Clearance and creepage for high voltage PCB design 

Instinctively, hardware engineers want to have larger spacing between conductors with higher voltages between them during PCB design. But how should that spacing actually be defined? 

Usually, space on the PCB is tight, and it is desirable to occupy as little space as possible while maintaining functional and safe design, so arbitrarily enlarging the spacing is not the best approach. Are there any formulas or standards that could help define the spacing? Spacing cannot be the same for every PCB and every situation, right? There must be more variables to this; it seems too complicated to derive a formula for it. 

Luckily, many engineers before us have asked these questions and conducted experiments that resulted in the standards we have today. It is important to know which standard to use in which case and how, as we will see that functional and safety isolation are not the same and that every situation and the environment of the device can bring different isolation requirements. 

In this blog, we will cover the basic definitions of parameters, industry standards, and ways of defining your isolation requirements – or even reducing them. 

What are clearance and creepage? 

For starters, how to define the spacing between conductors? It’s not the same case when looking at the spacing between conductors through air and over the PCB surface (for example, there might be a PCB slot between the conductors). 

Clearance – the shortest distance through the air between two conductors. Required clearance is defined so it prevents air ionization, electric breakdown of air insulation, and arcing because of transient overvoltage (short-duration overvoltage of a few milliseconds or less) and working voltage. Factors that are important for defining clearance requirements are: 

• working voltage, 

• transient overvoltage category, 

• pollution degree, 

• altitude (air pressure). 

Creepage – the shortest distance over the insulating (PCB) surface between two conductors. Required creepage is defined so that it prevents isolation breakdown or tracking (formation of permanent carbonized conductive paths across the surface of the insulator) at working voltage. Working voltage being the highest RMS of AC or DC voltage that can occur across an insulator when the device is supplied at nominal voltage. Factors that are important for defining creepage requirements are: 

• RMS working voltage, 

• pollution degree, 

• material group (defined by CTI) of the insulator (usually of FR-4). 

The influencing factors will now be briefly explained. 

What is material group and CTI (comparative tracking index)? 

Material group of a particular insulator is defined only by its CTI (comparative tracking index). 

CTI is a measure of insulator’s tracking strength. The higher the CTI the higher voltages it can withstand without appearance of tracking. Simplified: CTI is measured as the maximum test voltage at which a material withstands 50 drops of a standardized electrolytic solution (0.1% NH4Cl) without exhibiting tracking (exact test parameters as timings of the drops, electrodes spacing, etc. are all defined by standards).  

Most FR-4 materials used in PCB fabrications are material groups IIIa or IIIb.

But creepage doesn’t have to be looked at only on the PCB (FR-4) surface – sometimes the creepage can go over the component body because of smaller distance over the component body than between pads over the PCB surface. That’s why some component manufacturers use higher CTI compounds (for example material group I) – so the creepage requirement on the component itself can be reduced. In other cases, they omit one pin or increase the pitch of the IC to increase both creepage and clearance. 

What is pollution degree? 

Pollution degree is the measure of pollution (dirt, possibility of moisture, contaminants – solid, liquid or gaseous that can change dielectric strength or surface conductivity of the insulator) on the insulating surface (PCB). 

Since lower pollution degrees require smaller clearance and creepage, a common “trick” to reduce clearance and creepage requirements and save space is to apply conformal coating. By properly coating the PCBA, you can effectively achieve pollution degree 1 at the board surface. This allows for much tighter component density while still meeting strict safety and functional standards, as the coating acts as a barrier against pollution. 

What is transient overvoltage category? 

Transient overvoltage category divides devices according to the way of connecting to the mains voltage, which gives a probabilistic implication on possible transient overvoltage. The closer a device is to the origin of the electrical installation, the higher the expected surge energy and the larger the required clearances. 

What are the isolation standards and categories?

Not every situation requires the same exact insulation, so there are some insulation categories and standards that can give you insulation requirements. Standards basically provide tables that tell you what requirement you need for a specific situation.  

Insulation categories are: 

• Functional insulation – insulation required for a normal and functional working of the device. This doesn’t provide safety insulation (protection against electric shock). 

