Electrical Clearance, Creepage Distance, and Insulation Levels

Electrical clearance, creepage distance, and insulation level are fundamental to safety-compliant PCB and power-supply design. They determine how far conductors must be separated through air and across surfaces, how thick insulation must be, and which insulation class applies at a given circuit location. This article explains the definitions, the practical spacing values used for primary and secondary circuits, the conditions for thin insulation, wiring and material requirements, and the relationship between operational, basic, supplementary, double, and reinforced insulation.

Highlights:

Clearance is measured through air, while creepage is measured along an insulating surface; both depend on working voltage and insulation class.
Typical primary-to-secondary spacing is larger than secondary-to-ground spacing because the insulation barrier must withstand the full potential difference.
Insulation-level definitions connect directly to accessible parts, grounded conductors, SELV, ELV, and TNV circuits, so spacing decisions must be made in the context of the whole equipment.

Distance and Related Safety Requirements

Safety distance evaluation starts with two geometric definitions and then applies voltage, insulation class, and mechanical worst-case conditions. The requirement is not only to meet a nominal spacing on a drawing, but to preserve that spacing after assembly, force, and processing.

Clearance

Clearance is the shortest distance measured through air between two adjacent conductors, or between a conductor and the adjacent enclosure surface. Creepage distance is the shortest distance measured along the insulating surface between two adjacent conductors, or between a conductor and the adjacent enclosure surface. The two values are often considered together, but they are measured differently and are checked separately.

PCB clearance

PCB clearance is determined from the measured working voltage and the insulation class.

In typical designs, the following primary-side and secondary-side values are used:

Primary AC section: before the fuse, L-N ≥2.5mm and L/N to PE (earth) ≥2.5mm. After the fuse, no specific requirement applies, but a reasonable distance should be maintained to avoid a short circuit that could damage the power supply.
Primary AC to primary DC section: ≥2.0mm.
Primary DC ground to earth: ≥2.5mm for a floating primary ground to earth.
Primary side to secondary side: ≥4.0mm, including components connected across the primary and secondary sides.
Between secondary-side parts: ≥0.5mm is sufficient.
Secondary ground to earth: ≥1.0mm is sufficient.

Before judging compliance, apply 10 N to internal parts and 30 N to the enclosure so that the spacing is reduced. The clearance must still meet the requirement in this worst-case condition.

Creepage Distance

Creepage distance is determined from the working voltage and insulation class. In typical designs, the following values apply:

Primary AC section: before the fuse, L-N ≥2.5mm and L/N to earth ≥2.5mm. After the fuse, no specific requirement applies, but a reasonable distance should be maintained to avoid short circuit damage to the power supply.
Primary AC to primary DC section: ≥2.0mm.
Primary DC ground to earth: ≥4.0mm, such as primary ground to earth.
Primary to secondary: ≥6.4mm. If the pin spacing of optocouplers, Y capacitors, and similar components is ≤6.4mm, a slot must be cut.
Between secondary-side parts: ≥0.5mm is sufficient.
Secondary ground to earth: ≥2.0mm or more.
Between transformer windings: ≥8.0mm or more.

PCB Creepage

Distance Through Insulation

Distance through insulation must comply with the following rules according to the working voltage and the insulation application:

For a working voltage not exceeding 50V (71V AC peak or DC value), there is no thickness requirement.
The minimum thickness of supplementary insulation is 0.4mm.
When reinforced insulation is not subjected to any mechanical stress that could deform or degrade the insulating material at normal temperature, the minimum thickness of the reinforced insulation is 0.4mm.

The thickness requirements do not apply to thin-layer insulating material, regardless of thickness, if the insulation is used inside a protective enclosure of the equipment, is not subject to knocks or abrasion during operator maintenance, and meets one of the following conditions:

For supplementary insulation, at least two layers are used, and each layer can pass the electric strength test for supplementary insulation; or
For supplementary insulation made of three layers, any combination of two layers can pass the electric strength test for supplementary insulation; or
For reinforced insulation, at least two layers are used, and each layer can pass the electric strength test for reinforced insulation; or
For reinforced insulation made of three layers, any combination of two layers can pass the electric strength test for reinforced insulation.

Wiring and Assembly Precautions

Flat-mount components such as capacitors must be mounted flat, without adhesive dots.
If applying 10 N to two conductors can shorten the distance below the required safety distance, the part may be fixed with adhesive to maintain clearance.
When PVC film is installed inside an enclosure, confirm that the safety distance is maintained after processing.
Adhesive fixing must not leave adhesive strings or other foreign material on the PCB.
Part processing must not cause insulation damage.

Flammability Requirements for Materials

Heat-shrink tubing, PVC sleeving, and Teflon sleeving must be rated V-1 or VTM-2 or better. Plastic materials such as silicone sheet and insulating tape must also be rated V-1 or VTM-2 or better. PCB material must be rated 94V-1 or better.

Insulation Levels

Definitions

The following insulation levels are used in the safety assessment:

Operational insulation: the insulation required for normal operation of the equipment.
Basic insulation: insulation that provides basic protection against electric shock.
Supplementary insulation: independent insulation applied in addition to basic insulation, to protect against electric shock if the basic insulation fails.
Double insulation: insulation consisting of basic insulation plus supplementary insulation.
Reinforced insulation: a single insulation structure that, under the conditions specified in the standard, provides a degree of protection against electric shock equivalent to double insulation.

Application of Each Insulation Type

A. Operational insulation

Between parts at different voltages.
Between an ELV circuit (or SELV circuit) and grounded conductive parts.

B. Basic insulation

Between hazardous-voltage parts and grounded conductive parts.
Between hazardous voltage and SELV circuits that rely on grounding.
Between primary-side power conductors and a grounded shield or the core of the main power transformer.
As part of double insulation.

C. Supplementary insulation

Supplementary insulation is generally used between accessible conductive parts and parts that may become hazardous after basic insulation failure, for example:

Between the external surface of a handle, knob, grip, or similar part and its unearthed shaft.
Between the metal enclosure of Class II equipment and the jacket of a power cord passing through the enclosure.
Between an ELV circuit and an unearthed metal enclosure.

Supplementary insulation is also used as part of double insulation.

D. Double insulation and reinforced insulation

Double insulation and reinforced insulation are generally used between the primary circuit and:

accessible unearthed conductive parts; or
floating SELV circuits; or
TNV circuits.

Double insulation is basic insulation plus supplementary insulation.

Circuit Definitions Used in the Assessment

ELV circuit: an extra-low voltage circuit. Under normal operating conditions, the AC peak voltage between any two conductors does not exceed 42.4V, or the DC value does not exceed 60V, in a secondary circuit.

SELV circuit: a safety extra-low voltage circuit. A secondary circuit designed and protected so that, under normal conditions or single-fault conditions, the voltage between any two accessible parts, and between any accessible part and the protective earthing terminal (Class I equipment only), does not exceed a safe value.

TNV circuit: a telecommunication network voltage circuit. A circuit that carries communication signals under normal operating conditions.

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