120VAC vs 24VDC control 2

[richanton]

Nowadays with all the excitement about arc flash hazards, people want to get away from 120VAC control. I'm looking for any info on disadvantages of 24VDC control. I know the obvious like voltage drop over long distances, but I was interested more in any experiences with spurious trips and state changes due to routing 24VDC through very RF frequency noisy areas.

 

[rbulsara]

120V circuits are not considered a arc flash hazard as it relates to wearing PPE (suites etc.). But it is not to say they pose no hazard. People deciding to change contorl voltage because of arc flash fear are ill-informed or inexperienced.

Besides the voltage drop, a reliability issue comes in with a DC source. If not backed up by station batteries, DC source obtained just through a rectifier power supply could be less reliable as the power supply itself is more prone to failures, as compared to say an ac control transformer. You can add redundant power supplies to address that I suppose. Just my opinion.

 

[140668]

A real issue between 120Vac and 24Vdc is the fact that at 120V we are working with the LV standards, which poses a shock hazard. Whereas with 24Vdc we are working at ELV (Extra Low Voltage) and the possible shock hazard is classed as minimal. With todays technology on Linear PSU's I would recommend working at 24Vdc and if you experience a possible voltage drop due to long cable runs then this can be easily resolved by carrying out a few simple cable calculations and upgrading your cable size to achieve a satisfactory installation.

 

[dpc]

I think shock hazard is a bigger driver for this than arc-flash. But also, it makes for a more economical interface with digital solid-state devices.

Also, if the capacitive effects of 120V ac conrol circuits are taken into account, you might find that 24 vdc is not at as much of a disadvantage regarding control circuit length as you might expect.

 

[waross]

Contact film on pilot devices with minimal wipeing action may be an issue on lower voltages.
GE did some tests a few generations ago and found that about 100 volts was needed on pilot devices to penetrate contact film and ensure reliable operation in some industrial environments.
The old GE Solid State Logic Modules operated at a 12 volt logic level.
GE recommended using over 100 volts for field wiring to ensure reliable operation. We used input interface devices which operated on 120 volts AC in and logic levels out. I seem to remember a 125 volt DC version also.
respectfully

 

[ScottyUK]

this can be easily resolved by carrying out a few simple cable calculations and upgrading your cable size to achieve a satisfactory installation.

That's easy to say, but not always easy to achieve: cable sizes quickly become unreasonable on an industrial plant. Economics and common sense would suggest that running very heavy cables sized solely on volt-drop over long distances is not really a good design.

Another consideration of 24V v's 110V is that 24V contactors are not widely available above a fairly small rating, about 37kW / 50HP being the cutoff in many manufacturer's ranges. Above that level the 24V signal would drive a pilot relay for the main contactor, adding an unneccesary additional component capable of failure and still requiring a higher control voltage.

It really is a question of picking the best solution for your application, considering the physical installation, the size of loads to be controlled, the skill set of those maintaining the equipment, etc.

----------------------------------

Sometimes I only open my mouth to swap feet...

 

[DaveScott]

I agree about the higher voltage being more reliable in dirty environments (arc furnace areas etc.).
Another point is if you are driving a solenoid valve. 24V dc doesn't have any significant change in current as the inductance changes, but 120Vac has a very significant change.
If there is a sticky solenoid, then with ac, the supply ought to have individual fused protection, since otherwise the higher current will burn out the coil.
With dc, the coil is not affected, but you don't have indication of the fault either.
For voltage loss, quite often I have used more than 1 core in parallel for common supply and return.
There seems to be a big drive for 24V dc systems and, although I prefer 110/120Vac for reliability, I have accepted that this is just the way the world is going (but aren't autos moving towards 48Vdc?).

 

[mkinnes]

"With dc, the coil is not affected, but you don't have indication of the fault either."


This can make for an interesting time trying to locate the fault depending on the amount of things being supplied.

 

[itsmoked]

[!]"(but aren't autos moving towards 48Vdc?)"[/!]

42V

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[DaveScott]

Ha, OK. But in my defence: 42V is something that would be passed if one were moving on to 48V.

 

[OperaHouse]

I question if there is a big empirical difference between switch oxidation problem with modern controllers. Both types of controllers have too little current to maintain clean contacts. I have a GE Profile front loader clothes washer that started getting stuck in a process loop. 3 hours running and still not finished. They use the timer as a function switch and turn the timer motor on and off to advance to the next function. Sone WD-40 (this works as good as any contact cleaner) on the contacts got it running again. This used 120VAC on the controller input. A guilotine paper cutter I work on has interlocks on the interlocks on the interlocks. It needs them, this is the scarriest machine I've ever worked on! One of the contacts is about 90V DC at very low current that go bad. No way to put the right thing in. I just switch the contacts with another set of contacts that have 120VAC at higher current on them. That keeps it working for another 4 years, then I switch them again. Adding some extra current with an extra resistor load can help at any voltage. This can add some new reliabity issues if an extra power supply is added. Proper switch contact material selection is key.

 

[davidbeach]

Why would you design a control system that wouldn't blow fuses on a short circuit? That is just plan bad design unless the system is so simple that a loss of all control voltage shuts everything off immediately and that shutdown doesn't cause more problems than it solves.

 

[Drivesrock]

It is common practice in steel projects to have plant devices in 24Vdc - such as the solenoid valves & proximity switches. Also, to have pyrometers, hot metal detectors, light barriers and so on also fed with 24V. In these applications there is water being controlled - for direct and indirect cooling - and the plant staff also wash the areas down with water hoses. So 24Vdc or ac is the solution to prevent accidental shocks. With distributed I/O for the PLC control system cable lengths tend to be short.

Obviously, cable segregation is important for the 24Vdc signals to avoid intereference but I believe it would not be good practice anyway to have 120Vac control cabling mixed with motor cables, for example - especially if controlled by VSDs.

By employing inteligent PLC programming and even two digital signals from a device (S and S bar, when available) and spurrious events can be avoided.

The control panels in the electric rooms will have a 120Vac or higher voltage (according to customer / country standards) for contactor coils, power-supply mains and so on.

Drivesrock