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I'm trying to build a circuit that uses 400 VAC mains, with a peak voltage of 56 5 V, but let's consider 625 V maximum to take into account mains voltage fluctuations.

I always see fuse holders soldered into PCBs. A question that always comes to my mind is that if the isolation on the board itself fails and it shorts out before the fuse, it could possibly cause a fire without any protection to the circuit.

What is the likelihood of the isolation on a PCB failing? If it is as much as the plastic of the fuse holders, then it's OK to put fuse holders on PCBs. But if it is higher, then would it be a lot safer to chassis-mount fuse holders instead of soldering them into PCBs?

As far as the project I'm trying to build, and considering I'm going to be using a perfboard which has less spacing between holes, which would be the board isolation distance considered safe or which would be the number of holes that I need to leave between the poles with 625 V maximum considering a perfboard with 2.5 mm pitch (even with fuse holders not on the board)?

And should I chassis-mount the fuse holders or can I safely put 625 V on the perfboard without any protection before?

And how can I know the maximum voltage that I can put into the perfboard since no perfboard specifies it?

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    \$\begingroup\$ Perfboard != 400 VAC mains. \$\endgroup\$
    – winny
    Dec 27, 2022 at 21:00
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    \$\begingroup\$ If by "uses" you mean "is powered by", then at the risk of stating the obvious, I would recommend getting a fully-assembled power supply to bring mains voltage down to whatever it is you're using on the board. \$\endgroup\$
    – vir
    Dec 27, 2022 at 21:04
  • \$\begingroup\$ @vir It is powered by mains voltage, but not only. The actual core of the project involves relays that switch mains voltage and voltage measurement modules. \$\endgroup\$
    – Dinis
    Dec 27, 2022 at 22:34
  • \$\begingroup\$ Perfboard is what? FR2 or FR4 with holes, FR4 or FR2 with holes and plating, FR2 or FR4 with stipes of copper? \$\endgroup\$ Dec 28, 2022 at 3:16
  • \$\begingroup\$ @Jasen What I mean by perfboard is "FR4 or FR2 with holes and plating" \$\endgroup\$
    – Dinis
    Dec 29, 2022 at 2:31

2 Answers 2

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First and foremost, mains voltage of any kind is extremely hazardous. Only experienced people with the proper tools and safety knowledge should be doing this kind of thing.

To answer the question, there are two things to consider which are creepage and clearance. Creepage is when two conductive parts conduct along the surface of the PCB and is calculated by the distance along the surface of the PCB. Creepage does not apply to inner layers. Inner layers are based on the breakdown voltage of pcb material. Clearance is when two conductive parts conduct through air (arcing) and is defined by the distance of the air gap between the two parts.

The creepage, clearance, and spacing for inner layers based on the dielectric breakdown are all calculated by taking the parameters of your application such as voltage, PCB material, pollution degree, and frequency. And then applying them to an industry standard. A common industry standard for clearance calculation is IPC-2221B. The creepage standard that is common is the IEC 62368-1.

Example for calculating creepage: The voltage will be based off your RMS voltage, so in this case 400VAC. I would factor in an additional 5% to the nominal voltage which gives you 420VAC. Then you determine what material group you're using for the PCB. A standard FR4 is in material group IIa/IIb. Then determine your pollution degree. You can find which pollution degree you'll need to select by searching online the different levels and how they're defined. Once you have all of this, you can use a table or online calculator like this to calculate the required spacing along the surface.

Notice how big of a difference changing the pollution degree from 1 to 3. Pollution degree 1 is be 1mm while pollution degree 3 is 6.3mm. This is why there's no "general rule of thumb". That is unless you always design to the worst case scenario which isn't a practical thing to do.

Clearance calculation example: This is more straight forward and is calculated based on the Peak AC voltage or DC voltage. You can use an online calculator like this. The only thing to point out is that the "Coated" calculated distance is based on if the board has additional coating like conformal coating. A common misunderstanding is that solder mask counts as a coating. This standard does not consider solder mask as a coating, so only use that number if you are actually using an additional coating on the board.

Note that frequencies > 30KHz require a different standard.

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  • \$\begingroup\$ Thanks for the very detailed answer. In order to determine the pollution degree and material group of a perfboard, it needs to be specified by the manufacturer, right? The problem is that I cannot find any that contains that specification. Is there are general rule that denies the use of perfboards for mains voltage or it is possible to use perfboards for mains voltage? \$\endgroup\$
    – Dinis
    Dec 28, 2022 at 1:34
  • \$\begingroup\$ The material group for perf board will be IIIa/IIIb. The pollution degree is determined by you. The pollution degree will depend on what environment it is in. Pollution degree 1 is when basically no dust or debris will come in contact with the pcb or parts. Whereas pollution degree is enviroments that can accumulate conductive debris. Look up the definitions of each pollution degree to determine what yours falls under. Pollution degree 2 is common for general DIY stuff. You can also bring sections or the entire board to pollution degree 1 by potting it. \$\endgroup\$ Dec 29, 2022 at 15:23
  • \$\begingroup\$ Got it. Thanks. \$\endgroup\$
    – Dinis
    Jan 2, 2023 at 0:40
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Depending on what type of perfboard you have it might be fine.

The bare stuff with no metal should be fine so long as you leave plenty of space between your wires, (eg 2 empty holes gets you about 6mm between wires.)

image source: Twin Industries - Protoboards - 7100 series data sheet:
enter image description here

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  • \$\begingroup\$ Thanks for the answer. I don't have experience working with those unperforated board and I can't find any info on the internet, so allow the noob question: How do I solder the components after drilling the holes if it has no metal parts? Should I add the metal parts myself and make the PCB traces or just use wires and not make any PCB traces myself? \$\endgroup\$
    – Dinis
    Dec 29, 2022 at 2:30
  • \$\begingroup\$ this board has holes, just no metal. just use wires threaded through the holes. \$\endgroup\$ Dec 29, 2022 at 8:19
  • \$\begingroup\$ Yeah, I understand it has no metal, that was exactly my problem. Alright, wires threaded through the holes, thanks for the suggestion. \$\endgroup\$
    – Dinis
    Dec 30, 2022 at 1:20

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