CableDataSheet, Cable and Wire Technical Consulting Service

 
                                                                                                                         

What are creepage and clearance?
                                                                           

Creepage is the shortest distance along the surface of a solid insulating material between two conductive parts.

Clearance is the shortest distance in air between two conductive parts.

                                                                                                                                                               











Source: You are not allowed to view links. Register or Login

Hello, and welcome to TI's Precision Labs, an online video curriculum for engineers. This Precision Lab's video will provide a brief explanation of isolation creepage and clearance. This video will answer the following questions.

What is creepage and clearance and how are they different? How is minimum creepage and clearance determined for a system? What is working voltage and why does it matter to creepage and clearance? And how does operating environment affect creepage and clearance requirements?

What is creepage and clearance and how are they different? Creepage and clearance requirements are industry standard specified spacing requirements between metallic or conductive contacts that are external to the isolation device. Creepage is defined as the shortest distance between two conductive paths measured along the surface of a solid insulation.

The shortest path is typically found around the end of the package body between conductive pins or leads. Clearance is the shortest distance between two conductive parts measured through air, typically measured on a device package between pins under the body of the package. Both of these parameters are important because when an electric field is present across the isolator, electrical breakdown of either the package or of the air that is below the package can cause electrical failure or even arcing.

Selecting the right creepage and clearance for the design environment and end use case is critical. Minimum creepage and clearance distances are defined by the IEC standards bodies as guidance intended to prevent air arcing during operation. Arcing is a phenomenon that is caused by charge buildup and is especially a concern with higher operating voltages applied continuously over time.

The creepage and clearance guidance includes considerations for working voltage, device material classifications, and several environmental conditions such as altitude and pollution readings. We we'll take a quick look at the relationship between these factors and give some brief guidance on how to determine the correct creepage and clearance needed for your isolator.

The minimum distance for creepage and clearance will depend on whether the design requires functional, basic, or reinforced isolation ratings. These are the industry standard terms used to describe levels of voltage isolation performance and minimum safety requirements. The level of component isolation rating required is determined by the end equipment standards body. For example, the IEC standard for medical devices has different requirements than the standard used for test and measurement.

Before you select a basic or reinforced isolator for your design, you will need to consult with the end equipment standard for rating. Let's do a brief review of the differences between functional, basic, and reinforced insulation ratings. Functional insulation does not protect or isolate against electrical shock. But it is required for a product to function.

A good example may be the PCB material between metal traces on a board. The circuit would not function without separation of the traces. So this functional installation is needed to operate.

Basic installation is a single level of protection that provides a basic protection against electrical shock. A very common example here would be the plastic insulation of an electrical cord, which is a single level of protection from easy voltages. If the basic insulation fails, there is exposure to hazardous voltage. For integrated circuit isolators, the term basic isolator is used to describe isolation that acts as a single level of protection for voltages typically between 2 and 1/2 to 3 KB and occasionally as high as 5 KB.

Reinforced insulation, or double insulation provides the equivalent of two basic level protection devices in a single device and is also considered to have failsafe functionality allowing for direct user access. This does not mean that there are two devices in a single package but that the tested installation of a reinforced isolator performs at twice the level of a basic isolator.

For reinforced digital isolators, the level of protection is significantly higher, on the order of 8 KB to 10 KB or more. Reinforced isolator creepage and clearance distances are twice that of basic isolation devices, though both basic and reinforced certifications have minimum requirements for creepage or clearance.

In order to achieve a reinforced isolation reading, semiconductor manufacturers are required to provide safety and functionality according to industry standard test methodologies. The classification of reinforced isolation for a digital isolator IEC is achieved and documented through the certification process with most recent standard for digital isolators at the time of this video release being VDE0884-11.

Now that we know the role of isolation rating, we can look at the relationship between isolator components and creepage and clearance requirements. Working voltage, device material level, and environmental levels of altitude and pollution will all play a role in minimum creepage and clearance requirements. Working voltage is a device level specification used to quantify the ability of an isolator to handle high voltage across its barrier on a continuous day-to-day basis throughout its lifetime. Creepage and clearance requirements are related to isolator working voltage due to the possibility of arcing that may result from charge buildup and transfer on the surface of the device after an extended period of time during operation under high working voltages.

The quality of the mold compound and the materials used for a digital isolator are important factors in creepage and clearance requirements. Comparative Tracking Index, or CTI, is used to describe the maximum voltage that an insulating material can operate under in the presence of contamination, whether air, surface, or humidity without resulting in tracking.

