What will happen if I press the GFCI button on an electrical tester when it is connected to a regular socket?
The tester will just leak enough current to Protection Earth to trigger a RCD. Since RCDs are available in different sensitivities (ie, trip current) maybe you got several buttons on your tester. What is inside is probably just a resistor.
If you keep the button pressed long enough and nothing trips, I guess whether the resistor burns or not depends if you got it for $2 on aliexpress or not.
Still that's 230V*30mA = 7W so that needs a pretty beefy resistor. Feel free to adjust for local current and voltage. You can open the device to check, unless that voids the certification, otherwise... If you have doubts, only press it for like 0.5 seconds. RCD should trip in a couple tens of milliseconds anyway.
Maybe it has a manual? I mean, if the legislation says the RCD should trip in say, 20 milliseconds, then the thing should have a timer that only applies the current for that time, which would have the side effect of not cooking the resistor that sets the test current.
Here (France) there is a 500mA RCD in the main panel from the utility, and 30mA RCD have been mandatory in new residential installations for a while, it's a real lifesaver. So if you install a special 10mA RCD in the bathroom for the &quot;electric razor&quot; socket near the sink, and you use a RCD tester set to 30mA, most likely two RCDs will trip, but sometimes the faster one will trip first, so the second one no longer has current, so it won't trip, and you have to test it at a lower current to know if it works.
Likewise, when the tenants wash their cooktop in the sink and then plug it in, or they fish out a toast from a toaster with a fork while it's on, sometimes the 500mA RCD trips before the 30mA, because it's faster.