What you're overlooking is you keep measuring from something to ground. Neither of the power switch terminals are connected to ground...
PRR, I agree with you, when you say if you measure the voltage potential across a open switch, you measure line voltage. Yet if you measure to ground when the switch closed ...
I'm not PRR, but I think a picture is worth a thousand logical fallacies...
We've been arguing which of a switch's 2 voltage ratings matter: The contact (voltage) rating or the dielectric strength of the switch housing. See the attached image; with the switch open the voltage between the contacts is always the line voltage, regardless of which power cord line the switch is connected. If the switch is closed, the voltage between contacts is zero (because they're connected).
What you keep referencing is ground, when the switch only sees ground in the form of the chassis holding the switch housing. In that case, the dielectric strength of the switch is in play.
An
actual switch data sheet for a switch with a 120vac contact rating shows its dielectric strength (and therefore rating from contact to case) to be 1kV.
... if you measure to ground when the switch closed and the amp operating, the voltage is very low. on the neutral line, and line voltage on the hot wires. ...
To paraphrase PRR, demonstrate the courage of your convictions: With an amp power switch on, lick your thumb and bridge from the power switch contact connected to Neutral to the grounded chassis. You'll only need to test that once.
When the switch is closed, the switch contacts are connected and so the hot and neutral lines are connected. Both lines should be considered hot, because the power company enables current to pump through that circuit preventing the Hot side from collapsing to zero volts.
I think you're conflating the ability to use a choke with a lower insulation resistance from the choke coil to (grounded) case when the choke is inserted in the earthy side of a power supply, with changing ratings allowed for a switch depending on which line is switched. The former is true, but the latter is false.
... If you wire a switch into the neutral line, the material selection for the switch contacts are not as critical, You need a enough mass to keep from melting the contacts during the initial surge. ... My experience working with 480v three phase, indicates high content silver contacts are usually specified. While 125v contacts in a switch are hardened copper.
Contact material is about power/current passing through the switch contact and is not relevant to what we're discussing.
... However, if you wire a switch into the hot side, the contacts in the switch see peak to peak voltage, while the switch is closed. ...
When the switch is closed, the contacts are at exactly the same voltage (because they are directly-connected).
With the contacts open, the Hot side continually varies from 0v relative to Neutral, a positive peak voltage relative to Neutral, 0v again, and negative peak voltage relative to Neutral. Therefore the switch contacts only need to be rated for the line a.c. voltage (which implicitly addresses the peak voltage).
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