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Topic: Practical clearance for 220 VAC on PCB (Read 5 times) previous topic - next topic


I find all kinds of things on the web when trying to figure out the minimum clearance to 220 VAC on a PCB.  This relay from Sparkfun is made for 220VAC:

If I add this relay to my eagle design using their footprint, the clearance between the two output pins is 4mm.  So if it's only 4mm there, I don't need to have 8mm (like one site suggested) everywhere else.  (I'm tight on space or I'd be happy to put lots of clearance)  I just don't want to make something too unsafe.


How are you planning to connect the relay? If you have the 220V supply going to the common contact and one or both of the other contacts connected to devices to be powered, then if the worst happens and a large mains transient causes a discharge between the two pins, all that will happen is that the transient will reach the device that is supposed to be off. Depending on the application, this very likely doesn't matter.

It's much more important to maintain the 8mm clearance between the 220V pins and the relay coil pins/all the other non-mains circuitry on your PCB/anything the user might be able to touch.
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People normally cut a slot in the PCB as well as using a minimum clearance (otherwise moisure/dirt/ants can cause tracking and flashover). 

Make the space as large as possible (4mm is not adequate for a safety critical isolation).  An alternative is potting
the back of the PCB in silicone (electrically rated, not bathroom!), excluding moisture, insects and fingers all in
one go.

220Vac can have multi-kV spikes on it in real life (fridge motors with failed snubber, lightning strikes), so this is why a large
clearance is required.
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You have to clear all the copper over your mains to if using a double side board


You have to clear all the copper over your mains to if using a double side board

...so the board isn't enough of an insulator for 220VAC?

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