AC Routing Trace Width and Clearance


  • Hardware Contributor

    I'm attempting to trace out some AC connections on my latest PCB and would like some advice on trace width and spacing. I use circuitcalculator.com and desmith.net for my trace width calculations and then I use advancedmsinc.com for the spacings.

    The specifications for part of my AC portion is 4.3A @ 240VAC. The board is 1.6mm and 1oZ Copper. From using the calculators i'm getting the following results;

    Trace Width: 88.4-90.68mill (2.25-2.3mm)
    Trace to trace spacing: 15mil (0.381mm)
    Trace to external component exposed lead: 30mil (0.762mm)

    I'm starting to doubt those spacing result though, however i'm unsure on what to use because i keep seeing different figures floating around on forums as to what spacing to use. I do know that the trace width is defined by the current and the spacing is defined by the voltage, but that is about all i know regarding this, could someone through any light onto the situation?


  • Hardware Contributor

    @Samuel235 0.3mm for 240VAC sounds insane. I don't think your source of info is valid for anything above logic level voltages. There are some pretty good info about creapage and clearance by @sundberg84 in the thread https://forum.mysensors.org/topic/1607/safe-in-wall-ac-to-dc-transformers/317. There were some calculators there too.

    Width calculators you got are probably ok. Solder reinforce if necessary.


  • Hardware Contributor

    @m26872 This is exactly why i made this thread to be honest. I had bad feelings about that information and wanted to do two things; make others aware of this, and get some more information for myself. I have read that post from Sundberg before, i forgot all about it. I will take another read tonight. Thank you.


  • Hardware Contributor

    Found another article, regarding PCB safety regulations, this time by Silicon Labs. Link here. I would have liked to have this module safe for external use, but even inside of a sealed compartment (Which i don't feel too comfortable about with there being so much chance for heat to be generated in this) it would still be classed as pollution level 3/4 In my opinion. Then it would need around 3.2-4mm clearance, depending on the material group. Which i'm not too clued up on, more research is needed in this area. There is not a chance in the world i'm working with 4mm clearance on this board, not while trying to get it under 50x50mm.

    I'm starting to feel that depending on the material level, my clearance would be around 0.56-1mm. This is just first quick workings based on 20 minutes of research. I will need to put more time into this aspect i think. I can't be taking chances when it comes to AC Powering.


  • Hardware Contributor

    So from reading a provided link thanks to a previous post from @sundberg84, https://www.ieee.li/pdf/essay/safety_considerations_in_power_supply_design.pdf, it shows that we need to asses the amount of insulation needed to determine any advanced safety measures that need to be taken for AC power supply routing.

    What level of insulation do we choose here, Functional, Basic/Supplementary, or Reinforced?

    0_1464796048163_Insullation.jpg

    The way i have understood this information is that if a Power Supply board is going into an enclosure, does this mean that it only has to conform to the secondary insulation level, or have i misunderstood the type of insulation being discussed?


  • Hardware Contributor

    @Samuel235 I depends. Bare high voltage parts on PCB means no safety insulation. If inside it's inside a completely sealed (no connecting wires) enclosure, then the enclosure is the "supplementary" or "reinforced" type. Some PBC coating could probably provide the "basic" type.
    If there are any wires going in or out through enclosure and/or connecting to PCB inside, then these must be considered in the "system". Typical a touchable USB connector in the wire end means the need for full safety insulation on your PCB between that conductor and high voltage conducting traces. Usually provided by transformers an safety (X or Y) capacitors and the creepage/clearance distances you got above. >3mm for 240Vac.


  • Hardware Contributor

    @m26872
    So basically, if the container is not 100% sealed I need to have >3mm separation between the AC lines, is this what you're saying?
    OR
    Do i only need the >3mm between the AC line and the connection that is coming out of the module?

    I do plan on having some ventilation on my container and also there are 5 screw terminals on the board, one is the input AC, two are the relay output AC (x2) and then 2 more for external switches.


  • Hardware Contributor

    @Samuel235
    Sorry, but "sealed" was not the best word. Of course IP-class affect your pollution level, but sure you can have a vented and still 'safe' enclosure.

    I think this document has been linked to before and has some info: https://www.ieee.li/pdf/essay/safety_considerations_in_power_supply_design.pdf
    Important to understand the different aspects of safety (direct, indirect etc). It was several months since I read it, but I 'think' you should follow the creepage/clearance PCB design standards equally for line-2-line, line-2-earth (earthed user). For parts classification there're differences though (eg. X or Y cap).

    I'm no expert and I try to avoid guessing and don't have time to research more. Nor do I remember what I've learned. Perhaps someone else knows about this?


  • Hardware Contributor

    @m26872, that is the article that my image has been taken from 😉

    I have read through that but again, i'm confused regarding the pollution and the separation needed for such designs. Its not making complete sense to me.


  • Hardware Contributor

    @Samuel235 haha, sorry about that.. embarrasing, now you see how focused I'm... 😖


  • Hardware Contributor

    @m26872, don't be sorry, it shows that i'm at least looking through the correct documents haha.


  • Hardware Contributor

    @Samuel235 Why don't just accept the challenge and design with >3mm creepage?


  • Hardware Contributor

    I'm tempted to just do 3mm to be honest. But i would like to know the theory of why and how its worked out to be that. I'm also attempting to get it all located on a 50x50mm board along with the two relays, screw terminals for them and 2 switches too. Oh and all the DC circuitry too. So the smaller the creepage i'm allowed the more possible it would be. But i'm not attempting to sacrifice safety for size at all.


  • Hardware Contributor

    Does anyone have any opinions of voltages and current through screw terminals? I'm looking around for terminals for AC power, i can get some 3.5mm pitch that handle >250VAC and then other 3.5mm will only handle >100VAC. I know there is different grades of plastic and metal, but can there seriously be that much difference or is this an instance of the manufacturer just covering their backs and i would be fine to use any terminal?


 

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