I'm designing a pcb for battery management. There are -250 V, 0 V, and 250 V traces.

The most conservative results of the calculator on this website tell me to leave 3.1 mm for 500 V and 1.85 mm for 250 V, so I started drawing traces with spacings of 5 mm and 2.5 mm.

I was wondering if this is safe enough, or maybe too conservative. These minimum spacings might be enough to prevent an arc from forming, but my fear is that if one somehow forms, it won't get extinguished since ionized air has a lower resistance.

Is this justified or is it unrealistic at these 'low' voltages and already accounted for by the design standards? I'm trying to find a balance between trace spacing and trace width (lower losses).


Appreciate the input so far. I've made a temporary design of which I'll post some screenshots to make this question a little more specific.

I've left 3mm between 250 V and 6mm between 500 V and plan to coat all exposed pins in conformal coating.

pcb_with_guides.png includes the yellow guides (lines and circles) I used to maintain spacing.

  • V+ is +250 V, V- is -250 V, V0 is gnd. So the IC's and relay coil voltages sit around V0.
  • J1 and J3 are 1 kV rated screw terminals for wires coming from the battery pack.
  • K1 is a 1 kV DC DPST relay.
  • The ICs are hall-effect current sensors (isolation rated for 1414 VDC working voltage).
  • J2 and J4 are 1kV 3-pin connectors for power in/output and parallel connecting to a second pack.

Looking for general thoughts/feedback on this. I was also planning to mill out the pcb along the yellow lines. How thick should I make the slots? Does it hurt to leave in a couple mm of pcb at the center of the longest slot to maintain strength?

Edit 2: Updated pcb layout

PCB with spacing guides


Manufactering preview top

Manufactering preview bottom

  • \$\begingroup\$ I don't feel confident giving an answer since this is a question I've had in the past as well, but it would be helpful to have a bit more info, specifically what you're isolating against. Are you concerned about simply arcing across the terminals of an HV system, or are you isolating a low-voltage control circuit (or worse, SELV or medical applications)? \$\endgroup\$
    – nanofarad
    Commented Aug 26, 2021 at 17:42
  • \$\begingroup\$ 3.1mm = 122mil. I would say it is good, on FR4, for the initial run. The main factor is the dielectric between two traces. You can cut out the FR4 (air gap) between two signal paths. BTW, that is way way too close for me. Once arc starts, and there are good chances of, for that with 122mil space @500V AC, it will burn everything around and your customers town. One of the standards defines 60V is high for safety (forgot the detail), and keep the space (creepage) over 6mm \$\endgroup\$
    – jay
    Commented Aug 26, 2021 at 17:51
  • \$\begingroup\$ @nanofarad Yes, I'm just concerned about arcing across the battery terminals. \$\endgroup\$
    – Cedric
    Commented Aug 26, 2021 at 18:23
  • \$\begingroup\$ @jay I'm not sure I understand what you're saying. Do you think 3.1 mm is enough, or do you feel it's way too close? Why would you cut out the FR4 between the traces given that the dielectric strength of FR4 is higher than air? \$\endgroup\$
    – Cedric
    Commented Aug 26, 2021 at 18:32
  • \$\begingroup\$ I had the same question about air gaps at one point: electronics.stackexchange.com/q/74244/2028 \$\endgroup\$
    – JYelton
    Commented Aug 26, 2021 at 18:52

1 Answer 1


IPC-2221, "Generic Standard on Printed Board Design", has a table titled "Electrical Clearance" (paragraph 6.3 in the 1998 version). You can find old PDFs of IPC-2221 on-line.

For 301 - 500V they suggest a minimum spacing of:
B1 Internal conductors: 0.25 mm
B2 External conductors, uncoated, sealevel to 3050m: 2.5 mm
B3 External conductors, uncoated, over 3050m: 12.5 mm
B4 External conductors, with permanent polymer coating (any elevation): 0.8 mm
A5 External conductors, with conformal coating over assembly (any elevation): 0.8 mm
A6 External Component lead/termination, uncoated: 1.5 mm
A7 External Component lead termination, with conformal coating (any elevation: 0.8 mm

Bn is bare board, An is assembly.

This can change due to regulatory issues (UL, VDE, ...) and environmental conditions (humidity, dirt, dust, ...). I won't give a suggestion since you'll need to decide for your particular situation, however, 0.8mm would be too small of a gap for my tastes at 500V.

You're in a danger voltage range where a little tiny arc can turn in to a big arc that vaporizes metals which can cause a chain reaction of arcs throughout your system. I've had high current DC arcs melt stainless steel bolts and and AC arcs vaporize copper in a vacuum chamber (chamber operator forgot to turn off heater power when pumping down). Those were not fun events to clean up.

  • \$\begingroup\$ Thanks for the information. Could I get your thoughts on the tracing layout I added to my question? \$\endgroup\$
    – Cedric
    Commented Aug 28, 2021 at 12:19
  • \$\begingroup\$ @Cecemel As others stated, hall effect sensors can be influenced by external magnetic fields. If you rotate a hall effect sensor, you can see the output change due to the Earth's magnetic field. There are hall effect sensors that have reduced sensitivity to external magnetic fields. Follow the recommended layout in the data sheet. Is your relay rated for 250V DC? If it is only rated for AC operation at your operating voltage, it will have a very short life span due to arcing. HV DC is difficult to mechanically switch. \$\endgroup\$
    – qrk
    Commented Aug 28, 2021 at 17:21
  • \$\begingroup\$ @Cecemel Your layout looks reasonable with regards to spacing. Adding an air gap is helpful since dust can't settle on the gap. If you do have an arc, the gap won't have charred bits. Avoid pointy corners on your pours. Some PCB drafting programs allow you to control the width of the draw lines which allows you to control corner radius of your pours. \$\endgroup\$
    – qrk
    Commented Aug 28, 2021 at 17:26
  • \$\begingroup\$ Thanks. The sensor datasheet talks about differential sensing to minimize the influence of external fields so I assume that's what you mean. The relay is indeed rated for 1000 V DC. (As reflected by its cost of €85..) I'll add a 2-ish mm air gap. I'd already looked for ways to smooth the corners of the pours. I'll redraw them with a thicker line. \$\endgroup\$
    – Cedric
    Commented Aug 28, 2021 at 18:25

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