DC

What is DC?

DC (direct current) is electrical current that always flows in the same direction, because the voltage between the positive and negative poles never reverses polarity. Whereas AC (alternating current) changes direction 50 times per second in Europe, a DC source such as a battery, solar panel or switched-mode power supply delivers a fixed polarity. In OT environments DC is everywhere: virtually every PLC, sensor, actuator and IO module runs on 24 V DC, and UPS systems, variable frequency drives and battery storage all work internally with a DC bus.

🧠 What is the difference between DC and AC?

The power grid uses alternating current because transformers can easily step AC up or down to another voltage. That gave AC the upper hand in the late nineteenth century in the so-called war of the currents between Edison (DC) and Westinghouse/Tesla (AC). Electronics, batteries and solar panels, however, are naturally DC, so every installation contains a conversion from AC to DC.

An ideal DC voltage is constant over time. In practice a power supply adds a small ripple: residual AC from rectification or switching. The lower the ripple, the less it disturbs sensitive analogue inputs and communication.

🔋 Which DC voltages do you find in industry?

Why 24 V? It sits well within extra-low voltage (ELV): IEC 61140 sets that limit at 120 V ripple-free DC, and with SELV/PELV in dry conditions no protection against direct contact is generally needed up to 60 V DC. At the same time 24 V is high enough to drive relay coils and solenoid valves and to absorb the voltage drop across tens of metres of cable. The PLC standard IEC 61131-2 is built around 24 V DC with a permissible range of about 20.4 to 28.8 V (-15 % / +20 %), a window manufacturers state in their datasheets.

🔧 How is a 24 V DC supply in a control cabinet built?

A typical control cabinet converts 230 V or 400 V AC into a 24 V DC bus:

230/400 V AC → switched-mode power supply → (redundancy module / buffer / DC UPS) → electronic circuit breakers → 24 V DC bus → PLC / IO / sensors

• Switched-mode power supply - rectifies the mains voltage, converts it at high frequency and regulates the output to 24 V; efficiencies above 90 % are common.
• Redundancy - two supplies connected through a redundancy or diode module, so the failure of one supply does not stop the installation.
• Buffer or DC UPS - capacitor modules bridge short mains interruptions of hundreds of milliseconds; a DC UPS with a battery keeps the control system running for minutes to hours, so a PLC can shut down in a controlled way.
• Electronic circuit breakers - a switched-mode supply limits its current during a short circuit, so an ordinary fuse often does not blow. Electronic branch protection selectively disconnects only the faulty circuit.
• Earthing the 0 V - the 0 V is usually earthed at a single point (PELV), so an earth fault is detected rather than causing unintended switching.

IEC 60204-1, the standard for the electrical equipment of machines, recommends blue for DC control circuits, black for power circuits and light blue only for the neutral. That makes a 24 V circuit in the cabinet instantly recognisable.

📐 How do you calculate power and voltage drop in DC?

With direct current the relationship is simply P = V × I: there is no phase shift and therefore no reactive power. A 24 V, 10 A power supply delivers at most 240 W.

The biggest practical pitfall is voltage drop. A worked example: a 2 A solenoid valve 50 m away, wired with 1.5 mm² copper (resistivity about 0.0175 Ω·mm²/m). The total cable length out and back is 100 m, so R = 0.0175 × 100 / 1.5 ≈ 1.17 Ω. The voltage drop is 2 A × 1.17 Ω ≈ 2.3 V. With a 24.0 V supply only 21.7 V reaches the valve, close to the 20.4 V lower limit. That is why installers often trim the supply up to 24.5-25 V, choose a larger cross-section or place a local power supply in the field.

After voltage drop, EMC interference causes most problems in 24 V circuits: switched-mode supplies, PWM outputs of drives, ground loops and long parallel cable runs produce noise and voltage dips. The result is a PLC that restarts, jumping analogue readings or communication errors. Separate cable trays for power and signal, shielded cables, a single central earthing point for the 0 V and EMC filters at the supply input solve most of it.

🔄 Where is DC used in power electronics and motors?

Much equipment connected to the AC grid works internally with direct current:

• Variable Frequency Drive - rectifies the mains into a DC link (about 560 V DC on a 400 V AC supply) and uses an inverter to create an AC voltage with variable frequency for the motor. Several drives can share a common DC bus, so braking energy from one axis feeds another.
• Servo systems - work the same way; in robots and packaging machines the DC bus is often shared between axes.
• DC motors - brushed DC motors offer high starting torque and simple speed control, but need maintenance because of brush wear. Brushless DC motors (BLDC) and AC motors with drives have largely replaced them.
• UPS - an online UPS converts AC to DC, uses it to charge the battery and converts it back to AC.
• HVDC - for long subsea cables DC is more efficient, because there is no capacitive charging current in the cable. The first commercial HVDC link, to Gotland in Sweden, entered service in 1954. The Netherlands is connected by NorNed to Norway (2008, 700 MW, ±450 kV), BritNed to the United Kingdom (2011, 1,000 MW) and COBRAcable to Denmark (2019, 700 MW, ±320 kV).

