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| Hence, even though the directional or wattmeter element may close its contacts upon the flowing of current in the reverse (from normal) direction, the tripping circuit will not be completed until both elements close their contacts. |

| In another type of directional overcurrent relay, only one set of contacts exist. Here, the two relay elements, the overcurrent and directional, are combined into one. In this induction type of relay, the magnetic field of the overcurrent coil is made to induce a current in the disc of the directional element. The tendency for the disc to rotate in one direction is balanced against a spring that also acts to restrain the action of the overcurrent coil. When a fault current in the opposite direction develops, the directional element turns the disc in the reverse direction; it is aided by the magnetic field of the overcurrent coil and the force of the spring, all joining to produce a rapid closing of contacts that actuates the associated breaker. This type of relay is often used in three-phase systems; only two discs are mounted on a single vertical shaft to which one set of contacts is mounted. |

| Differential Relays |

| This type of protective relay operates on the difference between the currents entering and leaving the protected line or equipment. It utilizes the most positive selective principle. |

| With the differential principle, the current flowing into any part of an |

| Figure 13-9 Directional overcurrent relays: (A) Rear view of internal wiring. |

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