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[link] shows how a GFI works. If the currents in the live/hot and neutral wires are equal, then they induce equal and opposite emfs in the coil. If not, then the circuit breaker will trip.

The figure shows an L shaped structure having four terminals. This is represented as circuit breaker. The upper two ends are connected to a sensing coil wound on an iron ring shaped core. The lower two terminals of the circuit breaker have connecting wires than run through the center hole of the ring shaped core. Current is shown to flow to and fro across the two wires. The currents in the iron ring are shown as concentric circles.
A GFI compares currents by using both to induce an emf in the same coil. If the currents are equal, they will induce equal but opposite emfs.

Another induction-based safety device is the isolation transformer , shown in [link] . Most isolation transformers have equal input and output voltages. Their function is to put a large resistance between the original voltage source and the device being operated. This prevents a complete circuit between them, even in the circumstance shown. There is a complete circuit through the appliance. But there is not a complete circuit for current to flow through the person in the figure, who is touching only one of the transformer’s output wires, and neither output wire is grounded. The appliance is isolated from the original voltage source by the high resistance of the material between the transformer coils, hence the name isolation transformer. For current to flow through the person, it must pass through the high-resistance material between the coils, through the wire, the person, and back through the earth—a path with such a large resistance that the current is negligible.

The figure shows an A C source, one end of which is connected to earth and the other end is connected to a circuit breaker. The other end of the circuit breaker is connected to the primary of an isolation transformer. The secondary of the transformer is connected to an appliance shown as a resistance enclosed in a case. The current is shown to flow through the appliance. A person is shown in contact with the appliance. He is safe as the transformer induces a high resistance between the original voltage source and the device.
An isolation transformer puts a large resistance between the original voltage source and the device, preventing a complete circuit between them.

The basics of electrical safety presented here help prevent many electrical hazards. Electrical safety can be pursued to greater depths. There are, for example, problems related to different earth/ground connections for appliances in close proximity. Many other examples are found in hospitals. Microshock-sensitive patients, for instance, require special protection. For these people, currents as low as 0.1 mA may cause ventricular fibrillation. The interested reader can use the material presented here as a basis for further study.

Section summary

  • Electrical safety systems and devices are employed to prevent thermal and shock hazards.
  • Circuit breakers and fuses interrupt excessive currents to prevent thermal hazards.
  • The three-wire system guards against thermal and shock hazards, utilizing live/hot, neutral, and earth/ground wires, and grounding the neutral wire and case of the appliance.
  • A ground fault interrupter (GFI) prevents shock by detecting the loss of current to unintentional paths.
  • An isolation transformer insulates the device being powered from the original source, also to prevent shock.
  • Many of these devices use induction to perform their basic function.

Conceptual questions

Does plastic insulation on live/hot wires prevent shock hazards, thermal hazards, or both?

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Why are ordinary circuit breakers and fuses ineffective in preventing shocks?

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A GFI may trip just because the live/hot and neutral wires connected to it are significantly different in length. Explain why.

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Problems&Exercises

Integrated Concepts

A short circuit to the grounded metal case of an appliance occurs as shown in [link] . The person touching the case is wet and only has a 3.00 kΩ resistance to earth/ground. (a) What is the voltage on the case if 5.00 mA flows through the person? (b) What is the current in the short circuit if the resistance of the earth/ground wire is 0.200 Ω ? (c) Will this trigger the 20.0 A circuit breaker supplying the appliance?

The figure describes an appliance connected to an AC source. One end of the AC circuit is connected to a circuit breaker. The other end of the circuit breaker is connected to an appliance. The appliance is shown as a resistance enclosed in a rectangular metal case known as the case of appliance. The other end of the resistance is connected back to the AC source through a connecting wire. The connecting wire and the A C source are grounded. The ground terminal at the appliance case is connected to a resistance R g equals zero point two ohms. A person is shown to hold one hand on the appliance case. Since the resistance of ground is not zero the voltage is shown to flow through the person to the ground.
A person can be shocked even when the case of an appliance is grounded. The large short circuit current produces a voltage on the case of the appliance, since the resistance of the earth/ground wire is not zero.

(a) 15.0 V

(b) 75.0 A

(c) yes

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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