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It may be, but voltage alone cannot establish that a PCB is safe. A credible assessment needs the product’s applicable safety standard, calculated mains insulation requirements, verified component ratings, enclosure and earthing details, fault analysis, and test results. Without the schematic, layout, bill of materials, intended load and finished enclosure, no responsible reviewer can certify a particular board. If those fundamentals are undocumented, treat it as unverified—not ready for deployment.
What “220 V AC” means for a PCB
“220 V AC” usually describes nominal mains voltage, not the full electrical stress a board must withstand. A 220–240 V RMS sine wave reaches roughly 311–339 V peak. A bridge rectifier and smoothing capacitor can therefore create a DC bus near that peak, while switching events and mains surges can impose higher transient stress. Frequency, supply tolerance, earthing arrangement, installation location and jurisdiction also matter.
That is why there is no universally safe PCB spacing number for “220 V.” UL Solutions’ guidance on IEC 62368-1:2023 explains that mains clearance depends on transient voltage and overvoltage category, not just nominal RMS voltage. The applicable insulation type, pollution degree, material group, altitude, coating and construction also affect the assessment.
Identify the safety standard and boundary first
The applicable product standard depends on what the finished equipment does. Examples include IEC/UL 62368-1 for many audio/video, IT and communications products; IEC/UL 61010 for measurement, control and laboratory equipment; IEC/UL 60335 for household appliances; IEC/UL 60598 for lighting; and IEC/UL 60601-1 for medical equipment. Industrial controls and power supplies may fall under other product-specific requirements. Confirm the standard and edition for the target market before fixing layout rules.
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Define the hazardous mains domain, any isolated secondary, protective earth, accessible conductive parts and user-accessible connections on both the schematic and PCB. Determine whether the barrier needs basic or reinforced insulation under the relevant standard. A 5 V or 12 V label does not make a circuit safe to touch: the question is whether a fault can put hazardous voltage on a connector, shield, sensor, programming port or accessible metal.
IPC-2221 is IPC’s generic printed-board design standard, and IPC-2152 addresses current-carrying capacity. These are useful PCB design references, but they do not replace the end-product safety standard that defines the insulation system and required testing.
Clearance and creepage are different checks
| Check | What it measures | Common ways a design can fail |
|---|---|---|
| Clearance | The shortest path through air between conductive parts. | Component leads, mounting hardware, protruding solder or a nearby enclosure part shortens the air path. |
| Creepage | The shortest path along an insulating surface between conductive parts. | Pads, contamination, moisture, a component body or an ineffective slot leaves too short a surface path. |
| Insulation system | Whether the construction provides the required basic or reinforced protection under the product standard. | A visible gap is treated as proof despite an unverified component, material, barrier or test method. |
A board may have adequate clearance but inadequate creepage, or vice versa. UL’s cited IEC 62368-1 guidance says creepage cannot be less than the required clearance in the situation it addresses; it also describes separate requirements for PCB inner-layer insulation, including electric-strength testing. Do not infer from that guidance a single spacing value for every board or standard.
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Do not count soldermask as a safety barrier unless the applicable standard accepts the specific material and construction. Slots can lengthen a surface path, but they do not automatically solve air clearance, component-lead, contamination or mechanical problems. Dust, flux residue, humidity, condensation, mounting screws, washers and enclosure surfaces can all change the effective path.
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Inspect the isolation barrier across the whole assembly
Mark the barrier on the schematic and layout, then trace every possible bridge across it. Review copper pours and planes, vias, unused pads, through-hole leads, test points, connectors, transformer and optocoupler pins, relay footprints, mounting holes, screws, shields and heatsinks. Mains routed under a low-voltage component, or a metal screw placed near a mains trace, can defeat an otherwise clear-looking split.
Check every connection to USB, Ethernet, UART, sensors, antennas and programming equipment. Verify the transformer, AC/DC module, optocoupler or digital isolator’s insulation ratings and conditions of use, then confirm that the installed footprint preserves the required spacing. A component’s datasheet spacing does not guarantee that the PCB around it does.
Audit the mains components and their failure behavior
Fuse and fuse holder
Verify the fuse’s voltage rating, interrupt rating, time-current behavior, ambient derating and inrush tolerance. Confirm its location protects the downstream parts that require protection, and check the holder’s ratings and construction. A fuse chosen only because its current rating matches the normal load is not a complete selection: fault energy, conductor protection and start-up current matter too. A fuse primarily addresses overcurrent and fire risk; it does not replace isolation or protection against electric shock.
Check whether the fuse is in the line conductor and whether a single-pole fuse can leave hazardous parts energized after a fault. Do not confuse supplementary protection inside equipment with branch-circuit protection. UL’s IEC 62368-1 guidance discusses IEC 60127 and UL/CSA fuse requirements in context.
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X and Y capacitors
Class X capacitors are generally used across line and neutral; Class Y capacitors are used where a failure could create an electric-shock hazard, such as across an isolation barrier or from line to accessible earth or chassis. Select the safety class and approval appropriate to the circuit location. A capacitor’s capacitance and ordinary voltage rating do not make it an X or Y safety capacitor. UL describes these application distinctions in its engineering guidance.
Surge protection, switching and power conversion
For an MOV or other surge device, verify continuous mains rating, surge capability, coordination with the fuse and behavior if the device fails short. Consider whether thermal protection is needed and whether the enclosure can contain heat, smoke or fragmentation. A surge suppressor does not substitute for spacing, enclosure protection or a suitable fuse.
For a relay, check contact voltage and current against the real load—resistive, motor, capacitive, LED-driver or transformer—not just a headline rating. Verify inrush handling, contact-to-coil insulation, dielectric withstand, creepage, clearance, switching duty and expected life. Contacts can arc, weld or wear. Solid-state switches avoid mechanical wear but require attention to leakage, heat, dv/dt, snubbers and short-circuit failure behavior; their use does not remove the need for verified isolation of control circuitry.
