usb extension cord avoid electric shocks how?

Hi

do you use usb extension cord ? for external devices ?

how can i avoid potential electric shocks ?

should i keep it on the floor or on the case or?

i know there are only 3v but they could i guess damage devices like car navigators, ipod , mobiles and maybe the motherboard too

i have 2 usb extension cable , usb 2 abd usb 3
i keep them on the desktop
is it wise ? or is there a bettere solutions

thanks

 

Only use approved products.

 

and which are these products?

i bought at obi , if you are in europe i know it , brand polypool made in china

thanks Cudni