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23.8 Electrical Safety: Systems and Devices
You can learn how to make a bulb light using magnets.
There are two dangers to electricity.
Both dangers have been discussed.
We will focus on systems and devices that prevent electrical hazard.
The schematic for a simple AC circuit has no safety features.
This is not how power is distributed.
The circuit breaker is the first thing to be done to prevent overload.
There is a protective case around the appliance.
The case's safety feature helps prevent shocks by preventing a person from touching exposed wires and coming into electrical contact with the circuit.
A single appliance is represented by the resistance in the schematic of a simple AC circuit.
There aren't any safety features in this circuit.
The neutral wire is connected to the earth at the source of the current and the user location, so that it can return the current to the source.
The appliance is also grounded to zero volts.
It is important to check the wire insulation colors in your area to determine which color codes are used.
An earth/ground connection is a low-resistance path to the earth.
The neutral wire is named after it because of the two earth/ground connections on it.
Even if the insulation is missing, this wire is safe to touch.
The return path for the current is the neutral wire.
An alternative path through the earth, a good conductor, is provided by the two earth/ground connections.
The generating plant could be the location of the earth/ground connection.
The case of the appliance is forced to be at zero volts by the third earth/ground through the green earth/ground wire.
The live wire is used to supply the appliance's voltage and current.
To assure proper function of the three-wire system, the standard three-prong plug can only be inserted in one way.
Insulating plastic is color-coded to identify live/hot, neutral and ground wires but these codes vary around the world.
Live wires can be brown, red, black, blue or grey.
Blue, black or white neutral wire can be used.
It is important to determine the color code in your region since the same color may be used in different parts of the world.
The earth/ground wire is usually green but may be yellow or just bare.
Those who are colorblind can benefit from striped coating.
The older two-wire system did not have an earth/ground wire.
Under normal circumstances, insulation on the live/hot and neutral wires prevents the case from being directly in the circuit, so that the earth/ground wire may seem like double protection.
More than one problem can be solved by grounding the case.
A severe shock is possible if you don't have an earth/ground connection.
This is particularly dangerous in the kitchen, where a good connection to the ground can be found through water on the floor or a water faucet.
The circuit breaker will trip if the earth/ground connection is not intact.
Power tools with impact resistant plastic cases are doubly insulated and have nonconducting cases.
Modern two-prong plugs can be inserted into the asymmetric standard outlet in one way to ensure proper connection of live/hot and neutral wires.
The metal case of this appliance allows the live wire to come into contact with it.
In this situation, the appliance may operate normally.
A more subtle problem can be solved by grounding the case.
An emf can be caused by the AC current in appliances.
Current driven by the case emf is referred to as a leakage current.
An emf can be caused by AC currents.
It can be large enough to cause a shock.
The emf is kept near zero if the case is grounded.
A ground fault interrupter is a safety device found in updated kitchen and bathroom wiring.
The currents in the wires are compared.
When live/hot and neutral currents aren't equal, it's usually because the neutral current is less than the live wire.
Some of the current is coming back to the source by a path other than through the neutral wire.
The accepted maximum harmless shock is usually set to interrupt the circuit if the leakage current is greater than 5 mA.
If the leakage current goes through an intact earth/ground wire, it will trip, forcing the repair of the leak.
If the difference between the currents in the live and neutral wires exceeds a safe value, a ground fault interrupter will trip.
An intact earth/ground wire could have prevented the leak current here.
The figure shows how aGFI works.
If the currents in the live/hot and neutral wires are equal, the emfs in the coil will be equal.
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