Synchroscope is a device that indicates the degree to which two systems (generators or power networks) are synchronized with each other. Any two electrical systems is considered synchronized, if both systems are operating at the same frequency, and the phase angle between the systems is zero. Synchroscopes measures and displays the frequency difference and phase angle between two power systems. It is not possible to connect two AC power systems unless the systems are synchronized. Since it is necessary to synchronize AC power systems before connecting them, synchroscopes are used in any power plant that connects to an outside power grid and also in isolated plants containing more than one generator, as each generator must be synchronized with the others before being connected to the plant bus.