Power Distribution and the Control Supply
Main isolators, branch protection, motor circuits, trip curves and coordination, sizing the 24 V supply and ride-through.
From the supply to the load
Power usually enters the panel through a lockable main isolator, then branch protection to each load: drives, motor starters, the 24 V supply, heaters and sockets. Control power is normally 24 V DC from a switched-mode supply. Each device protects the one downstream of it, with the smallest rating that still lets the load run, so that a fault trips the nearest device and leaves the rest alive.
| Device | Protects against | Notes |
|---|---|---|
| Main isolator | Nothing: it isolates | Lockable (lock-out), load-breaking, handle on the door with a defeat for qualified people |
| Miniature circuit breaker (MCB) | Overload and short circuit of a cable | Characteristic B, C or D sets how much inrush it tolerates |
| Motor protection breaker (MPCB) | Overload, phase loss and short circuit of a motor circuit | Adjustable thermal setting; often combined with a contactor |
| Fuses | Short circuit and overload | High breaking capacity; gG for cables, aM for motors, semiconductor fuses for drives |
| Thermal overload relay | Motor overload | Class 10, 20 or 30 sets the time to trip at 7.2 × the set current |
| Residual-current device (RCD) | Earth leakage and shock | Drives leak current: use type B or A+ as the manufacturer says |
| Surge protective device (SPD) | Voltage spikes | At the incoming supply and, where needed, on signal lines |
Main isolator and incoming supply
- Size the incoming supply from the sum of the loads with a diversity factor (not every motor runs at full load at once), plus the largest motor’s starting current, and agree it with the site electrician or the utility.
- Know the supply: voltage and tolerance, frequency, number of phases, earthing system (TN-S, TN-C-S, TT, IT), the prospective fault current and the supply impedance. All of them affect protection, drives and the choice of RCD.
- Phase failure and sequence relays protect pumps and compressors from reverse rotation and single-phasing.
- Provide a main isolator that can be locked off and is the only means of isolation for the whole panel, and label any parts that stay live (a separate supply from another source) with a warning.
- Provide a service socket and a lamp in the panel on their own protected branch (and an RCD), so that maintenance is possible with the machine off.
A motor circuit
I = P ÷ (√3 × V × η × cos φ) (three-phase, P = shaft power)
Worked example: a 5 kW motor at 400 V
- With an efficiency of 0.88 and a power factor of 0.86, the full-load current is 5 000 ÷ (1.732 × 400 × 0.88 × 0.86) = 9.5 A. Use the nameplate value (here 9.5 A) for the overload setting: it is the motor’s real figure.
- Starting current of a direct-on-line motor is about 6 × 9.5 = 57 A for a few seconds.
- The cable and short-circuit device are chosen for 1.25 × the full-load current = 11.9 A or more (the local code gives the factor); a type C breaker of 20 A tolerates the start and, with a 4 mm² cable, protects it. The overload relay is set at the nameplate 9.5 A to protect the motor itself.
- Cable current rating depends on the installation. A 4 mm² cable may be rated about 32 A in free air on a tray, but in a hot duct (derating 0.87) bunched with other circuits (0.80) it carries only 32 × 0.87 × 0.80 = 22.3 A: just above the 20 A breaker, so the breaker still protects the cable. A 2.5 mm² cable would not be enough (see Wiring, Cables and Voltage Drop). (Figures here are typical; use IEC 60364-5-52 or the local code.)
- Type 1 or type 2 coordination: after a short circuit, type 2 lets the contactor and relay be used again, type 1 may need parts replaced. Use the manufacturer’s coordination tables for the combination.
- Use a variable-speed drive with its own protection; the motor cable, the line fuses and the output filter are given in the drive’s manual (see Motion Control and Actuators).
- Emergency stop and isolation must be able to remove power from the motor (see Machine Safety Systems).
Trip curves and coordination
A trip curve shows how long a protective device takes to act at each multiple of current. The aim of coordination is that the motor starts without a trip, a small overload is cleared by the overload relay, and the breaker acts only for faults and large overcurrents.
