Will there be a problem if I stay almost in the border in trace spacing when dealing with line voltages?

Question

The IPC-2221 standard shows that 1.25mm will be enough at less than 3050m at 250V (220V is the line voltage here in Turkey), even if the coating gets damaged. However, people generally recommend 5mm even 10mm. Why is this precaution taken? What is going to happen if I stand in the border and make the trace spacing 1.25mm or 1.5mm or 1.75mm or 2mm? That is a lot of options :)

Here is the table for IPC-2221. (I just saw IPC-2221 in the first line in Google that's why I've chosen that. My question is not standard specific meaning it could be other standards too.)

1.25 mm =  50 th
1.5  mm ~= 60 th
1.75 mm ~= 70 th
2    mm ~= 80 th
5    mm ~= 200 th
10   mm ~= 400 th

Answer 1

There are various standards. Some companies I have worked for have claimed to go thru them and determined that 5mm spacing for "line voltage" was going to cover all the cases. That is deliberately a superset, so of course there will be individual standards less tight than that.

If your design can tolerate 5mm space, then do it and you're done. If not, you have to spend some time figuring out which standards apply, and then check them carefully to see what the minimum required spacing is for your particular type of device and target usage. For example, home and industrial often have different limits.

There are several reasons spacing requirements might be larger than you would think. The different levels of importance placed on these has something to do with different specifications.

1. Line voltage spikes. You say your line voltage is 220V, but that is nominal. At least 10% should be expected, which brings it to 242V RMS. The peaks of that are +-342, and that's with everything working normally. In reality power lines can have fairly large spikes on occasion. How large depends on what you believe and what probability you want to protect against.

2. Rectification. From above we have to expect +-342V on a good day. However, some line circuits may couple things such that one peak or the other is held at ground, so the worst case the high voltage may be double the ground-referenced line peak, or 684V in your case. Whether this is considered, allowed, or made a separate catagory varies.

3. Dirt. A perfectly clean PC board has quite high resistance and therefore would support very little leakage accross even a small gap. However, dirt is a fact of life, and accumulated dirt will significantly lower the insulation resistance, especially with high humidity. You may think that 10 MΩ/square is pretty good, but if two such traces are running next to each other for 10x their spacing, then you've got 10 squares in parallel and you're down to 1 MΩ. Again, how this is considered in a spec varies. How dirty is dirty? What are the consequences of a few 100 µA leakage? The answers differ, and thereby so do the specs.

Answer 2

What will happen is a function of the quality of the power applied to the board. Keep in mind that while 220V is the nominal value of the supplied power, virtually all power lines are subject to transients or surges. This can be caused from such things as spikes being induced into the electric grid by nearby lightning strikes, back EMF form large motors (such as compressors on a refrigerator) etc.

This is why it is generally recommended that expensive and/or sensitive electronic equipment be protected by surge suppressors installed between the equipment and the wall outlet. One way to achieve transient voltage suppression is to use a Metal Oxide Varistor (MOV).

Will you be including some kind of transient voltage suppression into your product? If not, consider what would happen if you use a 1.25 mm trace spacing and 2,000V "spike" appears in your input power. While one might think engineering for a 2,000V spike is excessive, consider that if a lighting bolt has a direct strike on the power line in front of your house you will likely have a spike much, much greater than that.

The risk here is not simply that your board fails, you need to consider fire hazards and risk of electrocution as well.