Creepage Vs. Isolation Cutouts

[nrclark]

Hi all,

I'm working on a high-voltage DC design which needs to consistently be able to switch 400V on and off. In prior designs, I've always made sure to space and traces reasonably far apart, which I suppose would be relying on the 'creepage' method of arcing. After watching Dave's video on multimeter protection, I got pretty curious about using slots instead. Is there much gain? Can a 0.100"-spaced trace be packed tighter if there is a slot in-between traces? Should I put one there anyways? I'd like to hear people's experiences.

[jeroen74]

Maybe this link helps, it's from a certification point of view, which after all, you need to comply to anyway (assuming it's a commercial design)

http://www.ce-mag.com/ce-mag.com/archive/01/03/ProductSafety.html

[Lightages]

The creepage distance for practical purposes will not change because you have a slot cut into the board between the conductors. The distance is the "insulation". The difference when you cut a slot is that there is no physical path for charred material to provide a conductive path between the conductors should something blow up or vaporize.  Without the slot, the material can deposit a nice semi-conductive, or fully conductive, path from one conductor to the other. When you have a slot this cannot happen. When you put insulating material into this slot or between conductors then the you are insulating at the same time as maintaining a creepage distance.

[krish2487]

IIRC the minimum distance for 230/240V countries for creepage is 7mm.

As lightages said, a slot will only help prevent formation of a low resistance path between the high voltage points in case it arcs.

It is a good engineering practice to provide for 2x minimum distance AND give a slot.
(that is unless of course, your design has other limitations)

Almost all on this forum will concur with me when i say, when it is concerned with mains then overkill in terms of absolute safety is much better than making the end project look pretty or fit into a particular enclosure.

I d also suggest using conformal coatings. All the conformal coatings for PCB use are non conductive in nature. They provide an additional layer of "insulation" and do not readily allow the exposed solder joints to arc readily. BUT it is no way a tradeoff for minimum creepages.

[jeroen74]

Conformal coatings increase the CTI, or Comparative Tracking Index.

[T4P]

How about some silicone gunk covering up something? 

[krish2487]

@jeroen

I am not familiar with CTI but after reading what you suggested, I googled it.

There is only a specified ranges and PLCs assigned CTI rating and performance.

From what i infer - A higher CTI allows for a smaller creepage distance.

Did I understand it right??

or in the exactly opposite way???

Thanks for the information !

For anyone else who is interested in CTI this page sums it well :-

http://www.patentstorm.us/patents/6563056/description.html

[jeroen74]

I tore down a DVD player a while ago, and found this on the power supply PCB.



Note the shark teeth and the black stuff. I assume the teeth are spark gaps and that the black stuff likely is a CTI 'improver'.

[Neilm]

The creepage distance for practical purposes will not change because you have a slot cut into the board between the conductors. The distance is the "insulation". The difference when you cut a slot is that there is no physical path for charred material to provide a conductive path between the conductors should something blow up or vaporize.  Without the slot, the material can deposit a nice semi-conductive, or fully conductive, path from one conductor to the other. When you have a slot this cannot happen. When you put insulating material into this slot or between conductors then the you are insulating at the same time as maintaining a creepage distance.

The creepage distance between two points will increase if there is a slot in the PCB. The clearance distance between the same two points will not increase. Clearance is defined as a straight line distance in air between two points. Creepage is the shortest distance alone a material between two points.

Neil