If you have spent any time in a control cabinet on a factory floor, you might have noticed that 24 volts of DC electricity is used for controlling relays, contactors, and controlling/powering other automation devices. The automation industry uses this 24 VDC as a standard for low-voltage power and signalling, but why that particular voltage, and why DC? What makes 24 VDC so special that it can be used for so many different applications?In this article, we’ll explore the different reasons why we use 24 VDC and compare this standard to other voltages we might find in our control panel.Safety Within The Control CabinetNot just operators, but even the devices within the control cabinet need to be safe, and a way to make these devices safe is for them to use a voltage range that is considered extra-low-voltage or ELV. An ELV is a voltage less than 120 VDC and less than 50 VAC. Voltage within this range is considered less likely to cause an electrical shock in the event someone interrupts the circuit with their body. The classification of extra low voltage is defined by IEC 60038 and UK IET.This, then, is the first reason for choosing 24 VDC: it provides a safer working environment.Voltage DropIf you have ever taken an electrical theory class in school or watched videos online, you have likely learned about Ohm’s law, which defines the relationship between voltage, current, and resistance. A voltage drop is the amount of voltage lost due to current traveling through a resistance, and a common source of resistance is cable length itself. The longer the cable, the more resistance in the circuit, and so, the higher the voltage drop will be.To reduce the cable resistance, we would need to increase the cross-sectional area of the cable (essentially, use a larger diameter cable), but this is costly and not practical in high-flex situations.An alternative is to increase the voltage. This lowers the current required by the load (proven by Watt’s Law, which explains the relationship of voltage, current, and power) and therefore lowers the voltage drop (proven by Ohm’s Law).Through research, engineers have determined that 24 volts is the perfect voltage to accommodate relatively long cable runs without using large-diameter wires.Noisy EnvironmentsNoisy industrial environments don’t refer to audible noise, but electrical noise.Any equipment that has VFDs, servo drives, or large contactors will generate significant electrical noise or EMI within the electrical system. If your control voltage is small, such as 5 VDC, this electrical noise could cause receiving devices to interpret the noise as a trigger signal.With 24 volts as the control level, it is less likely that the electrical noise will reach an amplitude high enough to mimic a control signal.One might ask, then, “Why not make the control voltage even higher, which would make it even less susceptible to noise and voltage drop?” This is true, but as you approach the unsafe (non-ELV) 120 VDC level, you will start to run the risk of electrical shock.A 24 VDC power supply connected to a relay board. Image used courtesy of UnsplashWhy DC?In an electrical system, there are two types of voltage: alternating current (AC) and direct current (DC).In an AC system, the direction of current switches between positive and negative, while DC current travels in only one direction, from positive to negative (conventionally). It is the alternating frequency of the AC voltage that makes it dangerous in the event someone becomes shocked by the system. The AC voltage is known to cause muscle spasms or interfere with heart rhythms, even at lower levels.Another problem with AC voltage is that most modern control devices (PLCs, VFDs, etc) operate with the use of semiconductor devices, which operate on DC only. If the cabinet supply is AC, then each of these control devices must have an AC-DC converter inside, raising the cost. Instead, the DC conversion is done only once, benefiting all of the semiconductor devices.For safety and design efficiency reasons, DC voltage is chosen as the industry standard for low-voltage control and signalling.When to Use ACAlthough DC may be a standard voltage, that’s not to say there is no place for AC.Large Load DevicesIn some situations where you need to control a large load, 24 VDC would draw too much current, so a different control voltage needs to be used. Some large motor contactors use 120 or even 220 VAC for this reason. This voltage is already available in the motor control cabinet, so it makes sense to use it for these larger controls.Space and Cost SavingAnother reason to use an AC control voltage is when there is simply not enough room in the control cabinet for a power supply. Main power needs to be converted to 24 VDC, and if there is no room physically to fit the power supply, you may need to look into an AC contactor. Also, if there are no control devices that need DC, it might be cheaper to simply omit the power supply and make use of AC relays and solenoids.StandardizationJust like with software or programming languages, standardization is the key to success. The electrical industry has standardized on 24 VDC for control voltage. This means you can purchase a PLC from one company made in China and an I/O block from another company made in Brazil, and both will work with 24 VDC.By standardizing, designers and engineers save time because they do not have to search for different power supplies or ensure they are using the correct voltage.Measuring electrical voltage across contactors in a control cabinet. Image used courtesy of UnsplashVoltage: Not Just an Arbitrary NumberAs you can see, there are many reasons why we use 24 volts for signalling and control of devices. 24 VDC is safe to work around, it is reliable in noisy environments, and it is capable of being used over long distances.While there are situations where you might need to use an AC voltage, the preferred method is most often to use a control voltage such as 24 VDC. As always, when working with electrical components, read the documentation and know what voltage supply is required by your devices.
