An electrical device like this requires a reverse leakage current of 5μA @ 38.8V.While the device is operating, the maximum voltage - Zener (Nom) is reduced to 51V.
1N4757A_T50A Features
reverse leakage current of 5μA @ 38.8V 5μA @ 38.8V is the maximum voltage (Tol)
1N4757A_T50A Applications
There are a lot of ON Semiconductor 1N4757A_T50A applications of zener single diodes.