What color is the plus pole? In the regular color-coding standard, the PLUS pole is marked by RED color and the MINUS by the BLUE color.
However, in Asia, sometimes there is a reversed color coding, where PLUS is marked by the black color and MINUS by the red color.
Keep this in mind when working in DC installations of equipment from Asia, that in some cases the MINUS may be marked by the red color.
The best way it to verify the polarity by measuring, because human and manufacturing errors may occur at any time.