5

I am building a test probe PCB to test 75-115V. What is the minimum distance between traces that I should adhere to when building my PCB? I will have ground, 3v and 115V traces running next to each other, there will be low amperage, (1ma max), and the board is single sided.

Jason S
  • 13,950
  • 3
  • 41
  • 68
Reid
  • 2,030
  • 1
  • 21
  • 38
  • What's limiting the current from the 75V-115V source? – Jason S Apr 21 '12 at 18:58
  • It is 4 battery packs. I don't know the limiting current. – Reid Apr 23 '12 at 02:05
  • rough size per battery pack = ??? – Jason S Apr 23 '12 at 02:35
  • 18*6*6in, 30 batteries per pack. – Reid Apr 23 '12 at 18:43
  • 2
    @Reid I think what Jason S is saying is that battery packs can generally supply far more than 1 mA. Your intended load might be 1 mA, but if something goes wrong (say you tie the output wires together) your circuit board might see a lot more than that. You should generally put some sort of fusing between the battery and anything else. Otherwise, your PCB traces might themselves fuse in a fault situation. – Stephen Collings Sep 06 '13 at 12:51
  • When I have occasioan to lay traces for signals I dont want touching or need a bit more metal I make use of ALL the board space and place wide traces spaced far apart and very far from other traces that they should be isolated from, preferably on opposite ends of the board. I would extend wires to the other side of a board rather than making connections between 3V and 115V circuits in close proximity. A simple short between terminals or tracks can easily ruin your day. – KalleMP Nov 06 '18 at 11:37

2 Answers2

4

Obviously there are different standards for different environments and applications, so please don't take this as any sort of universal wisdom. But for the UL and CE clearance specs I work with, PCB trace clearance should be at least .2 mils per volt. So if you have 115V difference between two conductors, clearance should be at least 23 mils. Unless your board needs to be ridiculously small for some reason, you shouldn't have a problem quadrupling that.

If it still worries you, the PCB itself is an excellent insulator. Put the high voltage trace on the opposite side from the low voltage.

Remember, batteries can supply enough current to blow traces and components off your board, arc, and potentially explode the battery itself. If you're hooking stuff up to a battery, fuse the link.

Stephen Collings
  • 17,373
  • 17
  • 92
  • 180
  • 2
    Saw this after posting my answer: http://electronics.stackexchange.com/questions/22796/creepage-distance-for-pcbs-handling-line-voltage-ac Much more detail! – Stephen Collings May 25 '12 at 21:47
-2

Is it AC or DC that will be running on the trace? If DC then don't worry because that low of voltage won't arc. If it's AC then try to run AC perpendicular to the DC (opposite sides of the board would be ideal) so your collecting as little EMF as possible. Otherwise if your going to run parallel you'll need to do math to find the radius of the magnetic field produced by the AC lines and that would be your gap.

Stephen Collings
  • 17,373
  • 17
  • 92
  • 180
Emery King
  • 275
  • 2
  • 7
  • 1
    Note: electric fields extend to infinity (photons are massless outside plasmas). – tyblu Apr 21 '12 at 04:59
  • I was wondering about that actually. After i wrote that i thought i remembered that they extend infinitely like gravity. – Emery King Apr 21 '12 at 05:28
  • 2
    -1: 75VDC will DEFINITELY arc. – Jason S Apr 21 '12 at 14:32
  • To be precise: 75V may not be enough to initiate an arc (I'm trying to find this), but all you need is a momentary fault that causes current to flow and get interrupted, and trace inductance will initiate an arc, and it will be maintained until the arc is extinguished because the source of energy is gone or the traces burn up far apart. – Jason S Apr 21 '12 at 14:42
  • I've seen plenty of boards with power traces of voltages in that range at normal reasonable gaps. So I'm not going to go as far as explaining about atmospheric pressures and circuit faults on this one and assume it will be fine at any reasonable gap. – Emery King Apr 21 '12 at 20:25
  • And a normal reasonable gap is... ? – Jason S Apr 21 '12 at 21:42
  • .05 inch seems reasonable. I'm saying i probably wouldn't use any kind of math to solve it and just use the same gaping i'd use on anything else. – Emery King Apr 22 '12 at 00:39
  • -1: DC is as big if not a bigger arcing hazard at 75V than AC. Also any surface contamination or breaks in the soldermask combined with humidity will cause possible progressive electrical corrosion that can cause tracking and arc formation, more likely with DC. While 75V is not a huge hazard the DC aspect does not make it safer. – KalleMP Nov 06 '18 at 11:33
  • OP has a single sided PCB. – KalleMP Nov 06 '18 at 11:39
  • 1
    Thanks @KalleMP for bringing back this post that I forgot I wrote back in 2012. UPDATE: The first version of the board caught on fire multiple times. Not due to trace spacing, but due to poor isolation protection (Ahh, to be young, dumb, and working on a 38.4KW battery pack). Subsequent boards got significantly better as I progressed through my EE degree, and the final version ended up in commercial use =) – Reid Nov 26 '18 at 06:08