4kV high pot test

[limct]

Hi,
can anyone shed some light on this...what should be the minimum spacing between the +ve and -ve traces on a FR4 PCB, in order to pass in a 4kV high pot test.

Thanks!

Best regards,
ct

 

[BobbyNewmark]

The real question is the purpose of the isolation barrier. Do you want life-critical safety isolation from a source that can supply thousands of amps? Are you just wanting static electricity to not destroy a sensitive part? Does it have to pass at 95% relative humidity in a marine environment? Does it have to work at high altitude?

 

[logbook]

Any standard that calls for the 4kV test would also specify the creepage and clearance distances, depending on the comparative tracking indexes of the materials involved. These are all written down in EN61010 and/or EN60950, both of which are approved safety standards. Unfortunately you have to buy the standard(s). Cost >£100 each.

I would 'guess' at a creepage distance in excess of 4mm.

 

[limct]

Hi Bobby and Logbook,
Thanks for the reply. Or perhaps let me rephrase my question, say if I see the voltage breakage happen to a distance of 1mm for a given material at 1kV, would it be right to estimate (approximate) that when the distanec is 2mm, the breakage should then only happen at 2kV?

Thanks a lot.

Best regards,
ct

 

[jimkirk]

Breakdown is non-linear and dependent on many things. It depends on the geometry of the electrodes. For example a smooth, spherical electrode might have 35 kV breakdown with a 10 mm gap, but only 10 kV with a needle gap.

It also depends on air pressure. Google "Paschen curves".

For insulating materials, it somewhat follows a square root law, so 1 mm material might break down at 1 kV, but it could take 4 mm to withstand 2 kV...

According to NASA Technical Note D-7948, “inhomogeneity of leakage resistance within the body of the dielectric coupled with poor heat conductivity can produce high temperatures with attendant chemical changes. These changes can cause a decrease of resistivity by several decades of a portion of the material. The thickness of the dielectric is therefore effectively reduced and can lead to complete failure. This is the probable cause of the thickness effect-the variation of material dielectric strength with thickness in which the corona threshold voltage increases with dielectric thickness as expected, but the dielectric strength drops markedly.”

 

[OperaHouse]

Try to find the book GASEOUS CONDUCTORS by James Dillon Cobine. This is the bible on things that go arc in the night. Even though the last copyright is 1958, I have a paperback version and believe it is still in print.

 

[coconutalley]

According to a table in "Printed Circuit Board Design Techniques for EMC Compliane", spacing for 2,000Vrms is
8.4 mm [0.33"] and 5,000 Vrms is 17.5mm [.69"]. Your case the spacing is between these two. The Hipot voltage
is determined usually by adding more voltage on top of
the normal voltage. For over 1kV, they add 1kV on top.
If you put 0.6 inch it'd probably be fine.

 

[zurc]

I have a "rule of thumb" when dealing with tracks isolation. DIL-6 packages of some optocouplers are certified for 5KVrms isolation (under certain condicitons of humidity), and the pitch between isolated pins is 7.62mm. Therefore, 7.62mm must be enough for 5KV track isolation.