Why a Negative voltage (-48V ) is being used in telecom Network equipment?

BECAUSE WHEN undesired voltage add will decrease the total voltage -48+8=-40 the current will be in range(decreased) if we use positive like +48 any incraes in voltage  the total volt will increase +48+8=56

this will increase the current 

The best comprehensive answer for this I came across was this: 

(i) Positive voltage courses comparatively more corrosion in metal then Negative voltage.

(ii) Negative voltage is safer for human body while doing Telecom activities.

(iii) Thundering may cause positive voltage in the equipment circuit. In the case negative voltage (Lack of electrons) neutralizes positive charges and protects producing heat. 

(iv) Negative voltage is safer for long telephone line for transmitting power trough it.

(v) A standard lead-acid battery provides6V. This number corresponds to8 of those, which is pretty standard. It's the highest, safest compromise voltage to run over long wires.Negative voltage is used so that leakage currents to ground caused by moisture do not electroplate away the copper in the wires.

Old people were thinking that the Positive voltage causes the rust on copper, so they suggested that using a negative voltage will stop that rust !!Of course it did not, but they kept using it till these days.

C'est pour des raisons de protection contre la corrosion. Le positif étant mis à la terre, les parties actives des équipements sont toujours plus ou moins négatives, ce qui procure une protection cathodique en cas d'humidité, etc.