• Basic insulation – single layer of safety insulation required for protection against electric shock. 

• Supplementary insulation – “a little extra” of insulation on top of basic insulation. 

• Double/reinforced insulation – “twice” the basic insulation. 

Safety insulation is needed if there is danger of electrical shock to people (voltage is larger than around 60 VDC or 30 VRMS,AC). In that case the device/PCB usually has a low voltage (LV) or secondary and high voltage (HV) or primary side with an insulating spacing between them with the user “imagined” on the LV side. Safety insulation, its parameters and requirements are described in the IEC 60664-1 standard (with some derivatives for specific cases). There are basic, supplementary and reinforced safety insulations, but how to know when I need basic or reinforced insulation? Before giving a practical guide on this, here is another simplified circuit categorization based on its voltage (energy source): 

• ES1 (safe) – voltage up to 60 VDC or 30 VRMS,AC, 

• ES2 – voltage up to 120 VDC or 60 VRMS,AC, 

• ES3 (hazardous) – voltage above 120 VDC or 60 VRMS,AC. 

Here is a practical rule of thumb (for full details refer to the IEC standards): 

• Basic insulation between HV and LV parts is enough if the metal casing of the device on LV side is earthed (earthing the casing of the device adds another layer of protection against electrical shock regardless if the HV part is ES3 or ES2 circuit – in that case a fuse or circuit breaker goes off). These are known as Class I devices.  

• Reinforced insulation between HV and LV part is required if the metal casing of the device on LV side is not earthed (reinforcing insulation adds another layer of protection against electrical shock regardless if the HV part is ES3 or ES2 circuit). These are known as Class II devices and are often identified by the “double square” symbol. 

Side note: if the whole device is ES1 (safe) circuit, then no safety insulation is required and these devices are known as Class III. 

Functional insulation is needed for proper and functional operation of electronic devices. Functional spacing requirements can be seen in the IPC-2221B standard. It is used: 

• Inside safe ES1 circuits: where voltages are below 60 VDC or 30 VRMS,AC, the primary concern is preventing malfunctioning of the device. 

• Within the same voltage side: for spacings between conductors inside HV or inside LV side (not between HV and LV side) whose shorting would not affect safety, only functionality. Two conductors that are both part of HV side can have a potential difference between each other that should not affect the functional operation of the electronic device because of arcing or tracking. 

IEC 60664-1 defines clearance and creepage for safety, while the IPC-2221B defines spacing between the conductors for functionality. 

Example of determining clearance and creepage requirements 

Let’s have a simple example where we need to determine clearance and creepage requirements for some device with these parameters: 

• Power source: 230 VRMS,AC without PE (Class II). 

• This goes to a full bridge rectifier (so the DC voltage on the HV side is about 325 V) and into a flyback power supply that outputs 24 V on the secondary (LV) side. Flyback transformer should provide galvanic isolation between the primary (HV) and secondary (LV) part. 

• PCBA is not being conformally coated and is inside a plastic enclosure. 

• The device will be hardwired to the mains voltage. 

• PCB is made of standard FR-4 material. 

• Besides power, there are signals that go from HV to LV side. 

Important information that can be extracted from here: 

• HV side is an ES3 circuit and the LV side is an unearthed ES1 circuit, so this means we need reinforced safety insulation between the HV and LV sides. 

• PCBA will not be conformally coated and are inside of a plastic enclosure, so it’s a good assumption that the pollution degree is PD2. 

• The device will be hardwired to the mains voltage which means they are in the transient overvoltage category III. 

• The PCB is made of FR-4 material that is material group IIIa. 

First, let’s define the functional spacing inside the HV and LV part. We will use IPC-2221B for this. This standard defines spacings for tracks on bare board (internal layers or external with or without solder mask) and spacings between pads of the assembly (with or without conformal coating) with the second being same or larger. It is important to identify what voltages can appear between each conductor.  

• For example, between the rectified DC link and the primary (HV) ground, we know there can be at least 325 V. If we look up our specific case in the IPC-2221B table, we find out that the required spacing between external layer tracks below solder mask is 0.8 mm while the required spacing between the pads of uncoated components is 1.5 mm. 

