Creepage, CTI, conductor spacing and the Saturn PCB toolkit

[MrLikerBiker]

Hello, I've been looking into how to ensure conductors are properly spaced on a PCB to withstand HiPot testing.

The Saturn PCB kit allows the calculation of conductor spacing for a specified voltage level. This is presumably the creepage distance. However, it doesn't seem to take into consideration the CTI (comparative tracking Index) of the PCB substrate that is used.

How can this be?

Surely the conductor spacing should vary depending on the CTI value of the substrate used. For example, we currently use a standard FR4 which has a CTI of about 150. However, we could use a substrate that has a CTI of about 600 which should allow closer spacing of the conductors so long as clearance isn't being compromised.

Any advice?

Thanks.

[Neilm]

Are you looking for the creepage distance or the clearance distance. The creepage is what is dependent on the CTI, clearance isn't. Hi-Pot testing tests the clearance required for safety, not the creepage. The latter is more for longer term voltage exposure.

[Smokey]

Any of these design specs are going to be based on very specific safety certification requirements.  They have very specific product applications, and running configurations/condititons for the product to be certified.  The problem is that unless you know exactly who your customer is going to be and exactly what certifications you will need to comply with, it's damn near impossible (and also mostly arbitrary) to try to just throw darts and come up with anything realistic.
Most of the calculators and "general tables" are usually based on IPC2221 or IPC9592, but unless you know that is the only thing you need to comply with (and it never is) you will end up mis-specifying your thing and it will never pass when that time comes.

I've found unless you want to make a career of being a safety certification expert and work in a cert lab, you should really contract an actual safety certification expert that works in the cert lab you plan on using when you take your product to test.  If there is one guy that can tell you exactly what tests you need to pass, it's the guy that will be running those tests.

Now if this is just a personal project, then don't worry and just make the creepage and clearance as much as you can.  Follow the IPC guidelines if you want but don't sweat the CTI or pollution level or any of that junk.  If you are REALLY paranoid, just get a can of conformal coat and spray the crap out of your board when it's done.  Problem solved.