🔥 What are the safety risks of direct current?

The biggest difference from AC is the arc. Alternating current crosses zero 100 times per second, so an arc drawn when a contact opens extinguishes by itself. Direct current does not: a DC arc keeps burning as long as the voltage is high enough. Switches and fuses therefore need an explicit DC rating (for example under IEC 60947); an AC switch on a 1,000 V PV string cannot interrupt the arc.

In the Netherlands NEN 1010 governs installation design and NEN 3140 safe working, and IEC 60204-1 applies to machinery. The Low Voltage Directive (2014/35/EU) covers equipment between 75 and 1,500 V DC, so not ordinary 24 V components, which do fall under the EMC Directive among others. Lithium-ion energy storage is also subject to the Dutch PGS 37 fire safety guidelines.

🏭 How is DC changing the factory and the energy system?

DC is gaining ground beyond the control cabinet. Solar panels, batteries and fast chargers (EV charging infrastructure) are natively DC, and every conversion between AC and DC costs a few percent of energy. In Germany the DC-INDUSTRIE2 project (2019-2023, 39 partners, six pilot plants), building on the earlier DC-INDUSTRIE project, investigated a DC factory grid: a single central active infeed converter supplies a DC bus with a nominal 650 V, to which drives, PV and storage connect directly. Braking energy from drives stays in the system. Since November 2022 the work has continued in ZVEI’s Open Direct Current Alliance (ODCA) and is documented in VDE SPEC 90037. Partners report energy savings of up to 10 % and much less copper in the cabling.

For Dutch companies grid congestion also matters: combining solar power and batteries on a local DC bus reduces peak demand from the grid. In the transmission grid, TenneT connects new offshore wind farms with 2 GW HVDC links at 525 kV.

🛡️ Why do DC systems matter for OT security and monitoring?

Modern power supplies, DC UPSs, battery management systems and inverters have a network connection for diagnostics and monitoring. That makes them part of the attack surface: whoever can switch off a UPS or manipulate a battery system affects the availability of the entire control system. Vulnerabilities in solar inverters show this is not theoretical.

• Zones and conduits - include power and energy management in the zoning under IEC 62443 instead of placing it on the office network.
• Defense in Depth and Zero Trust - restrict management access to UPS and battery systems to known engineering workstations.
• Monitoring - measure DC voltage, current, ripple, battery state and temperature and send them to SCADA and the historian. Slowly rising ripple often points to ageing electrolytic capacitors in a power supply: a classic signal for condition monitoring and predictive maintenance, possibly supported by Industrial AI.

❓ Frequently asked questions

What is the difference between DC and AC?

With DC (direct current) the current always flows in the same direction, while with AC (alternating current) the direction reverses periodically, 50 times per second in Europe. AC is easy to transform to other voltages and therefore dominates the power grid. DC is the natural form for electronics, batteries and solar panels.

Why does industrial automation run on 24 V DC?

24 V DC lies well within extra-low voltage, so touching it is safe under normal conditions, yet it is strong enough for relays, valves and cable runs of tens of metres. IEC 61131-2 and PLC manufacturers’ datasheets assume a permissible range of roughly 20.4 to 28.8 V. As a result PLCs, sensors and actuators from different brands are interchangeable.

Is 24 V DC dangerous?

A 24 V DC voltage is not dangerous to people on contact in dry conditions. The risk lies in the current: a 24 V supply or battery can deliver tens to hundreds of amperes in a short circuit and overheat cables. Proper fuses or electronic circuit breakers therefore remain necessary.

Why is a DC arc more dangerous than an AC arc?

Alternating current crosses zero 100 times per second, so an arc drawn when a contact opens extinguishes by itself. Direct current has no zero crossing, so a DC arc keeps burning as long as the voltage is high enough. Switches and fuses in DC installations such as PV strings must therefore carry a DC rating.

What is a DC bus?

A DC bus is a shared DC voltage rail from which several devices draw or to which they return energy. In a variable frequency drive it is the DC link between rectifier and inverter, in a control cabinet it is the 24 V rail. In DC factory grids such as those of the ODCA, a 650 V bus supplies drives, PV and storage at the same time.

Why is HVDC used for subsea cables?

A long AC subsea cable behaves like a large capacitor, so much of the current is used up as charging current. HVDC does not have this problem and therefore loses less energy over hundreds of kilometres. The Netherlands is linked by HVDC to Norway, the United Kingdom and Denmark, and offshore wind farms are brought ashore with HVDC.

📌 In summary

DC is the form of current on which virtually all control systems, electronics and energy storage run. In industry 24 V DC is the standard for PLCs, sensors and actuators, supplied from the AC grid through switched-mode power supplies with redundancy, buffering and electronic circuit breakers. Direct current requires attention to voltage drop, arcs and battery safety, and DC components with a network connection belong in your OT security approach.

DC is the invisible backbone of every OT installation: designing, monitoring and securing the 24 V supply properly prevents a large share of faults and downtime.