Electric shock and earth
A literal meaning of the earth is the earth. Grounding it (earth) is a thing connected with the earth. As for the earth, potential is steady to the connected thing because very big shape is had, and potential is very steady if it connects it with the earth by low resistance.
What is the earth resistance (ground resistance)?
The earth resistance (ground resistance) is resistance from the earth electrode (ground electrode) to the depth of the earth.
This resistance cannot be measured by putting the electrode in the depth of the earth. Please see the page of "Measurement of the ground resistance" about the measuring method of the ground resistance. Please see the page of "Calculation of the ground resistance" about the concept of the ground resistance.
Necessity of earth
For what is the ground bar of electrical machinery and apparatuses necessary?I think that there is coming thinking it is necessary because it is said it is dangerous if it doesn't ground it, too.
The earth considers to be chiefly necessary for safety in this page, and, besides, to prevent an unnecessary radiation of the stability of the work of the equipment and the electromagnetic radiation, it might be necessary.
There are two kinds of signs of the earth as showing in this figure. There is a chassis earth connected with the case of the earth, the radio, the television connected with the earth (earth), and the wireless machine (cabinet and chassis). However, it is likely not to distinguish clearly. I think that I often chiefly do the chassis earth by the purpose of the prevention of an unnecessary radiation of the stability of the movement of the equipment and the electromagnetic radiation etc.
Necessity of earth of transformer on pillar
The high voltage of 6600V has been converted into 100V or 200V of the domestic use with the transformer on the utility pole in left figure. The second side of this transformer and the core of the transformer are grounded.
It thinks about the case without this earth. 6600V of the first side enters 100V/200V circuit. When the insulation of this transformer worsens. The leak of the appliance is exist or not, it becomes unusually dangerous conditions.
If the second side is grounded at this time, the high voltage can be prevented from the current's flowing to the earth through the earth of the transformer on the pillar when the insulation of the transformer worsens, and the second side of the transformer on the pillar is safe.
What is the reason to ground (white line)?Even if a part of 200V(for instance, red) is grounded, the above-mentioned purpose can reach. However, even if a black line is 100V circuit, the voltage against ground becomes 200V in this case. If the neutral point is grounded, the voltage of ground is 100V in 200V circuit. It can be said as there is no danger when getting an electric shock that the voltage against ground is low.
It is necessary to ground the transformer on the pillar absolutely for safety like this.
(The voltages discussed here are all the effective values. The peak value becomes these about 1.41 times. )
Necessity of earth of appliance
100V in the voltage of ground is generated in a domestic both terminals of one side and 200V of 100V because the transformer on the pillar is grounded. When the person touches the metallic portion of the exterior when insulation worsens and the V of the exterior of the product of the appliance hangs to the metallic portion 100(When the appliance leaks electricity in a word), it gets an electric shock as shown in the drawing on in a word. At this time, a lot of currents (current leakage and ground-fault current) might flow when feet from which the person is standing is damp and the person die.
Electricity flows to the earth even if the person touches the metallic portion of the exterior if in this case, there is a good earth for the appliance and the human body is safer. Moreover, the leak breaker of the switchboard works by the thing that this current of the earth (current leakage and ground-fault current) flows and it cuts off the power supply.
The earth of the exterior metal of the appliance etc. used in a moist place is very important like this.
Then, even if the microwave oven is used in a dry place, should I ground it?The reason for the microwave oven is that there is a possibility that the high pressure hangs to the metallic portion of the exterior according to how to break when breaking down because the electric power treated by the high pressure is large (In 500W?1000W). Because the electric power treated when the re-becoming it instigating is large, person's possibility of dying is large. It seem to be though dying while repairing the microwave oven is rare.
Case that need not be grounded
All appliances You in the following cases, may be by the shock hazard and not exist the earth few.
- When establishing it in the place where the one used with the circuits of the DC 300V or AC150V or less in the voltage against ground in the working voltage was dried---(If it is a dry place, even the equipment of the single phase 200V such as the televisions and videos is unnecessary the earth.)
- When you establish the machine apparatus of the double insulation structure to receive the application of Electrical Appliances and Materials Safety Law---(power tool etc.)
- When you establish the high sensitivity high-speed type leak breaker (15mA or less in the ratings sensitivity current is limited to the one of the current operation type of 0.1 seconds or less in the operation time) in the cable run that supplies electricity to the machine apparatus for the low pressure established besides the place with the water---(Because it is difficult for the washing machine to set it up in the place without the water, the earth is necessary with this earth leakage breaker. )
- It omits it though besides this.
Electric shock and dangerous current value
Even a little current might not only be felt an unpleasant current by the electric shock but also dies when passing over an important part of the body of the heart etc.
There are a lot of cardiac standstills (ventricular fibrillation) and respiratory arrests in the cause of the death by the electric shock by the low voltage, and is a burn by generating heat because of the current that flows in the arc heat and the human body by the electrical discharge in addition to it in the electric shock accident by a high voltage. Because the current passes the depth of the body, the burn by the electric shock is very dangerous.
How to feel the electric shock is different by DC and AC. It goes into convulsions in AC as the muscle trembles while DC shrinks the muscle and stiffens. It is said that about 1000V of DC is very dangerous because of the muscle's stiffening gripping the electric wire when it touches the electric wire by the hand and not parting.
When fiddling with the transmitter of the vacuum tube type (It was a power supply of 500?1000V in direct current), it was written that not working alone in the middle of the night was important in old times in a wireless magazine. I decided not to touch in the transmitter of the vacuum tube type alone in the middle of the night after having seen this.
It gets an electric shock even if the leak breaker has adhered and there is a considerable impact in the translation where the current of about 30mA flows to the human body for about 0.1 seconds if the ground-fault current flows. It makes it to [zo] when thinking of the thing if the leak breaker has not adhered though I experience such an electric shock accident, too.
|Electric shock current||Reaction of human body|
|0.5mA||Smarting do not you feel anything, and it feels it slightly according to the place where the current flows.|
|1mA||It ..dindling.. feels it.|
|5mA||Feel the blitz painful|
|50mA||The muscle might shrink and it die.|