Talking about AMEA socket system and 120V.
High power demand devices like A/Cs do have bigger plugs with some switches on them.
What exactly is their purpose?
Seems like a fuse but isn't the socket cirquit fused against overload?
What are the big plugs for that you find on high demand devices in North America (120V-Areas)
acdevicefusessocket
Related Solutions
LittleFuse provides a very nice app note with a lot of details for the design of these kind of devices.
As for measuring the current, you could use a very small resistor (1 or 0.1 ohm resistor) and use an oscilloscope to measure the current. Be careful not to ground the device. You need to use two probes for doing this measurement and subtract the value of one from the other - DO NOT CONNECT THE GND OF THE PROBE TO EITHER SIDE OF THE RESISTOR- you will in effect cause a short circuit if you do this. Just get 2 probes, connect the tip to each side of the resistor, and the grounds to the actual ground. use oscilloscope math capability (or just yourself) to do a difference between the two channels and this will be your current.
If the short-circuit current of your transformer is less than 40A you could use a Polyfuse such as the Littlefuse 30R500UMR. They're cheap (under a dollar in singles), and automatically reset when the fault is removed.
It needs to have a DC voltage rating of more than 22V, so this model is okay (30V rating).
Here is the trip time curve for this series of parts:
If the fault current can exceed 40A, it might not shut off reliably (and could damage the Polyfuse). You might want to put a somewhat higher current rating 5x20mm 250VAC fuse in series if you're not sure about this detail. It's possible to estimate the maximum fault current if you have the ability to measure the transformer secondary resistance accurately, but that's another question.
Another option, probably the one I would choose, would be to use a thermal circuit breaker such as the TE Connetivity W51-A122B1-5 or the W58-XB1A4A-5. Those items require manual reset, but have interrupting capacity in the 1000A range, which is not going to be a problem unless you are using a transformer sitting outside on a concrete pad for your train set.
Best Answer
It sounds like you are talking about the plugs with a built in Ground Fault Interruptor designed to automatically disconnect the circuit if current flow from the "hot" line is diverted anywhere other than the "neutral".
It's not really the "high power" which suggests their presence, but rather the possibility of a current leakage path developing.