Safety Rules, Preventing the Electric Shock
While you are working on electric circuits, there is often the possibility of receiving an electric shock by touching the “live” conductors when the power is on. The shock is a sudden involuntary contraction of the muscles, with a feeling of pain, caused by current through the body. If severe enough, the shock can be fatal. Safety first, therefore, should always be the rule.
The greatest shock hazard is from high voltage circuits that can supply appreciable amounts of power. The resistance of the human body is also an important factor. If you hold a conducting wire in each hand, the resistance of the body across the conductors is about 10,000 to 50,000 ohms. Holding the conductors tighter lowers the resistance. If you hold only one conductor, your resistance is much higher. It follows that the higher the body resistance, the smaller the current that can flow through you.
A safety rule, therefore, is to work with only one hand if the power is on. Also, keep yourself insulated from earth ground when working on power-line circuits, since one side of the line is usually connected to earth. In addition, the metal chassis of radio and television receivers is often connected to the power line ground. The final and best safety rule is to work on the circuits with the power disconnected if at all possible, and make resistance tests.
Note that it is current through the body, not through the circuit, which causes the electric shock. This is why care with high-voltage circuits is more important, since sufficient potential difference can produce a dangerous amount of current through the relatively high resistance of the body. For instance, 500 V across a body resistance of 25,000 Ω produces 0.02 A, or 20 mA, which can be fatal. As little as 10 uA through the body can cause an electric shock. In an experiment on electric shock to determine the current at which a person could release the live conductor, this value of “let-go” current was about 9 mA for men and 6 mA for women.
In addition to high voltage, the other important consideration in how dangerous the shock can be is the amount of power the source can supply. The current of 0.02 A through 25,000 Ω means the body resistance dissipates 10 W. If the source cannot supply 10 W, its output voltage drops with the excessive current load. Then the current is reduced to the amount corresponding to how much power the source can produce.
In summary, then, the greatest danger is from a source having an output of more than about 30 V with enough power to maintain the load current through the body when it is connected across the applied voltage. In general, components that can supply high power are physically big because of the need for heat dissipation.
RESISTANCE OF EARTH
The earth, no not the ground, I am speaking of planet earth, is not made of metal (in any great concentrated amount) so one may expect that it is not a good conductor. However if you recall the equation R = ÏL/A, where A is the cross sectional area – well the earth indeed does have a huge cross sectional area. This means for many applications the earth itself can be used as a conductor to save us having to run two conductors from the source to the load. Such circuits are called earth return and they have been used for power distribution and telephone communications
While you are working on electric circuits, there is often the possibility of receiving an electric shock by touching the “live” conductors when the power is on. The shock is a sudden involuntary contraction of the muscles, with a feeling of pain, caused by current through the body. If severe enough, the shock can be fatal. Safety first, therefore, should always be the rule.
The greatest shock hazard is from high voltage circuits that can supply appreciable amounts of power. The resistance of the human body is also an important factor. If you hold a conducting wire in each hand, the resistance of the body across the conductors is about 10,000 to 50,000 ohms. Holding the conductors tighter lowers the resistance. If you hold only one conductor, your resistance is much higher. It follows that the higher the body resistance, the smaller the current that can flow through you.
A safety rule, therefore, is to work with only one hand if the power is on. Also, keep yourself insulated from earth ground when working on power-line circuits, since one side of the line is usually connected to earth. In addition, the metal chassis of radio and television receivers is often connected to the power line ground. The final and best safety rule is to work on the circuits with the power disconnected if at all possible, and make resistance tests.
Note that it is current through the body, not through the circuit, which causes the electric shock. This is why care with high-voltage circuits is more important, since sufficient potential difference can produce a dangerous amount of current through the relatively high resistance of the body. For instance, 500 V across a body resistance of 25,000 Ω produces 0.02 A, or 20 mA, which can be fatal. As little as 10 uA through the body can cause an electric shock. In an experiment on electric shock to determine the current at which a person could release the live conductor, this value of “let-go” current was about 9 mA for men and 6 mA for women.
In addition to high voltage, the other important consideration in how dangerous the shock can be is the amount of power the source can supply. The current of 0.02 A through 25,000 Ω means the body resistance dissipates 10 W. If the source cannot supply 10 W, its output voltage drops with the excessive current load. Then the current is reduced to the amount corresponding to how much power the source can produce.
In summary, then, the greatest danger is from a source having an output of more than about 30 V with enough power to maintain the load current through the body when it is connected across the applied voltage. In general, components that can supply high power are physically big because of the need for heat dissipation.
RESISTANCE OF EARTH
The earth, no not the ground, I am speaking of planet earth, is not made of metal (in any great concentrated amount) so one may expect that it is not a good conductor. However if you recall the equation R = ÏL/A, where A is the cross sectional area – well the earth indeed does have a huge cross sectional area. This means for many applications the earth itself can be used as a conductor to save us having to run two conductors from the source to the load. Such circuits are called earth return and they have been used for power distribution and telephone communications
No comments:
Post a Comment