Voltage Separation Standards - Page 1
Forum section: electronics / projects

author: sparkyinak
author_rank: Contributor
timestamp: August 30, 2025, 09:31:46 pm
content: Is there an “industry standard” in the U.S. with separation requirements based on a circuit’s voltage? Bare conductors like leads from resistors and compactors have to have separation to prevent the voltage from jumping across through the air. Low voltage I’d no big deal but like a project I’m working on now has VAC. That is why high voltage transmission towers get larger as the voltages get higher to hold the bare transmission wires further apart. Air is a poor insulator. Thank you in advance

author: floobydust
author_rank: Super Contributor
timestamp: August 30, 2025, 09:38:21 pm
content: The problem is dirt and moisture, and what the insulator is. This varies widely in applications and industries.Because of this, many safety standards have different clearance values for the same voltage. It's not just one equation or number.You can know when for example, air breaks down but this depends on (pressure) altitude, humidity, light UV, geometry etc.So a generous safety factor is also applied to the "science lab" values.

author: tooki
author_rank: Super Contributor
timestamp: August 30, 2025, 10:01:14 pm
content: The basic industry standard is IPC-2221. You can find tables and calculators for that, for examplehttps://resources.altium.com/p/using-an-ipc-2221-calculator-for-high-voltage-designandhttps://www.protoexpress.com/tools/pcb-conductor-spacing-and-voltage-calculator/It includes the “generous safety factor” floobydust was referring to. But he is absolutely correct about their being various standards. This TI white paper is a good overview:https://www.ti.com/lit/ml/slup419/slup419.pdf?ts=1756586065636

author: TimFox
author_rank: Super Contributor
timestamp: August 30, 2025, 11:00:58 pm
content: Why do you say air is a poor insulator?An old rule-of-thumb for voltage gradient in air, used when measuring high voltage with Frankenstein-lab type insulated spheres to get a spark gap, is 3000 V/mm or 75 kV/inch, under normal conditions (dry air, mean sea level, and 27oC).The TI document above concentrates on "creepage", where the path is along a boundary between solid (e.g., FR4) and gas (i.e., air), including crud "pollution" on the surface.Normally, the breakdown across such a surface (boundary between solid and gas) is at a considerably lower voltage than through the gas itself with no solids in the way.

author: Smokey
author_rank: Super Contributor
timestamp: August 31, 2025, 05:40:11 am
content: I've always found the creepage and clearance standards to be incredibly unintuitive and hard to interpret.https://www.smps.us/pcbtracespacing.htmlThe green line on this plot... 299V to 301V is a HUGE step! Like they think some EE magic happens at 300V. Yeah, it's probably just because 208VAC has a peak of about 300V and that gets you line operated industrial things with some serious energy behind them. But still.

author: floobydust
author_rank: Super Contributor
timestamp: August 31, 2025, 06:24:40 pm
content: Quote from: tooki on August 30, 2025, 10:01:14 pmI would never use the IPC PCB spacings values, who knows where they come from? What transients, dirt, moisture I don't see them spell that out as needed.It would give an unpleasant surprise - when putting a product through certifications, they do a "spacing analysis" to see if your creepage/clearances meet that safety standard. Nothing about IPC in that.Fundamental creepage/cclearance data comes out of many decades of study, by the IEC. IEEE also has their own data.Smokey, voltages are "stepped categories" for mains. Pic is an old example showing the (voltage step) mains categories, for global designs.They will include extra for the transients that are present. Cat. II, Cat. III Cat. IV you may have heard of those lol they also increase with voltage category. Who knows what this IPC-2221A green line is assuming but it might include that.Also, a factor is the voltage stress on the insulation material and aging effects. After years insulation degrades, ages - so again, extra space is given.Even with air, you get ozone buildup if you push the E-field intensity. So the 3kV/mm is not a great long term idea either. And there are other mistakes that can be made using that number.

author: TimFox
author_rank: Super Contributor
timestamp: August 31, 2025, 07:31:35 pm
content: Quote from: floobydust on August 31, 2025, 06:24:40 pm3 kV/mm is a value for voltage gradient, which is E-field.Ozone is complicated: it results when you have a spatial variation in that E-field, for example surrounding a needle point, or when the voltage gradient near a wire or other small feature breaks down the air, and the ionization grows under the influence of the field.In any event, that value does not hold when there is a surface involved.When looking at voltage gradient through an insulator (as opposed to along its surface), voids and bubbles will have higher field in air than the average through the material, enhanced by the dielectric constant of the insulator.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 02, 2025, 12:42:01 pm
content: It's unclear if the OP is asking aboutfunctional insulation(i.e. failure of the insulation affects device performance or reliability, but no direct hazard to the user) orprotective insulation(failure of the insulation can directly result in a hazard to the user via harmful leakage currents).AFAIK, IPC-2221A only deals with functional insulation, not protective insulation.For creepage/clearance for protective insulation, if you're planning to go through a real certification process, you need to determine the appropriate standard(s) that apply. IEC 60950-1 is probably a good starting point.