• For example, between the flyback output voltage and the secondary (LV) ground, we know that there can appear at least 24 V. If we look up our specific case in the IPC-2221B table, we find out that the required spacing between external layer tracks below solder mask is 0.05 mm while the required spacing between the pads of uncoated components is 0.25 mm. Here only 0.25 mm will be considered for simplicity and to have the same clearance in both cases. 

Second, let’s define the clearance and creepage between the HV and LV parts for safety. We will use the IEC 60664-1 for this. This standard defines relaxed creepage requirements for PCBs than for other insulators because of good, controllable and repeatable PCB manufacturing processes. 

• Creepage: if we look up at our specific case (PD2, material group IIIa, working voltage) in the IEC 60664-1 table, we find out that the required creepage is 2 mm. But this is just a single layer of insulation (basic) so if we want reinforced, we double it and we get 4 mm of creepage requirement. 

• Clearance: if we look up at our specific case (PD2, transient overvoltage category III, working voltage) in the IEC 60664-1 table, we find out that the required clearance is 3 mm. But this is just a single layer of insulation (basic) so if we want reinforced, we take the next larger clearance from the table which is 5.5 mm. 

Usually, the creepage requirements are larger, but because of a high transient overvoltage category, we get that the clearance requirement is higher. 

For power transfer across the isolation, a flyback transformer is used while for signal transfer optocouplers can be used. It is important to follow clearance and creepage requirements with these components also. 

Side note: if the intended altitude of the device is greater than 2000 m above sea level, then correctional factors should be used for safety clearance and if the intended altitude of the device is greater than 3050 m above sea level, then a different part of the IPC table should be used for functional spacing. 

How to implement this in Altium Designer? 

Dealing with clearance and creepage requirements can be done with not just layout work but also with some schematic work: 

• Schematic: a clean schematic with distinguished HV and LV parts and use of blankets or parameter sets for net classes helps for a proper layout and readable/automated design rules. This prevents manual entry of net names every time. 

• Layout: using the net classes defined in schematic inside design rules practically prevents you from violating the clearance and creepage requirements. Also, a good thing is to use some auxiliary layer in which you can draw lines with thickness of the required insulation between the HV and LV parts and to turn on the visual clearance boundaries. This provides a clear visual of the HV and LV parts on the PCB. 

Using the larger value (creepage or clearance) as the clearance rule inside Altium Designer for safety insulation is a safe approach. In our example, 5.5 mm inside the safety clearance design rule minimizes the chance of requirement violations. 

Side note: clearance “works” over air (arcing), while creepage “works” over the insulator surface (tracking), but what about inter-layer isolation? If the prepreg is thinner than the creepage or clearance requirement, can I route LV tracks on the internal layer below HV tracks on top layer? The answer is usually yes because the dielectric strength of the FR-4 material is in the range of 20 kV/mm – 50 kV/mm which prevents electric breakdown through the FR-4 material. But safety standards usually have a minimal prepreg thickness or dictate use of two prepregs together. Sometimes those two prepregs have perpendicular direction of fibers, used to achieve better isolation. 

Conclusion 

Clearance and creepage requirements cover different physical occurrences to prevent different faulty behaviors – clearance prevents electrical breakdown through air because of transient voltages and peak working voltages while creepage prevents tracking over the surface of the insulator while the device operates on its nominal voltage. Pollution degree (environment), location of connection of the device to the mains (transient overvoltage category), altitude and material group (CTI) are factors that dictate the insulation requirements besides the voltages.  

IEC 60664-1 covers the safety insulation requirements while the IPC-2221B covers the functional spacing requirements. Using these standards boils down to recognizing the specific context of your device and reading the proper tables. The outputs from these standards can be used inside the CAD tools (design rules with net classes, visual boundaries, etc.) to achieve the insulation requirements easier. 

This blog gives a practical high-level overview of the topic, for more detailed information and product-specific requirements, always refer to the applicable standards for your equipment.