Panels Articles
Longer-form technical writing, published as it is finished and revised as field experience corrects it. Every entry carries its published and updated dates.
74 entries
Control Power Supplies
Sizing 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.
8 min readUpdated Sep 5, 2026
Short-Circuit Current Rating (SCCR)
What a control panel’s short-circuit current rating means, how it is determined under UL 508A, why the NEC requires it on the label, and why the number is usually smaller than people expect.
9 min readUpdated Sep 5, 2026
Enclosure Selection
Choosing the box: NEMA and UL 50 type ratings, materials for corrosive and outdoor sites, sizing for heat and wire bending space, and the details at the conduit entries and the door that decide whether the rating survives installation.
8 min readUpdated Sep 5, 2026
Enclosure Heat Calculations
Estimating the temperature inside a control panel from the power its contents dissipate and the surface that carries it out, what to do when the answer is too hot, and why drives change everything.
8 min readUpdated Sep 5, 2026
Component Layout
Arranging a panel so it is safe to work in, cool enough to run, quiet enough for its signals, and possible to modify: power and control separation, heat, wireways, terminals, and the working space rules.
8 min readUpdated Sep 5, 2026
Hand-Off-Auto Selectors
What each position of an HOA selector does, how it is wired into the starter circuit, which protections must survive Hand, what the controller should see, and the mistakes that let a pump run with nobody watching.
7 min readUpdated Sep 5, 2026
Variable Frequency Drives in Pump Panels
What a drive adds to a pump panel and what it costs: speed control, soft start, reduced inrush, and diagnostics against heat, harmonics, cable and motor stress, bypass, and the minimum speed below which a pump does no work.
11 min readUpdated Sep 5, 2026
Soft Starters
Reduced-voltage solid-state starters: how they ramp a motor, what they do for inrush, water hammer, and mechanical stress, where they beat a drive and where they do not, bypass contactors, and the settings that matter on a pump.
9 min readUpdated Sep 5, 2026
Circuit Breakers in Control Panels
Miniature, molded-case, and supplementary protectors: what each is listed to do, how a trip curve is read, where the NEC and UL 508A set the rules, and how breaker choice drives the short-circuit current rating of the whole panel.
10 min readUpdated Sep 5, 2026
Fuses in Control Panels
Fuse classes, time-delay and fast-acting types, current limitation and what it does for the panel short-circuit rating, semiconductor fuses for drives, control circuit and DC fusing, and the habits that make a blown fuse a diagnosis instead of a mystery.
10 min readUpdated Sep 5, 2026
Panel Power Supplies
The 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.
9 min readUpdated Sep 5, 2026
Control Relays
The interposing and logic relays in a control panel: why a controller output drives a relay instead of a starter coil, contact ratings and the difference between resistive and inductive loads, coil suppression, ice cube relays versus terminal relays versus solid-state, and the failure modes that make a relay the first suspect in a dead circuit.
8 min readUpdated Sep 5, 2026
Terminal Blocks
The point where field wiring meets the panel: screw, spring, and push-in terminals, ratings and the UL 1059 groups, fused and disconnect terminals for instrument loops, grounding terminals, multi-level blocks, jumpers and marking, and the layout that makes a loop check or a field change a five-minute job.
8 min readUpdated Sep 5, 2026
UPS for Control Panels
Keeping 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.
8 min readUpdated Sep 5, 2026
Noise Problems
Electrical noise as a panel problem: the four ways it couples, the design features that keep it out, the sources people forget such as unsuppressed coils and unbonded doors, and the sequence for finding where noise gets into a panel that should be clean.
10 min readUpdated Sep 5, 2026
Ground Faults
Ground faults in and around control panels: what happens on grounded 120 VAC circuits, on grounded and ungrounded 24 VDC systems, and on drive outputs; where they hide, from chafed wire to flooded boxes; and the divide-and-conquer method that finds them.
10 min readUpdated Sep 5, 2026
No Control Power
The control power path from the disconnect to the last relay, and how to work along it when the panel is dead: transformer and fuses, control breaker, emergency stop and safety relay, DC supply and distribution, what trips them, and design that makes it easy.
10 min readUpdated Sep 5, 2026
NFPA 70 and the Control Panel
The National Electrical Code inside an industrial control panel: Article 409 and the listing it points to, the short-circuit current rating, overcurrent protection for control circuits, grounding and bonding, working space, and what an inspector asks.
9 min readUpdated Sep 5, 2026
UL 698A
Industrial control panels for hazardous locations: what UL 698A adds to UL 508A, a panel in a classified area or a safe-area panel with intrinsically safe circuits, barriers and isolators, the control drawing, and where utilities meet classified areas.
8 min readUpdated Sep 5, 2026
Wireways
Panel wireway design: fill limits and the space terminations need, separating power, control, and signal wiring, routing around heat and drives, sizing rules, the cases where wireway is the wrong choice, and the habits that keep a panel serviceable.
