The table 8.1 in UL840 doesn't consider clearances for voltages less than 50V (for example). I have to use a clearance of 0.2 mm on the PCB for the case of overvoltage category 1 and pollution degree 2. This is a problem indeed, since the distance between the microcontroller pins is only 0.175 mm. The standard doesn't say I am allowed to extrapolate. Does anyone know how to deal with this problem of UL 840 not considering voltages less then 50V? P.S.: Coating could be a solution but I don't want to use it since it's expensive compared to the price of the electronic.

  • \$\begingroup\$ What is the nature of your 12V that you're worried about the pin-to-pin clearance on your microcontroller, but not whether it happens to get much closer than that within the microcontroller package itself? \$\endgroup\$ – 1N4007 Jan 29 '19 at 17:18
  • \$\begingroup\$ Thanks for answering. The main problem is not the 12V that comes from an external power supply. The Problem is the clearance of the microcontroller. It has a pin to pin distance of 0,175mm. The max. voltage at µC Pins is only 5V and for UL840 there is no difference between 5V and 50V. According to UL840 als the µC has to have a min. pin to pin distance of 0,2mm. A coating would help me to reduce the pollution degree but coating would come with other problems that could occure for examle during temperature change test. \$\endgroup\$ – Haldun Jan 30 '19 at 10:23

I believe you are misapplying this table.

Instances where clearance (and creepage) is a concern are where the voltage/power source can be a shock or fire danger to the user. If your 5V is created by an internal regulator, then you don't need to worry about clearances to other traces powered by the same 5V regulator. Trace spacing at these voltages is more a function of PCB technology. That's why your microcontroller spacing is less than 0.2mm; it's more than enough.

Tables like you're dealing with are commonly (but not exclusively) applied to instances where you're dealing with mains voltage, where some insulation is required to protect the user from transients and the resulting stress on insulation. Certainly you can have a hazardous 5V rail if it's capable of overvoltage transients, but those are less common.

A final disclaimer: I don't know the full details of your design, and can not be responsible for the safety of your product. The responsibility falls first to you as the designer to determine whether your 5V is hazardous to the user.

  • \$\begingroup\$ Thank you for your answer 1N4007. I actually hope that I'm misapplying the table. There are actually 2 tables in UL840 that I consider. Table 8.1 and Table 9.2. The first one is about minimum clearances and the second one is about "minimum acceptable creepage distances on printed wiring boards". Chapter 6 says "creepage distances cannot be less than the associated clearances". Which means I have to apply Table 8.1. And the smallest voltage there is 50V. By the was: The application is an electrical vehicle (industrial/no passanger car). \$\endgroup\$ – Haldun Jan 31 '19 at 14:02
  • \$\begingroup\$ @Haldun Yes, you would be misapplying the creepage table for the same reasons I noted. \$\endgroup\$ – 1N4007 Jan 31 '19 at 14:17
  • \$\begingroup\$ I'm gonna discuss about it tomorrow with our quality guys. Just in case I need more arguments than the ones in your kind answer... Do you know which document or which UL standard explains when to use table 8.2.? \$\endgroup\$ – Haldun Feb 3 '19 at 19:58
  • \$\begingroup\$ Just look at the scope for UL840. At its heart its just laying out guidelines for trace spacing (exclusive of solid insulation) in an insulation system. If you don't need insulation, then these tables are not required. And when they say "insulation", they mean the protection required to protect the user from shock, not how far one ought to space a 5V trace from its own ground. Perhaps reading up more on the use of the term would be beneficial. \$\endgroup\$ – 1N4007 Feb 3 '19 at 20:59

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