How can NEMA electric plug be safe ?
How can NEMA [1-15, IEC Type A] electric plug [in Japan] be safe?


Nothing is safe. There are only degrees of safety.

European plugs incorporate more safety features, which makes them safer. But they also work with higher voltages, which are slightly more dangerous (230 instead of 120 in US or 100 in Japan). 

UK Plugs and outlets have many more safety features than any other design, so are safer still.

None of these designs can always prevent a determined or negligent person from electrocuting themselves.



I think it is better to regard them as less safe rather than unsafe, Just take extra care.


Do not use cords, plugs or outlets that appear damaged, replace them.
Always ensure plugs are fully pushed in.
Only grasp plugs by the plug body.
Keep fingers away from the front edge near the pins.
Do not pull plugs out by the cords
Do not let cords trail where they may be tripped over (causing plugs to be pulled partly out)
If you can do so, replace ordinary breakers and outlets with GFCI equivalents.