PCB News - Creepage distance and electric clearance of circuit board

Generally speaking,the required value of creepage distance is larger than the required value of electrical creepage and clearance table, and the requirements of both must be met when wiring (that is,the distance between the surface and the distance of the space must be considered), slotting (groove width Should be greater than 1mm) It can only increase the surface distance, that is,the creepage distance, but not the electrical clearance. Therefore, when the electrical clearance is not enough, slotting cannot solve this problem. When slotting, pay attention to whether the location and length of the slot are appropriate to meet the requirements. Creepage distance requirements.

Electrical isolation distance of components and PCB board: (Electrical isolation distance refers to the comprehensive consideration of electrical clearance and creepage distance)
For the switching power supply of class I equipment (most of the switching power supplies of our company are class I equipment), the isolation distance on the components and PCB board is as follows: (the following values do not include margin)

For AC-DC power supply (take no PFC circuit and input rated voltage range of 100-240V～ as an example)                     

Electric clearance creepage distance
L line-N line (before the fuse) 2.0mm 2.5mm
Input-ground (before the rectifier bridge) 2.0mm 2.5mm
Input-ground (behind the rectifier bridge) 2.2mm 3.2mm
Input-output (transformer) 4.4mm 6.4mm
Input-output (except transformer) 4.4mm 5.5mm
Input-magnetic core, output-magnetic core 2.0mm 2.5mm

For AC-DC power supply (take PFC circuit and input rated voltage range of 100-240V～ as an example)                     

Electric clearance creepage distance
L line-N line (before the fuse) 2.0mm 2.5mm
Input-ground (before the rectifier bridge) 2.0mm 2.5mm
Input-ground (behind the rectifier bridge) 2.2mm 3.2mm
Input-output (transformer) 5.2mm 9.0mm
Input-output (except transformer) 4.4mm 6.4mm
Input-magnetic core, output-magnetic core 2.2mm 3.2mm

For DC-DC power supply (take the input rated voltage range of 36-76V as an example)            

Electric clearance creepage distance

(DC+)-(DC-) (before the fuse) 0.7mm 1.4mm
Input-ground (before the fuse) 0.7mm 1.4mm
Input-ground (after the fuse) 0.9mm 1.4mm
Input-output (considering basic insulation) 0.9mm 1.4mm
Input-output (considered to be reinforced insulation) 1.8mm 2.8mm
Input-magnetic core, output-magnetic core 0.7mm 1.4mm

Electrical isolation distance inside the transformer:
The electrical isolation distance inside the transformer refers to the sum of the width of the retaining walls on both sides of the transformer. If the width of the transformer retaining wall is 3mm, then the electrical isolation distance of the transformer is 6mm (the width of the retaining walls on both sides is the same). If the transformer does not have a retaining wall, the isolation distance of the transformer is equal to the thickness of the adhesive tape used. In addition, for the AC-DC power supply, the primary and secondary windings of the transformer should be separated by three-layer adhesive paper. For the DC-DC power supply, only two-layer adhesive paper can be used for isolation. The following values do not include margins:
                                     

Required separation distance Minimum width of retaining wall
AC-DC (input voltage 100-240V~, without PFC circuit) 6.4mm 3.2mm
AC-DC (input voltage 100-240V~, including PFC circuit) 9.0mm 4.5mm
DC-DC (Voltage 36-76V) 2.8mm 1.4mm
Note: If the pin of the transformer is not covered with an insulating sleeve, the isolation distance at the pin may only be the thickness of the adhesive tape and the retaining wall. Therefore, the pin of the transformer needs to be covered with an insulating sleeve and the sleeve must be worn. Over the retaining wall.

Creepage distance: The shortest linear distance measured by air separation between two conductive components or between the conductive component and the object interface.
Clearance: The shortest distance between two conductive components or the interface between a conductive component and an object measured along the insulating surface.

When the clearance does not meet the standard distance: the circuit board can be slotted between two conductive components. If the distance between the conductive component and the shell and the accessible part is not enough, the conductive component can be wrapped with insulating material live.

Wrapping the conductive components with insulating materials not only solves the Creepage distance but also the clearance problem. This method is generally used when the distance between the transformer and the surrounding components on the power board is not enough, the transformer is wrapped.

In addition, the voltage difference between the two conductors can be appropriately reduced without affecting the function of the product

Determination of electrical creepage and clearance table:
According to the measured working voltage and insulation level, the distance can be determined
For the requirements of the electrical gap size of the primary circuit, see Table 3 and Table 4
See Table 5 for the requirements of the electrical gap size of the secondary side circuit
But usually: primary AC part: L-N≥2.5mm before the fuse, L.N PE (earth)≥2.5mm, after the fuse device, there is no requirement, but a certain distance should be kept as far as possible to avoid short-circuit damage to the power supply.
Primary side AC to DC part ≥2.0mm
Primary side DC ground to earth ≥2.5mm (primary floating ground to earth)
The primary side part to the secondary side part ≥4.0mm, the components that are connected 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 the table 6 to determine the creepage distance

But usually: 

(1) Primary AC part: L-N≥2.5mm before the fuse,L.N earth ≥2.5mm,there is no requirement after the fuse, 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 ≥6.4mm, such as optocoupler, Y capacitor and other components of the foot spacing of ≤6.4mm should be slotted.
(5) ≥0.5mm between the secondary side parts is enough
(6)The secondary side ground to the ground ≥ 2.0mm or more
(7) Between the two stages of the transformer ≥8.0mm

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 value or DC value), and there is no thickness requirement;
The minimum thickness of additional insulation should be 0.4mm;
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 should be 0.4mm.

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;
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; 
For reinforced insulation, use at least two layers of materials, each of which can pass the electrical strength test for reinforced insulation;
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.

There are some points to note about the wiring process:
Flat-mounted components such as capacitors must be flat-mounted without dispensing
For example, the distance between 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 shell equipment, pay attention to ensure 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 anti-flammable materials:
Heat shrinkable tubing V-1 or more than VTM-2; PVC tubing V-1 or more than VTM-2
Teflon casing V-1 or VTM-2 or more; plastic materials such as silicone sheet, insulating tape V-1 or VTM-2 or more
PCB board above 94V-1

About the insulation level
(1) Work insulation: the insulation required for the normal operation of the equipment
(2) Basic insulation: insulation that provides basic protection against electric shock
(3) Additional insulation: independent insulation applied in addition to basic insulation to protect against electric shock in the event of basic insulation failure
(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

The applicable situations of various insulations are as follows:
A.Oprational insulation
a.Between two parts with different voltages
b.Between ELV circuit (or SELV circuit) and grounded conductive parts.

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

C.Supplementary insulation supplementary insulation
a.Generally speaking, it is between the accessible conductor parts and the parts that may carry dangerous voltage after the basic insulation is damaged, such as:
1.Between the appearance of handles, knobs, handles or the like and their ungrounded shafts.
2.Between the metal shell of the second type of equipment and the outer skin of the power cord passing through the shell.
3.Between the ELV circuit and the ungrounded metal casing.

b.As part of double insulation
1.Double insulation
2.Double insulation Reinforced insulation

Generally speaking, between the primary circuit and
a.Between accessible non-grounded conductive parts, or
b.Between floating 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.

TNV: Communication network voltage circuit
Under normal working conditions, a circuit that carries communication signals.

The above is the introduction of the creepage and clearance table distance and electrical clearance of the circuit board. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.