Worked example: does the motor start, and who trips first?
- The start: 57 A for 3 s. The overload relay would take 14.5 s to trip at that current and the 20 A breaker 34 s: both are longer than 3 s, so the motor starts.
- A 50 % overload (14.3 A, a jammed conveyor): the overload relay trips in about 313 s, while the 20 A breaker does not trip at all (14.3 A is below its rating). The relay trips first and protects the motor; the breaker protects the cable and clears faults.
- At class 10 the relay trips in about 10 s at 7.2 × the set current (a locked rotor).
- A short circuit of several hundred amperes goes past the breaker’s magnetic threshold (5 to 10 × 20 A, 100 to 200 A for type C) and trips it in milliseconds: the relay is not fast enough, which is why the breaker is there.
- The mistake to avoid: a breaker too small for the start (a 16 A breaker trips on the start of this motor) or an overload set too high (the motor burns before the relay acts).
Fault level and short-circuit rating
A short circuit near a strong supply draws thousands of amperes. Every device must be able to break that current and the panel must withstand it (its short-circuit current rating, SCCR in UL 508A, and the rated short-time withstand in IEC 61439). The available fault current falls with the impedance of the supply and the cable to the panel.
Isc = V ÷ (√3 × Z)
Worked example: a 400 kVA transformer
- Transformer rating 400 kVA at 400 V: full-load current = 400 000 ÷ (1.732 × 400) = 577 A. With an impedance of 4 %, the fault current at its terminals is 577 ÷ 0.04 = 14.4 kA.
- A cable run to the panel adds an impedance of 0.012 Ω per phase. The source impedance is 0.016 Ω, so at the panel Isc = 400 ÷ (1.732 × (0.016 + 0.012)) = 8.2 kA.
- If the panel and its branch devices are rated 10 kA, the rating is enough. Put the available fault current and the panel’s rating on the nameplate; it is a requirement of the codes.
Starting methods and voltage dip
| Method | Starting current | Starting torque (of the direct-on-line value) |
|---|---|---|
| Direct on line (DOL) | 6.0 × (57 A) | 100 % |
| Star–delta | 2.0 × (19 A) | 33 % |
| Soft starter | 3.5 × (33 A) | 50 % |
| Variable-speed drive | 1.2 × (11 A) | 100 % |
Worked example: the voltage dip
A direct-on-line start draws 57 A. With a supply impedance of 0.15 Ω the line-to-neutral voltage falls by 57 × 0.15 = 8.6 V out of 231 V: a dip of 3.7 %. A dip of more than about 5 to 10 % flickers lights and can drop out contactors and PLC supplies. A soft starter or a drive cuts the starting current and the dip (see the table), at a cost in torque or money.
The 24 V control supply
Supply rating ≥ (sum of DC loads) × 1.2 to 1.3 then round up to the next standard size
Worked example: sizing a 24 V supply
| Load | Current |
|---|---|
| PLC CPU, communications and I/O card supply | 2.0 A |
| 16 sensors at 0.1 A each | 1.6 A |
| 8 solenoid valves at 0.25 A each (6 W) | 2.0 A |
| HMI panel | 0.8 A |
| Relays and other | 0.5 A |
| Total (worst case, everything on) | 6.9 A |
Add 25 % margin for growth, aging and temperature derating: 6.9 × 1.25 = 8.63 A. The next standard size is a 10 A supply (240 W). It then runs at 69 % of its rating with everything on, and at 86 % with the margin.
- Keep the supply below about 80 % of its rating in normal use, and check the datasheet for derating at high temperature (often above 50 °C).
- Allow for inrush: capacitive loads and motors can exceed the supply’s limit at start. Check its overload behavior (constant-current or hiccup).
- Use a supply with a power-good output and monitor it in the PLC; log its voltage.
- Check input tolerance: many 24 V devices work from about 19 to 29 V; check the lowest voltage at the far end of a long cable (see Wiring, Cables and Voltage Drop).
