Proper implementation of creepage and clearance on high-voltage PCB

Question

I am designing a 3-phase voltage detector based on optocouplers as shown below:

^{simulate this circuit – Schematic created using CircuitLab}

For this I need to follow DIN 60664-1 and my biggest problem is to understand how to apply those rules for creepage and clearance. Namely, I figured out that with polution degree 2 I need to stick to the following measures: 3 mm minimal clearance and 4 mm minimal creepage. Fine, but how is that achieved in practice? From the circuit that I have shown, how do I determine the creepage and distance between L1_1 and L2_1, or between L1_2 and L2_2? Does that mean that for every point in the circuit I must calculate the voltage and then determine the creepage and clearance in the standard?

Take a look at the resistor connected to L1 and L1_1. It is 0805 size, but should leg L1_1 also have clearance and creepage of 3 mm and 4 mm with respect to the L1?

At which point may I bring lines closer to each other - legs of the capacitors connected with the optocoupler - there can never be a breakdown between them because the diode keeps that voltage low.

Answer 1

Disclaimer: I am a complete and utter n00b at EE that probably knows less than you do. Take this with an enormous grain of salt.

how do I determine the creepage and distance between L1_1 and L2_1

You don't. Clearance and creepage don't mean anything in a schematic. They come into play when you start to physically lay out a device... and hopefully your design software will help you out, here.

Clearance is the shortest air distance between exposed conductors. Imagine taking a string and pulling it taught between the nearest two points where conductor is exposed (e.g. a component lead or solder pad). Insulators may push the string aside, increasing clearance distance.

Creepage is the shortest distance along a surface (usually, your PCB). Similar principle, but the string has to stick to the surface, and now it is air gaps that may increase creepage.

This page and this other answer have some helpful illustrations.

Does that mean that for every point in the circuit I must calculate the voltage and then to determine the creepage and clearance in the standard?

Strictly speaking, I think the answer is "yes". Practically, if I'm using e.g. a 5V power supply, I would just use the creepage/clearance values for 5V everywhere on the theory that I won't ever see higher voltages... although this doesn't account for spikes due to inductive loads, so you might want to keep those in mind if applicable. (Doesn't look relevant to your specific example, but in general...)

Answer 2

To answer the first question about whether you should or should calculate voltage for every part.

Generally calculating voltage for every part and than editing the clearance accordingly is a mistake and a waste of time. Firstly, PCB designing software should have editable rules for this purpose so its checking the clearance for you. Secondly, if you know the clearance for the circuit, u use it universally for it. Think of it like this: if you have clearance calculated for every component separately, this can happen: if some component get broken and produces short, the higher voltage propagates further and can cause a chain reaction of shorts.

If you take a lok at the resistor connected to L1 and L1_1. It is 0805 size but should leg L1_1 also have clearance and creepage of 3mm and 4mm with respect to the L1?

The clearance is a distance, which conductive parts (like traces and pads) have to be separated with. If you think of it like this than the answer is yes. Whichever conductive part, that is on different potential, should have the clearance between it.