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Fig. 28-12 Short-Circuited Phase Faults.

Fig. 28-15 Grounded Wye/Delta.

A line-to-ground fault, which is common, is not always easy to detect. When the wire connects to ground, the line impedance drops, but not necessarily to zero. The change in current and voltage levels may sometimes be too modest to detect without grounded systems. This type of fault detection capability can be established through the connection of the distribution transformer grounded wye/delta, with measurement of the neutral current (Figure 28-15), or the connection of three potential transformers, connected wye/open delta (Figure 28-16), to the transformer primary (measuring voltage across the open delta).

Generally, when there is a fault or the line is de-energized, the voltage drop is quite significant. Therefore, voltage relays without great sensitivity are sufficient to provide adequate protection. Upstream faults (faults that occur between the facility and the network substation) can be sensed by undervoltage relays. For instance, if the utility trips off the line, the undervoltage relay at the facility will respond.

Downstream faults, which are not always easily detected, often result in both the facility and the utility feeding the fault. In some cases, a synchronous generator may carry the load and feed a dead line. Current, voltage, and frequency relays are used to sense this condition, depending on possible scenarios.

Reclosing breakers are automatic devices much like circuit breakers that are used to time and interrupt faulted

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