Minimum PCB trace spacing for 500 VDC?

Question

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).

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Edit:

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

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.