PCB Design: Getting Creepage and Clearance Righ

The page "PCB Process Design Specification" has shown, standardize the PCB process design of the product, specify the relevant parameters of the PCB process design, make the PCB design meet the technical specifications such as manufacturability, testability, safety, EMC, EMI, and build product process, technology, quality and cost advantages，this page talk about the distance and related safety requirements about PCB design.

1. Electric clearance: the shortest distance measured along with the air between two adjacent conductors or a conductor and the surface of the adjacent motor casing.
2. Creepage distance: the shortest distance between two adjacent conductors or one conductor and the surface of the adjacent motor casing measured along the insulating surface.

Determination of electrical clearance

According to the measured working voltage and insulation level, the distance can be determined.

• Please refer to Table 3 and Table 4 for the requirements of the electrical gap size of the primary side circuit
• See Table 5 for the requirements of the electrical gap size of the secondary side circuit

But usually: primary side AC part: L—N≥2.5 mm before the fuse, L. N PE (earth)≥2.5 mm, after the fuse device, it is not required, but a certain distance should be kept as far as possible to avoid short-circuit damaged power supply.

• Primary side AC to DC part ≥ 2.0mm
• Primary side DC ground to ground ≥ 2.5mm (primary floating ground to ground)
• The primary side part to the secondary side part ≥4.0 mm, which is connected to the components between the primary and secondary sides
• The electric gap of the secondary side part is ≥0.5mm
• The secondary side ground to the ground ≥1.0mm is enough

Note: Before deciding whether to meet the requirements, the internal parts should be applied with a force of 10N, and the outer shell should be applied with a force of 30N to reduce the distance so that the space distance still meets the requirements in the worst case.

Determination of creepage distance:

According to the working voltage and insulation level, check table 6 to determine the creepage distance
But usually:

• (1), Primary AC part: L—N≥2.5 mm before the fuse, and L. N earth ≥2.5 mm. After the fuse, there is no requirement, but try to keep a certain distance to avoid short-circuit damage to the power supply.

   

• (2), the primary side AC to DC part ≥ 2.0mm
• (3), primary side direct current ground to ground ≥4.0mm such as primary side ground to ground
• (4), the primary side to the secondary side ≥ 6.4 mm, such as optocoupler, Y capacitor, and other components of the foot spacing of ≤ 6.4 mm to be slotted.
• (5) The distance between the secondary side parts is ≥0.5mm
• (6), the secondary side ground to the ground ≥ 2.0mm or more

• (7), Between the two stages of the transformer ≥8.0mm or more

Insulation penetration distance

According to the working voltage and insulation application, the following regulations should be met:
——The working voltage does not exceed 50V (71V AC peak or DC value), and there is no thickness requirement;
——The minimum thickness of additional insulation shall be 0.4 mm;
-When the reinforced insulation does not withstand any mechanical stress that may cause deformation or performance degradation of the insulating material under normal temperature, the minimum thickness of the reinforced insulation shall be 0.4 mm. If the insulation provided is used in the protective enclosure of the equipment and will not be bumped or scratched during maintenance by the operator, and falls under any of the following conditions, the above requirements do not apply to thin-layer insulation materials regardless of their thickness ；
-For supplementary insulation, use at least two layers of materials, each of which can pass the electrical strength test for supplementary insulation; or:
——Additional insulation composed of three layers of materials, of which any combination of two layers of materials can pass the electrical strength test of the additional insulation; or:
-For reinforced insulation, use at least two layers of materials, each of which can pass the electrical strength test for reinforced insulation; or:
——Reinforced insulation composed of three layers of insulating materials, of which any combination of two layers of materials can pass the electrical strength test of the reinforced insulation.

Points to note about the wiring process

• Flat-mounted components such as capacitors must be flat-mounted without dispensing.
• For example, the distance between the two conductors can be shortened by applying a force of 10N. When the distance is less than the safety distance requirement, glue can be used to fix this part to ensure its electrical clearance.
• When laying PVC film in some enclosure equipment, pay attention to ensuring the safety distance (pay attention to the processing technology).
• Be careful not to make any foreign objects such as glue wires on the PCB board.
• When processing parts, it should not cause insulation damage.

There are requirements for flameproof materials

• Heat shrink tubing: V—1 or VT M—2 or more; PVC tubing V—1 or VT M—2 or more
• Teflon casing: V-1 or VT M-2 or more; plastic materials such as silicone sheet, insulating tape V-1 or VT M-2 or more
• PCB board: 94V—1 and above

About insulation class

• (1) Work insulation: the insulation required for normal operation of the equipment
• (2) Basic insulation: insulation that provides basic protection against electric shock
• (3) Supplementary insulation: independent insulation applied in addition to basic insulation to protect against electric shock once the basic insulation fails
• (4) Double insulation: insulation composed of basic insulation plus additional insulation

• (5) Reinforced insulation: a single insulation structure, under the conditions specified in this standard, the protection level it provides against electric shock is equivalent to double insulation

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The applicable situations of various insulation are as follows:

A. Operational insulation

• Between two parts with different voltages
• Between the ELV circuit (or SELV circuit) and the grounded conductive parts.

B. Basic insulation

• Between parts with dangerous voltage and conductive parts that are grounded;
• Between SELV circuits that have dangerous voltages and rely on grounding;
• Between the power conductor on the primary side and the grounding shield or the core of the main power transformer;
• As part of double insulation.

C. supplementary insulation

a. Generally speaking, there may be a dangerous voltage between the accessible conductor parts and the damage of the basic insulation
Between the parts, such as:

• Ⅰ. Between the appearance of handles, knobs, handles, or the like and their ungrounded shafts.
• Ⅱ. Between the metal shell of the second type of equipment and the outer skin of the power cord passing through the shell.
• Ⅲ. Between the ELV circuit and the ungrounded metal casing.

b. As part of double insulation

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D. Double insulation, reinforced insulation

Generally speaking, between the primary circuit and

• a. Between accessible non-grounded conductive parts
• b. Between floating (f l oat i ng) SELV circuits or
• c. Between TNV circuits

Double insulation = basic insulation + supplementary insulation

Note:

ELV circuit: extra low voltage circuit. Under normal working conditions, the AC peak value between the conductors or between any conductor does not exceed 42.4V or the DC value does not exceed 60V in the secondary circuit.
SELV circuit: safety extra-low voltage circuit. The secondary circuit is properly designed and protected so that under normal conditions or single fault conditions, between any two accessible parts, as well as any accessible parts and the protective earth terminal of the equipment (only for The voltage between Class I equipment) shall not exceed the safe value.
T NV: Communication network voltage circuit. Under normal operating conditions, a circuit carries communication signals.