If none of the equipment that you have connected to your NT1 requires a power supply through the interface, you do not have to plug the NT1 into the mains. 

The NT1, which is not a passive device, normally powers itself from PS1. Only if the NT1 contains a TA (so it is a 'NT1-plus' or 'Super-NT1') or if it can supply power to attached devices will it need to have a mains power supply.

Some ISDN phones can also be powered from PS1.  This allows you to have at least one working ISDN telephone on your line in the event that your local power supply fails. In some countries this is a regulatory requirement.

Note that there are no provisions for PS1, PS2 or PS3 in the case of the PRI.