What is the general rule of thumb regarding UL's creepage clearance regulation?

Question

I've read a whole bunch of material but I'm still quite confused regarding creepage clearance, specifically:

1. How to calculate the minimal clearance on a PCB board?

2. How to calculate the width of the air gap (pcb cutout, hollowed slot of pcb) to enhance clearance should you really, really not be able to meet the minimum clearance in question #1 and decide to use air gap instead?

Elaboration:

Based on what I've read, for 300Volts WORKING circuit, the minimum clearance is expected to be 3.2mm, so I assumed, since my working voltage is 220v and maximum input voltage is 305v, a 3.3mm clearance will suffice. But according to this online calculator (http://www.creepage.com/) the minimum clearance is only 2.1mm (I could have remembered it wrong in which case I apologize) so how should I actually crunch the numbers?

I also heard that UL test labs will tune the input all the way to 2kv, sometimes higher (though I don't know for how long), and apply it to your board and your board have to survive without giving out sparks or outright be reduced to nothing but a charred wreck. This further perplexed my understanding of the minimal clearance: there is no way I can cater 2kv???

My board is tightly packed and some places really can't enable me to afford the 3.3mm luxury, which bring us to question #2, how wide should I make the air gap to offset the negative effect of the small distance? Is there a way to calculate that too?

This is really all too overwhelming as there are just too much information to process. So what would be the general rule of thumb, speaking particularly from experience?

BTW my pollution level is 2.

Answer 1

The important thing to remember about this sort of things is that these are ratings. Ratings are about safety in the edge cases of a big mains spike and heavy board contamination and high humidity and ... you get the idea.

Ratings make all sorts of assumptions about the expected safety level required, the likely board contamination, the likely spikes expected on mains supplies (1500v is not uncommon!) etc. This is why you should not expect exact agreement between UL figures, and a website doing calculations to IEC60950. Slightly different standards, slightly different assumptions.

You may think you can scale between 300v and 220v? For Ohm's law, yes. For ratings, not really. Ratings tend to specify bands, though they'll sometimes have formulae.

Obviously the safest way is to calculate using both UL and IEC60950, and choose the largest. You say you haven't the space for that. It's quite common to see a routed slot in a PCB underneath things like optocouplers and transformers used for isolation. This increases the creepage distance to the perimeter of the slot rather than the diameter. The clearances for air and creepage surface will usually be different. Read the standard carefully.

So the next question is, is this a one-off for personal use, or is this a commercial product, that you hope to sell?

If you are going to sell this thing, then you need to find the space for the rated clearance. Your product won't get approval, and you'll find it difficult to sell without. If a customer's factory burns down, you may get sued by them, as their insurance would be void for using non-rated equipment, even if it wasn't yours that started the fire.

If this is a one-off for personal use, then the previous paragraph doesn't apply. Obviously, larger is better. It's quite possible to make something that works, but would not get approval under UL or IEC60950. The risk is yours.

Answer 2

This further perplexed my understanding of the minimal clearance: there is no way I can cater 2kv???

As far as I understand, the creepage only needs to be to the nominal voltage, not whatever might be thrown at the supply.

This is from a testing report from an ETL for AC mains:

Testing was performed at ±0.5 kV with surges coupled between L1 and L2 and at with surges coupled ±0.5 kV, ±1 kV between each line and ground. A minimum of 5 surges of each polarity was applied.

This was done with a Keytek emc pro

As far as spacing goes, it really depends on what equipment the supply will go into. Industrial and commercial devices have different voltage requirements. It also matters what IEC requirement the supply will fall into as shown in the following table:

Source: http://www.epsma.org/EPSMA_Guidelines%20for%20Safety%20Requirements%20for%20AC-DC%20Power%20Supplies%20-%20DIN%20rail.pdf

It looks like they are using 1kV (table here) for a creepage baseline as they use 1.5mm if your under UL508 (well above 240V, so they must be including the spike voltages in the spec). The other standards use 4.0mm or 2kV (well above 400V of industrial equipment)

At any rate, if this is your first go around a regulator consultant will save you time and money if you have questions. Or you could just design for the max standard and hope that it passes through regulatory.

How to calculate the minimal clearance on a PCB board?

Find the regulations that your product will be tested to and look through the specs (which are expensive and paywalled FYI).

How to calculate the width of the air gap (pcb cutout, hollowed slot of pcb) to enhance clearance should you really, really not be able to meet the minimum clearance in question #1 and decide to use air gap instead?

Use the table in this question: Minimum distance between two connectors in a high voltage PCB design

Most off the shelf connectors conform to these standards (and will need to be UL or equivalent tested, and you will list them in your design so when you go to regulator they won't have to be retested. Untested connectors may need to be tested). So look for the standards that the connectors were tested to (sometimes available on the product's webpage or with the manufacturer)