Tracking is the creation of conductive pathways on the surface of an isolation material and is characterized through standardized material testing. If conductive paths are established on the surface of an isolator, the potential for arcing increases. For this reason, working voltage and package material level with a known CTI are directly related to end equipment creepage and clearance distance requirements.

A device with higher CTI levels equates to a higher quality material level. Material level one, for example, is the highest quality material for mold compound. With this in mind, higher CTI found in material group 1 requires smaller creepage and clearance distances than those made with lower CTI, or higher material groups mold compound. TI isolators use material group 1 and offer the highest quality mold compound.

As mentioned earlier, material CTI is tested with applied contamination which in the test environment simulates environmental risks of surface contamination. In similar fashion, environmental conditions will also influence minimum creepage and clearance distances with dependence on the end equipment use case, altitude, and environmental pollution degree. At higher altitudes of 2,000 to 5,000 meters above sea level, the air pressure is lower causing peak over voltages such as surge or temporary overvoltage to more readily cause arcing between isolator pins.

Equipment operating at high altitudes require greater spacing between pins or higher clearance distances based on IEC60664-1, which provides multiplication factors by which clearance must be increased at higher altitudes in order to protect from altitude induced arcing. Similarly, in operating environments where there is a higher likelihood of particulates collecting on the surface of the device thereby creating conductive pathways, the consideration of the pollution degree will also influence the minimum distance requirements. Both altitude and pollution levels are rated for the end equipment operating environment from the end equipment standards. So it's wise to always check IEC guidelines for the latest standard.

It is worth noting that there are some techniques that have been used to reduce creepage and clearance requirements. For example, conformal coating, or potting techniques involve the deposition of insulating polymer or other material over the printed circuit board and have historically been used to reduce pollution degree around the isolator. This reduces the requirement on creepage and clearance by eliminating the influence of pollutants. Other techniques, such as groove cutting can be used to attain a lower PCB creepage distance. Groove cutting effectively extends the creepage distance.

For a group greater than one millimeter wide, the depth requirement is that existing creepage distance plus the width of groove and twice the depth of the groove must equal or exceed the required creepage distance. The groove must not weaken the substrate to a point that it fails to meet mechanical test requirements. And therefore all layers under the isolator must be free from traces, vias, and pads in order to maintain the maximum creepage distance.

While these methods may reduce required distances, they do add cost and are less reliable. And they also have additional needed inspection steps in PCB manufacturing. One effective way to reduce creepage and clearance without conformal coating or potting is to select a wide body isolator manufactured with high quality mold compound.

This image shows two identical isolators in two different packages. The wide body package meets IEC creepage requirements of 14.5 millimeters. And because many new isolators use the higher quality mold compound, material group one, like the one shown here. They enable high altitude designs and tolerate higher pollution degrees without requiring additional steps in PCB manufacturing.

This concludes our introduction to isolation creepage and clearance. We outlined a general set of guidelines to follow when determining creepage and clearance requirements. These include understanding the required insulation rating of basic or reinforced rear end equipment. The IEC equipment standards are the most common starting point. And creepage requirements may double from basic to reinforced.

Identify the insulation working voltage of your system. This will directly influence the creepage and clearance requirements to maintain safe, long-term operation with minimal risk of electrical failure or arcing, identify the pollution degree of the system's end operating environment, levels one through four, and the peak altitude of your end systems operating environment.

Lastly, consider the material group category of the isolator being used in order to determine available package options. Newer wide body packages often offer the smallest solution size due to higher quality materials and higher CTI specifications. Please continue watching to take the online quiz.

Number one, true or false, creepage and clearance requirements for any design are determined by the isolation rating of basic or reinforced? False. Creepage and clearance requirements are determined by isolation rating, but additionally by IEC specification, altitude and pollution degree, and material levels.

You are designing a high voltage charging station that will operate at 400 volts rms of reinforced working voltage. The expected pollution degree is level 2. What material group for your isolation device will allow minimal creepage distance in your layout?

Material group 1 is the highest quality option and will minimize CTI associated with the device allowing for minimal creepage requirement. You can find material group information in the vendor data sheet. You're working to design the smallest possible solution for an integrated isolated I2C interface device.

It offers 8 millimeters of creepage based on the standard package. But your IEC guidelines require 10 millimeters of creepage distance under pollution degree 2. What techniques can you use to achieve your goals of integration and smaller package usage?

You could reduce the pollution degree to level 1 through potting and use groove cutting beneath the existing SOIC package to increase the existing creepage distance. This concludes TI's Precision Lab, what are Creepage and Clearance. You can browse more isolation topics and resources at You are not allowed to view links. Register or Login.