Underreaching transfer trip schemes include two variations: direct underreach and permissive underreach.

The communications for this type of relaying are generally the same as for the overreaching systems, using audio tones with frequency shift keying over microwave, leased line, or fiberoptic communications channels.

Direct Underreach

This form of protection requires only a single distance fault detector at each end. It has to be set short of the remote end and will simultaneously trip the local breaker and send a trip signal to the remote end, which then trips directly upon receipt of the signal.

Note that local confirmation is not required upon receipt of a trip signal.

Though this scheme is the least complex, it is seldom used because of the high risk of false outputs from the communication channel, which would result in false trips. This risk can be minimized by using a dual-channel transfer trip, which requires the receipt of two signals from the remote end to effect a trip.

See Figure 3.

Figure 3 – Direct Underreach Scheme

Permissive Underreach

This scheme is identical to the direct underreach scheme with the addition of an overreaching fault detector. The transfer trip signal requires local confirmation by this fault detector before tripping can occur. This increases the security of the scheme and the consequent range of application.

It is commonly selected when an existing step distance relay line is to have the pilot added. See Figure 4.

Carrier is not normally used since a fault could short out the communication signal and prevent the signal from reaching the remote terminal.

Figure 4 – Permissive Underreach Scheme