Generally, the limit of the permanently permissible shock hazard voltage UL for
a.c. voltages is 50 V and for d.c. 120 V. In electrical circuits containing socket
outlets and in electrical circuits containing class I mobile equipment normally held
permanently in the hand during operation, higher shock hazard voltages, which can
arise in the event of a fault, must be disconnected automatically within 0.4 s. In all
other electrical circuits, higher shock hazard voltages must be automatically disconnected
within 5 s.