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For terminal blocks and connectors, verify voltage and current ratings, wire range, torque, finger protection, flammability, retention and spacing after the intended wiring is installed. For transformers, optocouplers, digital isolators and AC/DC modules, check the safety approval and insulation conditions—not only the nominal voltage rating. A recognized or pre-certified AC/DC module can reduce design work, but it does not certify the finished product’s fuse, layout, thermal performance, enclosure or installation.
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- Input and output are highly isolated.
- Four corners have a fixed mounting hole to prevent movement 1 vibration lead to productpower, fixed hole straight by 3.2mm.
Check earthing, enclosure access and fire risk
For Class I equipment, inspect the protective-earth terminal, conductor and bonding of accessible conductive parts. Bonding should be mechanically secure and corrosion-resistant; do not rely on a fragile PCB trace as the sole protective-earth connection. Check earth continuity, and ensure a fuse or switch cannot improperly interrupt protective earth. EMI filters and capacitors to earth also need assessment for touch current.
Class II or double-insulated equipment is not simply equipment with earth omitted. Its insulation system and enclosure must provide the required protection. In either approach, assess whether a finger, probe or tool can reach mains through openings, ventilation slots or service access. Review spacers, brackets, wire routing, loose-wire behavior, mounting orientation, discharge provisions, labels and installation instructions. A high IP or NEMA rating alone does not replace the relevant pollution-degree assessment; UL makes that qualification in its IEC 62368-1 guidance.
Electrical safety includes preventing fire. Assess heat from rectifiers, switches, resistors, relays, transformers, connectors, fuse holders and MOVs at maximum load and worst-case ambient temperature. Consider blocked ventilation, component tolerances, aging, thermal deformation and nearby insulation. Analyze abnormal operation and faults such as a shorted output, stalled motor, failed fan, shorted semiconductor or failed surge device. Normal operation at room temperature does not establish safe fault behavior.
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- Review the design: Select the applicable product standard and check the schematic, layout, insulation barrier, BOM and component approval documentation against it.
- Inspect the unpowered board: Look for solder bridges, whiskers, burrs, contamination, damaged insulation, exposed mains test points and assembly defects. Check continuity and isolation with power disconnected.
- Set up controlled energization: Use an appropriate current-limited, isolated development arrangement and properly rated instruments and procedures. Isolation does not make exposed mains safe to touch.
- Characterize operation: Measure inrush and steady-state current, then check temperatures at maximum intended load and relevant ambient conditions.
- Test protective measures: Where applicable, test protective-earth continuity, insulation resistance, leakage current and dielectric withstand using procedures and equipment suitable for the product standard.
- Exercise fault conditions: Evaluate abnormal operation and relevant single-component faults, then repeat appropriate checks with the finished enclosure, wiring and mounting arrangement.
- Obtain qualified review: Have a safety engineer or certification laboratory review the product before workplace use, sale or other deployment requiring approval.
A multimeter check or successful power-on cannot establish dielectric strength, leakage-current limits, fire behavior, endurance, abnormal-condition safety or production consistency. Do not apply improvised high-voltage tests: dielectric-withstand testing can itself injure people or damage equipment if performed without the correct procedure and safeguards.
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Separate design review, testing and certification
| Stage | What it can establish | What it does not establish by itself |
|---|---|---|
| Engineering review | Can identify design errors in the schematic, layout, components and intended installation. | It is not a laboratory test or product certification. |
| Prototype safety testing | Provides evidence about insulation, leakage, temperature and fault behavior for the tested build. | It does not automatically establish production consistency or authorize a certification mark. |
| Certification and listing | A recognized laboratory evaluates the complete product against applicable requirements and, as relevant, production controls. | Approval applies only within its scope, conditions and instructions. |
In the United States, OSHA’s NRTL program recognizes private-sector laboratories to test and certify certain products to appropriate safety standards. OSHA says that many types of electrical equipment may require NRTL approval in specified workplace circumstances; consult its products requiring approval information for the relevant category and setting. Listed or labeled equipment must be installed and used according to its listing instructions, as OSHA states in this interpretation.
A component approval does not certify the assembled product. Nor is CE an OSHA NRTL mark where an NRTL approval is required; OSHA addresses this distinction in its safety bulletin. Requirements differ by country, product category and use, so identify the target market rather than assuming one mark or standard settles every obligation.
Go/no-go checklist for a 220–240 V PCB
- Do not energize: The applicable standard is unknown; the mains boundary is unclear; accessible low-voltage connections lack verified isolation; safety capacitors or other required components are ordinary parts; soldermask is the only claimed barrier; exposed mains points or unsafe enclosure openings remain; or fault and abnormal-operation behavior have not been considered.
- Controlled laboratory testing only: The architecture and component evidence are documented, but insulation, leakage, thermal, earth-continuity or fault testing is incomplete. Restrict work to people trained for mains hazards, using suitable equipment and safeguards.
- Ready for formal safety review: The standard, supply conditions, insulation type, spacing calculations, component ratings, earthing or double-insulation strategy, enclosure and thermal/fault analyses are documented, and appropriate prototype tests have been completed.
- Deployment requires market-specific approval: Confirm applicable certification, listing, installation and production requirements for the product and intended market. Do not equate a successful prototype test or approved component with approval of the finished equipment.
If placing mains on the custom board is not essential, a certified external AC/DC adapter or appropriately selected DIN-rail supply can keep hazardous voltage off the logic PCB. That reduces the custom board’s mains-design burden, but the supply’s installation conditions, enclosure, wiring and accessible output still need to be addressed.
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