The short answer
PLC Power
The controller section of a panel is powered in layers: a control transformer or a dedicated circuit provides 120 volts for the controller rack supply, the 24 volt supplies, and the 120 volt loads; one 24 volt supply powers the controller and its communications; a separate 24 volt supply powers the field I/O, so that a shorted field device cannot drop the controller; and a UPS carries the controller, the network, and the radio through sags and short outages while the outputs do whatever the process design says they should do without power. Each supply is sized for its measured load plus margin at the panel temperature, protected by its own fuse or breaker, monitored by a contact wired to an alarm, and its DC common is grounded at one point. The design decides in advance what restarts when power returns and in what order, because a panel that restarts every pump at once when the generator picks up is a panel that trips the generator.
Key points
- Separate the controller supply from the field I/O supply; a field short must not reboot the controller.
- Size each supply for the measured load plus a third, derated for the panel temperature.
- Put the controller, the switch, and the radio on the UPS; decide deliberately what else rides through.
- Fuse or breaker each supply and each output group; monitor every supply with a contact wired to an alarm.
- Ground the 24 volt common at one point and document the restart behavior of every output.
The layers
| Layer | Feeds | Notes |
|---|---|---|
| Panel disconnect and main protection | Everything | Rated for the panel full-load current and the available fault current |
| Control transformer or dedicated 120 volt circuit | Rack supply, 24 volt supplies, 120 volt loads | Secondary fused; sized for the sum of the connected loads plus inrush |
| UPS | Controller, network, radio, and selected 24 volt loads | Runtime sized for the outage the site must ride through; bypass for maintenance |
| 24 volt supply A | Controller rack, communication modules, touchscreen | Isolated from the field; monitored |
| 24 volt supply B | Field inputs, outputs, and instruments | Fused by group; monitored; sized for the field load |
| Loop supplies | Analog transmitters where a separate loop supply is used | Or from supply B through fused terminals |
| Output power | 120 volt or 24 volt to relays and solenoids | Fused by group; on or off the UPS by design |
Separation
A field cable shorted by a backhoe, a solenoid with a failed coil, or a wet junction box will drop the supply it is on into current limit. If that supply is also the controller supply, the controller reboots, every output drops, and the process is in its fail state until someone finds the short. With the field on its own supply, the field fuse opens or the field supply limits, the controller stays up, reports the fault, and keeps the rest of the process running. The separation costs one supply and repays it on the first field fault.
Sizing
I_supply ≥ (Σ I_loads × 1.33) ÷ D_temp
- Σ I_loads = sum of the measured or nameplate currents of every device on that supply
- 1.33 = margin for growth and inrush
- D_temp = derating factor at the panel maximum temperature from the supply data sheet
The load is the real load: the controller rack, each communication module, the touchscreen, the switch, the radio, each input and output at its worst case, each transmitter at 22 milliamps, each relay coil. It is summed at design and re-summed whenever a device is added, and the number is written on the drawing beside the supply. A supply at 90 percent of its rating in a hot panel is a supply that will fail in a few summers.
The UPS
- On the UPS: the controller, the communication modules, the switch, the radio or modem, and the touchscreen, so that the site stays visible and reports the outage.
- Off the UPS by default: motor starters, most solenoids, and anything the process should not run without power; on the UPS by design: a valve that must close, an alarm dialer, a critical sample pump.
- Runtime for the longest outage the site must ride through before the generator starts or the site can be visited, plus margin.
- A maintenance bypass so the UPS can be replaced without a shutdown, and a battery replacement schedule.
- A power fail input to the controller from the UPS or a separate relay, so SCADA knows the site is on battery.
Protection and monitoring
- Per supply
- Input protection sized for the supply inrush; output protection at the supply or by fused terminals on each branch.
- Per output group
- Fused terminals or a small breaker per group of outputs, so a shorted solenoid drops a group.
- Per loop
- A fused terminal on each analog loop.
- Monitoring
- Every supply has a contact wired to a controller input; the UPS has its status contacts wired; all alarmed.
- Indication
- Indicating fuse holders and supply status lights so a technician sees the failed item at the door.
Grounding the DC
The 24 volt negative is bonded to the panel ground at one point, at the supply, so that the common is at ground potential and a positive-to-ground fault is a short that opens a fuse rather than a floating system that reads strange voltages. Two supplies feeding separate circuits each have their negative grounded at their own single point; supplies that are combined have one. A floating 24 volt system is used only when the design calls for it, with a ground fault monitor. The instrument ground bar and the shield bar tie to the same ground reference.
Restart
When power returns, the controller boots in a few seconds, the outputs are off until the first scan, and then the logic decides. The design states, for every output, what it does on restart: pumps return to automatic and start by level with a staggered delay; a chemical feed waits for flow; a valve holds until commanded. The stagger keeps the generator from seeing every motor at once, and the documented behavior keeps the operators from being surprised.
Frequently asked questions
- Can the controller and the I/O share one supply on a small panel?
- They can, and many small panels do; the risk is a field short rebooting the controller. A second small supply costs little, and on any panel where the controller must stay up through a field fault, it is worth it.
- How much UPS runtime?
- Enough to bridge the generator start, or the time to visit a site with no generator, plus margin for the batteries aging. Thirty minutes is common for a plant panel; several hours for a remote site that must keep reporting.
- Should the outputs be on the UPS?
- Only the ones the process needs during an outage, decided one by one. A UPS that runs the pumps is a much larger UPS, and the generator is the right answer for that.
- Why ground the 24 volt common?
- So that faults are detectable and the system is at a known potential. A floating supply lets a single ground fault go unnoticed until a second one shorts the supply, and it lets noise ride the common. One bond, at the supply, on the drawing.
Related topics
- Panel Power SuppliesThe 24 V DC supply behind everything in a modern panel: sizing with real inrush and duty, single versus redundant with diode modules, how a switch-mode supply behaves under a short, distribution and fusing per branch, monitoring, and the failure modes that take out a whole panel at once.
- UPS for Control PanelsKeeping the controller, the radio, and the SCADA server alive through an outage: AC and DC UPS types, what to put on the UPS and what not to, sizing for runtime and inrush, the battery as a maintenance item, the alarms a UPS must provide, and what happens when the UPS is the thing that fails.
- How to Size a Control Panel Power SupplyAdd up the real load, account for inrush and simultaneity, add headroom, and check the heat you just added to the enclosure.
- Failed Power SuppliesDiagnosing the 24 volt supply in a control panel: dead, low, noisy, or cycling, a failed supply against an overloaded one, loads that grew, heat and capacitor aging, redundant supplies, sag on the input, and what to specify so the next failure is an alarm.
- Control Power SuppliesSizing the 24 VDC supplies that run a controller and its field devices, separating loads that should not share a supply, and keeping the controller alive through a power event.
- Lift Station Generator OperationStandby power at a lift station: what the transfer switch does, how the controls ride through the transfer, staggered pump restarts, what SCADA should see, load testing, and the portable generator connection.
Direct contact
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