- Redundancy: two supplies with a redundancy module give a second source; each must carry the full load.
- Use a separate supply for the safety circuit and the noisy loads where practical.
Protecting the 24 V branches
One shorted sensor cable should not take down the whole machine. Divide the load into branches, each with a DC breaker or an electronic fuse that trips fast enough, and group them by function.
Worked example: branches and a short circuit
| Branch | Load | Protection (about 1.25 × load, rounded up) |
|---|---|---|
| PLC and I/O | 3.0 A | 4 A (or the next size) |
| Sensors, group A | 1.0 A | 2 A (or the next size) |
| Sensors, group B | 0.8 A | 1 A (or the next size) |
| Valves | 2.5 A | 4 A (or the next size) |
| HMI and switch | 1.2 A | 2 A (or the next size) |
- The branch loads add to 8.5 A, within the 10 A supply (85 %).
- A short at the end of a 10 m cable of 0.75 mm² has a loop resistance of 0.46 Ω, so the fault current is only about 24 ÷ (0.46 + 0.05) = 47.2 A. A 2 A breaker with a trip at 3 times its rating (6 A) will trip. Long thin cables limit the fault current, so check that the protection works at the far end, or use an electronic fuse that limits and trips on current.
Ride-through and power loss
A mains dip or loss is a normal event. Decide what each part of the system must do: ride through a short dip, shut down in an orderly way, or run on a battery for a while.
Worked example: a small UPS and the hold-up time
- The PLC, switch and HMI draw 120 W. To keep them alive for 10 minutes, the energy is 120 × 600 ÷ 3 600 = 20.0 Wh. With an 80 % inverter efficiency and a 30 % allowance for aging and temperature: 33 Wh of battery.
- A switched-mode supply stores energy in its capacitors and typically holds the output for about 20 ms of lost input (the datasheet states it): 120 W × 20 ms = 2.4 J. That rides through a dip of one or two cycles but not a short outage.
- Decide the behavior after a power return: the PLC’s retentive memory keeps the state; the program must start in a safe, defined condition and never restart a motor by itself without an operator action where that is dangerous (see Machine Safety Systems).
- Test it: cut the power in the middle of a cycle, restore it, and check what happens (see Testing, Commissioning and Acceptance).
Related
- System Integration: the overview of this guide.
- Wiring, Cables and Voltage Drop: cable rating and voltage drop of the circuits.
- Panel Layout, Heat and Enclosures: the heat these devices produce and the enclosure.
- Machine Safety Systems: isolation and emergency stop.
- Motion Control and Actuators: drives and motors.
FAQ
How do I size a 24 V DC supply?
Add the worst-case currents of all loads, multiply by 1.25 for growth, aging and temperature, and round up to the next standard size. Keep it below about 80 % in normal use.
Why does a motor breaker trip when the motor starts?
The starting current (about 6 times the full-load current) is above the breaker’s instantaneous threshold, or the breaker is too small. Use a breaker with a suitable characteristic and rating for motor starting, and set the overload relay at the nameplate current.
What is short-circuit current rating?
The highest fault current the panel and its devices can safely withstand and interrupt. It must be at least the available fault current at the panel, which depends on the transformer and the cable.
Do I need a UPS?
Only if the machine must keep running or shut down in order when the mains fails. A small UPS for the controller and network is common; motors and drives usually stop.
References
- IEC 60204-1 (electrical equipment of machines: supply, protection, isolation); IEC 61439-1 and -2 (low-voltage switchgear assemblies); IEC 60364-5-52 (cable current ratings).
- UL 508A, Industrial control panels (SCCR); NFPA 70 (NEC) and the local electrical code.
- Manufacturers’ coordination tables for motor starters and the datasheets of the circuit breakers, relays and power supplies.
Last reviewed: October 2026. Currents, ratings and fault levels are illustrative: use the nameplates, the datasheets and the code of the country where the machine will run.