7 min readUpdated Sep 5, 2026
Terminals
Designing the terminal strips of a control panel: grouping by voltage and function, numbering that matches the drawings, fused and disconnect terminals for instrument loops, shield and ground terminals, spares, ferrules and torque, and the terminal schedule.
7 min readUpdated Sep 5, 2026
Surge Devices
Surge protective devices in a control panel: the types and where each belongs, the ratings that matter, supply protection, signal and network protection on conductors that leave the building, stage coordination, grounding, indication, and replacement.
8 min readUpdated Sep 5, 2026
Network Switches
Selecting and installing the Ethernet switch in a control panel: managed against unmanaged, industrial ratings and DIN rail mounting, ports and media including fiber, ring protocols, power and grounding, and the configuration documented with the panel.
8 min readUpdated Sep 5, 2026
Blown Fuses
Working a blown fuse in a control panel: reading the circuit before replacing anything, fault against overload, finding a short by sectioning, the transformer and power supply cases, fuse types that must match, indicating holders, and replacement rules.
8 min readUpdated Sep 5, 2026
Failed Power Supplies
Diagnosing 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.
8 min readUpdated Sep 5, 2026
Panel PLC Faults
Working a controller fault from the panel side: reading the indicators before opening the software, faults caused by the panel rather than the program, power and grounding, I/O module and wiring faults, heat, loose connections, and the order of checks.
8 min readUpdated Sep 5, 2026
Relay Problems
Diagnosing control relays in a panel: contacts that do not make or that weld, coils that fail or chatter, drop-out on sag, interposing relays on controller outputs, loads that burn contacts, suppression, sockets, and replacing against fixing the cause.
7 min readUpdated Sep 5, 2026
PLC Power
Powering the controller section of a panel: the control transformer and 24 volt supplies, separating controller power from field I/O power, sizing with margin, the UPS and what rides through, protection and monitoring, the DC ground, and restart sequencing.
8 min readUpdated Sep 5, 2026
Panel I/O
Arranging the I/O in a controller panel: module types for the field devices, sinking, sourcing, and isolated inputs, fused and relay outputs, the path from field terminal to module, spare capacity, and addressing that follows the I/O list.
8 min readUpdated Sep 5, 2026
Relays in the PLC Panel
The relays a controller panel needs and why: interposing relays between outputs and loads, input relays for high-voltage and remote signals, isolation and safety relays, monitoring relays, coil and contact selection, and keeping logic out of relays.
7 min readUpdated Sep 5, 2026
Isolation
Isolating the controller from the field: why grounds differ and what that does to signals, signal isolators and isolated channels, isolation transformers and their limits, relays as isolation, fiber as the ultimate isolator, and when isolation is worth it.
8 min readUpdated Sep 5, 2026
Panel Surge Protection
Surge protection of a controller panel as a system: a single entrance and the ground bar it lands on, staged protection on the power, protectors on every loop and data line that leaves the building, the radio feed, layout, and the checklist after a strike.
8 min readUpdated Sep 5, 2026
Panel Networking
The network inside a controller panel: which networks exist and how they are kept apart, where the switch sits, copper patching and fiber entry, the radio or cellular router as a panel component, cable management, shield grounding, and labeling.
7 min readUpdated Sep 5, 2026
Duplex Pump Panels
The duplex control panel: two pumps on one wet well and the sections every duplex panel contains, from power and starters through level control, alternation, hand-off-auto, alarms, and the backup float circuit to telemetry, with the failure responses.
9 min readUpdated Sep 5, 2026
Triplex Pump Panels
The triplex control panel: three pumps on one wet well, the alternation schemes that keep three pumps evenly used, staging levels and standby logic, power, drives and their coordination, the backup circuit for three pumps, and what a duplex never decides.
8 min readUpdated Sep 5, 2026
Alternation
Pump alternation at the panel level: what it is for, the alternator relay against controller logic, schemes from alternate-every-cycle to runtime balancing, manual lead selection, skipping a failed pump, alternation in the backup circuit, and the tests.
7 min readUpdated Sep 5, 2026
Lift Station Backup Control
How a lift station keeps pumping when the transmitter, the PLC, or the SCADA link is gone: float backup logic, the control transfer relay, redundant-off protection, and how to test it with the controller actually dead.
12 min readUpdated Sep 5, 2026
Industrial Firewalls
Firewalls built for control networks: how they differ from office firewalls, where they belong in a utility network, transparent versus routed modes, Modbus and DNP3 awareness, logging, and the ruggedized units that protect one controller or a remote site.
9 min readUpdated Sep 5, 2026
How to Calculate a Panel SCCR
Determine the short-circuit current rating of a control panel by the UL 508A Supplement SB method: list every power circuit component, find each rating, apply published series combinations and current limiting, and mark the panel with the lowest result.
