About Peter Sztano Expertise All questions on electric motors and drives, energy generation, magnetic fields, industrial automation (especially motion control, energy distribution, etc.).
Experience dr.univ. degree
20 years experience in research, development, manufacturing and education
Expert: Peter Sztano Date: 3/8/2004 Subject: Grounding
Question Hi Mr Sztano,
I would like to ask you questions on grounding in a electrical circuit. Thanks in advance.
I have read that grounding is a reference point in the circuit where the voltage is assumed to be zero.
1)My question is we often see grounding symbol drawn linked to the electric circuit, so is the grounding wire really attached to a circuit? If so, won't the current be constantly flowing back to the ground wire?
In a electric plug there are positive and a negative wires and there is usually a grounding wire.
Please correct me if I am wrong:
2)It is known that the positive and negative wire join up to form a closed loop and the circuit works. So is the grounding wire attached to the chasis of the equipment so that in the case of the broken live wire which touched the chasis, there is a return path to the ground wire
3)If in such a case, a person touched the
equipment at the same time, is the person safe?
4)What is the difference between common ground and chasis ground or are they the same?
Thank you very much.
regards,
Charles
Answer Grounding is for your safety. Electrical apparatuses usually have parts made of metal or other conducting material. Some of these parts are under dangerous voltage in normal operation, but others, those that can be touched, not, especially chassis, body, covering, frame, etc. are surely not.
If there is an insulation failure or other damage to the equipment, voltage can appear on parts that can be touched (wires or metal parts contact each other). This can be dangerous if one may touch the metal parts of this broken machine while standing on the ground or otherwise contacting an other conducting object like water piping, heating tubes or similar. The current in this case flows through the body and may kill humans and animals.
One way of avoiding dangerous voltages to appear on touchable metal parts is to make a direct metal contact (e.g. with a wire) between the part and the ground, this way, in case of a failure the potential (voltage) of the part can NOT go as high as being dangerous, as the grounding wire will direct the failure current straight to the ground instead of letting it flowing through your body.
Of course, the failure current is usually extremely high, so the network protection will switch off the power immediately after the failure (10..250 miliseconds) in most of the cases (there are lots of other failure types that can not be prevented by grounding).
This protection is called short circuit protection, and you can also identify the circuit breaker (MCB) in you home.
The main idea of grounding is (mostly) creating a short circuit in case of insulation and letting the protection switching off the supply, preventing humans, fires etc.
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In a standard 230V wall socket you will find three wires or poles:
L1 one of the live wires "Line" 230V to the neutral
N one of the live wires "Neutral" 0V to the PE (usually)
PE protective earth for "grounding" the equipment
(none of them is positive or negative, wall sockets use alternating currents)
