Trace width / spacing for Mains voltage

[IanB]

...has such narrow traces for 10A and it's certified by UL

I believe UL are only testing for gross safety violations that could pose a threat to life or property? I don't think they are concerned with good design or bad design. If your product self destructs but causes no harm, that is not their problem.

(Edit: Underwriters Laboratories essentially are acting on behalf of the insurance industry. They are testing whether use of a product may give rise to any insurance liabilities.)

[yanir]

I believe UL are only testing for gross safety violations that could pose a threat to life or property? I don't think they are concerned with good design or bad design. If your product self destructs but causes no harm, that is not their problem.

I guess I had put to much faith in them. I do want this to last, but I'll take causing no threat to life or property.

[ciccio]

I believe UL are only testing for gross safety violations that could pose a threat to life or property? I don't think they are concerned with good design or bad design. If your product self destructs but causes no harm, that is not their problem.
(Edit: Underwriters Laboratories essentially are acting on behalf of the insurance industry. They are testing whether use of a product may give rise to any insurance liabilities.)

I've never had a design certified by UL, but currently I have two prototypes under certification for CE marking by a Notified Laboratory.
They are big multichannel audio power amplifiers with a maximum ac drain of 15 A at 230V.
The mains 15 A fuse is on a PCB, that acts as a power distribution to  multiple SMPS. The track width is about 4 mm. The lab made a number of thermal tests, including one with a blocked cooling fan and one with blocked airflow, reading a lot of thermocouples inside the amplifier, and one thermocouple was taped over one of the tracks carrying mains current.
They said that the temperature rise should not exceed the maximum temperature allowed by the PCB laminate manufacturer, in the worst condition. There are no minimum sizes.
Obviously, with fiberglass  double layer laminate, this temperature is higher than with "cardboard" single layer, so your track will be allowed to carry an higher current value.

This is not a case with wires: there are standards that set minimum copper sizes in function of current (e.g 1.5 mm2 from 10 1o 15 amps,  0.7 mm2 for 5 to 7.5 A, etc...

[IanB]

They said that the temperature rise should not exceed the maximum temperature allowed by the PCB laminate manufacturer, in the worst condition. There are no minimum sizes.
Obviously, with fiberglass  double layer laminate, this temperature is higher than with "cardboard" single layer, so your track will be allowed to carry an higher current value.

This is not a case with wires: there are standards that set minimum copper sizes in function of current (e.g 1.5 mm2 from 10 1o 15 amps,  0.7 mm2 for 5 to 7.5 A, etc...

We can believe of course that exceeding specified thermal limits might lead to the possibility of failure with fire or shock hazards resulting. So this kind of testing seems quite reasonable.