PCB Trace Width : Common (LINE) Trace for 2 Relays
You are designing a product that has voltages over 60 VDC or 42 Cop inside.
That means that you should look for the right UL norm and read it.
An UL safety engineer would help you a lot.
The distances in air 4.2 mm or 8.4 mm that you mention are from the primary circuitry to the secondary circuitry (SELV circuit).
Those distances actually depends on the maximum voltage that your product develops. There is a table that gives you the minimum distance as a function of the maximum voltage that the UL engineer measures on your product.
In a dusty environment (class 3) also distances are further increased.
The relays must be UL recognized components.
From the Line trace and the Neutral trace the distance is solely regulated by the electric arc that may arise. At 230 VAC the minimum distance is 2.4 mm.
UL doesn't care if you draw those traces at a distance of 0.1 mm. The UL engineer says &quot;The fuse will blow but the end user is safe&quot;.