10 min readUpdated Sep 5, 2026
How to Select Surge Protection
Select the surge protective devices for a control panel and its field circuits: list every conductor that enters, classify its exposure, choose the device type and ratings for the power, the loops, and write it into the drawings and the I/O list.
8 min readUpdated Sep 5, 2026
How to Size an Enclosure
Size a control panel enclosure so that everything fits, cools, and can be worked on: lay out the components with their clearances and wireway, add the spare space the plant will need, and mounting before ordering.
8 min readUpdated Sep 5, 2026
How to Build Terminal Schedules
Produce the terminal schedule that lets a technician find any wire: define the strips and their numbering, list every terminal with its wire numbers, the device and the field destination, and keep it current through every change.
7 min readUpdated Sep 5, 2026
How to Design Panel Grounding
Design the grounding and bonding of a control panel and its site so that faults clear, signals are clean, and surges have a path: bond the enclosure and everything in it, and verify with measurements before the panel is energized.
8 min readUpdated Sep 5, 2026
Pump Will Not Start
A pump that is called and does nothing. The order to check: permissives and mode, control power, the starter or drive, the motor circuit, and the pump itself, with the measurements that settle each one in minutes.
10 min readUpdated Sep 5, 2026
Floating Reference Between Panels
Signals between two panels that misbehave because the panels do not share a DC reference: inputs that never turn on or half turn on, strange voltages to ground, and noise that comes and goes. How DC commons should be bonded and how to cross a panel boundary.
9 min readUpdated Sep 5, 2026
Missing Equipment Ground
An enclosure, motor, or device with no effective equipment ground: how it shows up as a tingle, a fault that does not trip, surge protection that does nothing, or instrument noise; how to test the path safely; and why a ground rod is no substitute.
9 min readUpdated Sep 5, 2026
High Neutral-to-Ground Voltage
What a neutral-to-ground voltage reading means, the values that are normal, and the causes of a high one: a long or heavily loaded neutral, harmonic current, a loose or open neutral, a missing service bond, or an extra bond downstream, and telling them apart.
9 min readUpdated Sep 5, 2026
Cable Routing Problems
Noise that comes from where a cable runs: signal cables sharing a conduit, tray, or wireway with power or drive cables, long parallel runs, and shallow crossings. How coupling works, the separations that prevent it, and how to find the shared path.
9 min readUpdated Sep 5, 2026
Radio Frequency Interference
Readings that jump when a handheld radio is keyed, inputs that flicker near a telemetry antenna, transmitters that drift beside a modem: how radio energy gets into control circuits, how to prove it with the radio itself, and the fixes.
9 min readUpdated Sep 5, 2026
Drive Causes Instrument Noise
When many instruments became noisy after a drive was installed, the drive installation is the place to look: the checklist for motor cable, shielding, grounding, filters, and carrier frequency that decides how much noise a drive puts into the plant.
10 min readUpdated Sep 5, 2026
No Control Power in Panel
The panel is dead: no pilot lights, no controller, no HMI. The order of checks from the disconnect through the control transformer and its fuses, the control breaker, the emergency stop chain, and the DC supply, with the readings that locate the break.
8 min readUpdated Sep 5, 2026
Fuse Blows Repeatedly
A control fuse that opens again after replacement has a cause the replacement did not touch: a shorted coil, a ground fault in a wet field device, an overloaded circuit, the wrong fuse type for inrush, or an intermittent short. How the timing identifies it.
8 min readUpdated Sep 5, 2026
Relay Chatter
A relay or contactor that buzzes, cycles, or will not stay pulled in: low coil voltage from an overloaded supply or a sag, an AC coil with a dirty armature or broken shading ring, a solid-state output leaking, or an input bouncing at its threshold.
8 min readUpdated Sep 5, 2026
Ground Fault Present
The ground fault indicator is on, the monitor alarms, or a fuse opens when a field circuit is energized: how to find which conductor touches ground and where, by dividing the circuit with the monitor or an insulation tester, and where faults hide.
8 min readUpdated Sep 5, 2026
Panel Overheating
A panel that runs hot: drives tripping on temperature in the afternoon, controllers faulting in summer, components discolored, a fan that never stops. The causes from an undersized enclosure to a clogged filter, measuring the rise, and the fixes.
8 min readUpdated Sep 5, 2026
Nuisance Breaker Trips
A breaker that trips without a fault anyone can find: inrush against an instantaneous trip, a circuit loaded beyond its rating by additions, the wrong trip curve, a breaker weakened by heat, a loose terminal, or harmonics. Reading the curve.
8 min readUpdated Sep 5, 2026
Loss of Control Voltage
Control voltage that drops out for a moment and comes back, resetting controllers and dropping contactors: utility sags when a large load starts, a transfer switch operation, a loose neutral, or an overloaded control transformer. Catching it with a recorder.
