How do you wire -48v dc power and ground to earth?

always think of any supply system so :

RED = MOST POSITIVE

BLACK = MOST NEGATIVE

in a telecom system the -48V supply is refferenced to ground and earth, so the supply wire is NEGATIVE,

this means a DC supply most positive signal the RED wire, go to EARTH = ZERO,

the BLACK wire is the MOST negative, so in this case it is -48V