice to use high current (conductor get heat/melt). it is a must thing that the current must not exceed 100A for any circuit in industrial instruments. so due to this reason, we can not reduce the voltage for keeping the current minimum. so voltage less than 24VDC is not possible.

3. In addition to the above reasons, the intrinsic safe concept of two wired analog devices with floating supply is most efficient for 24VDC system.

 

24VDC can indeed generate a spark. If you want to prevent sparks you need to use intrinsic safety barriers to reduce the amount of energy that can be in a circuit. That will prevent a spark.

24VDC is a relatively safe voltage as far as electrocution goes. You are unlikely to die from contacting live 24 VDC.

There is no rule I am aware of that restricts current to 100A. Wires are always sized for whatever current they will see and have some kind of protection from overload and short circuit so melting or getting hot should not be an issue with any wire that is properly installed.