8 min readUpdated Sep 5, 2026
Power Supply Failure
A 24 volt DC supply whose output is gone, low, cycling, or noisy: an overload that has put it into current limit, a supply killed by heat and age, input voltage out of range, or surge damage. One test tells an overloaded supply from a dead one.
8 min readUpdated Sep 5, 2026
UPS Not Carrying Load
The power failed and the controls went down anyway: a battery aged out or never connected, a UPS sitting in bypass since a fault, an overload, loads on the wrong receptacles, or a failed self-test nobody read. Finding out which, and testing properly.
8 min readUpdated Sep 5, 2026
Voltage Sag on Motor Start
Lights dim, contactors drop, and controllers reset every time the big pump starts: the starting current of an across-the-line motor pulling down a weak source. Measuring the sag, checking the source, and fixing it with a soft starter or ride-through.
8 min readUpdated Sep 5, 2026
Factory Acceptance Testing
Testing a control system before it leaves the shop: what must exist before the test, how the procedure is built from the narrative and the matrix, how the field is simulated, what is witnessed, and what the punch list and the sign-off mean.
9 min readUpdated Sep 5, 2026
Control Schematics
The ladder-style drawing of every control circuit in a panel: how a schematic is organized by line number, the conventions for contacts, coils, and cross-references, what belongs on a schematic and what belongs on a wiring diagram, and reading one under pressure.
9 min readUpdated Sep 5, 2026
NFPA Standards for Controls
The National Fire Protection Association documents that reach into a control system: NFPA 70, the National Electrical Code, and the articles that govern panels, motors, and instrumentation; NFPA 70E for electrical safety at work; NFPA 79 for industrial machinery; NFPA 820 for wastewater plant hazardous area classification; and NFPA 110 where standby power meets the controls.
9 min readUpdated Sep 5, 2026
UL Standards for Controls
The UL listings behind the components and the panels: UL 508A for industrial control panels and its SCCR supplement, UL 508 and UL 60947 for the components in them, UL 489 and UL 1077 for breakers and supplementary protectors, UL 248 fuse classes, UL 50 and 50E for enclosures, UL 698A for hazardous-location panels, and what a listing, a recognition, and a mark actually mean.
9 min readUpdated Sep 5, 2026
Wiring Diagrams
The drawing that says where every wire goes: panel wiring and interconnection diagrams, and the conventions of wire numbers, conductor colors, terminal numbers, and cable numbers that make them readable. What belongs here versus on the schematic.
9 min readUpdated Sep 5, 2026
Panel Layouts
The drawing of what goes where inside a control panel: enclosure, back plate with every component to scale and keyed to the bill of material, wireways, terminals, door devices, and entries. The conventions, the required spacings, and the checks.
9 min readUpdated Sep 5, 2026
Cable Schedules
The list of every cable with what it is, where it runs, and where it lands: the columns, the numbering convention, types and signal classes, routes through conduits and trays, lengths, terminations, spares, and status from design through as-built.
8 min readUpdated Sep 5, 2026
The Panel You Can Troubleshoot at Two in the Morning
Panels are designed for the day they are built and used for the nights they fail. The design choices that decide whether a technician with a flashlight and a meter can find the fault: labels and wire numbers, and the fuse that tells you it blew.
8 min readUpdated Sep 5, 2026
Fiber Between Buildings, Every Time
The copper Ethernet cable between two buildings works on the day it is pulled and costs the plant a controller, a switch, and how to specify it so it stays simple.
7 min readUpdated Sep 5, 2026
Heat Kills Panels Slowly
Nothing in a control panel fails the day it gets hot. Power supplies, drives, relays, and the controller lose life a summer at a time, and what to do about it that actually works outdoors.
9 min readUpdated Sep 5, 2026
As-Built Means Someone Walked the Panel
The drawings in the panel pocket are the ones from the bid set, and how a small utility keeps its drawings true after the integrator has gone.
9 min readUpdated Sep 5, 2026
UL 508A Industrial Control Panels
What the listing actually covers, what has to appear on the label, and the requirements that most often cause a panel to fail inspection.
9 min readUpdated Aug 25, 2026
Lead/Lag Pump Control
Which pump runs first, when the second joins, how they trade places, and the design decisions that determine whether a duplex station wears evenly.
9 min readUpdated Aug 18, 2026
How to Size a Control Panel Power Supply
Add up the real load, account for inrush and simultaneity, add headroom, and check the heat you just added to the enclosure.
7 min readUpdated Aug 14, 2026
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Content on CurryControls.com is general technical reference information provided for educational and reference purposes only. It is not engineering advice for a specific installation. All information must be independently verified against current codes, standards, manufacturer documentation, project requirements, and qualified professional judgment before it is relied upon or applied.