On circuits above all exposed metal parts is a fundamental safety and reliability principle that appears in everything from consumer electronics enclosures to industrial control panels. By ensuring that no conductive trace, component lead, or solder joint sits directly on or too close to bare metal surfaces, designers minimize the risk of short circuits, inadvertent grounding, and electromagnetic interference (EMI). This article explains why the rule matters, how to apply it during the design and assembly phases, and what standards and tests verify compliance And it works..
Why Keeping Circuits Above Exposed Metal Parts Matters
Exposed metal parts—such as chassis walls, mounting brackets, heat sinks, or shield cans—are often at ground potential or connected to other circuitry. When a circuit trace or component lead inadvertently contacts these surfaces, several failure modes can arise:
- Short circuits – A direct connection between a power node and ground can cause excessive current, blowing fuses, damaging components, or creating a fire hazard.
- Unexpected grounding – Signals that should float or reference a different node may be pulled to ground, corrupting data or causing logic errors.
- EMI coupling – Exposed metal can act as an unintentional antenna, radiating noise or picking up interference that degrades performance.
- Safety hazards – In equipment accessible to users, accidental contact with live circuitry through a metal enclosure can lead to electric shock.
Designers therefore treat the clearance between any conductive element of a circuit and any exposed metal as a critical design parameter, not merely a suggestion.
Design Considerations: Clearance and Creepage
Two related concepts govern the spacing needed to prevent electrical breakdown: clearance (the shortest distance through air) and creepage (the shortest distance along the surface of an insulating material). Both must be sized according to the working voltage, pollution degree, and material group of the insulating substrate The details matter here..
Determining Required Distances
- Identify the highest voltage that may appear between the circuit node and the exposed metal (e.g., mains AC 230 V RMS → ~325 V peak).
- Select the pollution degree – typically 2 for indoor office equipment, 3 for industrial environments.
- Choose the material group – based on the comparative tracking index (CTI) of the PCB laminate or conformal coating (Group I: CTI ≥ 600, Group II: 400 ≤ CTI < 600, etc.).
- Consult tables from standards such as IEC 60664‑1 or UL 840 to derive the minimum clearance and creepage values.
Here's one way to look at it: a 230 V AC circuit in a pollution degree 2 environment with FR‑4 (CTI ≈ 175, Group III) typically requires a clearance of ≈ 3 mm and a creepage of ≈ 4 mm. If the design uses a conformal coating with higher CTI, the required creepage can be reduced.
Practical Layout Tips
- Route high‑voltage traces away from board edges where metal chassis or shields are closest.
- Use slots or cutouts in the PCB to increase creepage distance when space is limited.
- Place components with exposed leads (e.g., TO‑220 packages) on standoffs that lift them above the metal surface.
- Apply insulating barriers such as Kapton tape, mica washers, or plastic spacers where a component must be near metal.
Materials and Enclosures: Choosing the Right Barriers
Even with adequate spacing, the choice of enclosure material influences how strictly the “circuits above all exposed metal parts” rule must be enforced Small thing, real impact..
Metallic Enclosures
- Advantages: Excellent shielding, mechanical strength, and heat dissipation.
- Challenges: Conductive surfaces demand strict clearance; any burr, sharp edge, or unfinished surface can reduce effective distance.
- Mitigation: Deburr all edges, apply a non‑conductive coating (e.g., powder paint, anodizing) on interior surfaces that face the PCB, and use insulating gaskets or foam tape where the board contacts the chassis.
Non‑Metallic Enclosures
- Materials: ABS, polycarbonate, fiberglass‑reinforced polyester.
- Benefits: Intrinsically insulating, reducing reliance on spacing alone.
- Caveats: Some plastics can develop surface conductivity when contaminated with dust or moisture; periodic cleaning and conformal coating help maintain insulation.
Hybrid Designs
Many products combine a metal shield for EMI with a plastic outer shell for user safety. So in such cases, the metal shield is treated as an “exposed metal part” from the PCB’s perspective, and the same clearance rules apply. The outer plastic layer then provides a secondary barrier against user contact.
Grounding and Shielding Practices
Grounding is often confused with the need to keep circuits away from metal. Proper grounding does not eliminate the need for clearance; rather, it works together with spacing to control potentials.
- Star grounding – Connect all grounds to a single point near the power entry to avoid ground loops that could lift local metal sections to unexpected potentials.
- Shield cans – When a shield is soldered directly to the PCB, make sure the solder mask defines a clear keep‑out zone around the shield’s edges. Use a solder‑mask defined (SMD) keep‑out of at least the calculated clearance.
- Ferrite beads and common‑mode chokes – Place these components on the side of the board opposite the metal enclosure to avoid creating a conductive bridge.
- Conformal coating – A thin UV‑cured or silicone coating can add dielectric strength, allowing slightly smaller clearances while still meeting safety margins.
Testing and Verification
Design rules are only as good as the validation that backs them up. Several tests confirm that a product maintains adequate separation between circuits and exposed metal.
