Understanding Electrical Grounding.

The electrical grounding system is a backup path for electric current to flow back to the power source in case of a fault.

Nature Of Electric Current Flow.

Electric Current flows from a power source through live wires to a load. The current then returns back to the power source through the neutral wire. The same is true for AC and DC power sources. current always returns to the power source. It is important to know that electric current always tends to flow through the path of lowest resistance.
Power sources include
1) Batteries - electrons flow from negative to load returns through positive.
2) Transformers - current flows from phase to load returns through neutral.
3) Alternators  - current flows from phase to load returns through neutral.

Short Circuit 

A complete and healthy circuit loop is formed when electric current flows from the source, to the load and back to the source through the neutral. This is the normal path in which electricity is expected to flow.
The live wire and the neutral wire are never expected to meet or touch each other before a load but if for any reason the live wire meets the neutral wire before a load, the electric current finds a short-cut(unwanted path) to return back to the source and this condition is known a short circuit.

A load in the circuit normally provides a degree of resistance or impedance to the flow of electrons but when a short-circuit occurs, the load is bypassed and there is little or no resistance to the flow of electrons. Consequently, a very high amount of current will flow.

Why Is Electrical Grounding Necessary?

Electrical grounding is very important for two main reasons
1) To protect the user of an equipment from receiving lethal shocks in the case of a fault.
2) To protect the equipment from damage in the case of a fault.

Protecting the User.
Many electrical appliances are covered with a metallic frame that can conduct electricity. From microwave ovens to refrigerators, ceiling fans and compressors. If you were to use a metal rod to touch a naked wire carrying voltage you will receive a good shock and potentially lose your life depending on the amount of current that flows through your heart.

The same situation is true if the live wire gets damaged due to aging and accidentally comes in contact with the metallic frame of your equipment. Touching your equipment has the potential to send you to the grave.
A piece of equipment is grounded by creating an intentional electrical connection with a conductor between the equipment frame and the earth's ground. When this is done there is a very low resistance path for current to flow back to the source such that even if you come in contact with an equipment during a fault you will not receive a lethal electric shock because electricity will naturally choose the path of least resistance. Copper wire has much much less resistance than the human body so a large part of the fault current will choose to flow through the copper wire to ground instead of your body.
Conductive materials used as an enclosure for electrical equipment must be grounded.

Protecting the equipment.
During a fault condition, if a live wire comes in contact with neutral or ground, excess current flows. In theory, the current that flows during a short circuit is infinite but in practice, the power source cannot deliver an infinite amount of current there is usually a limit to the current a power source can deliver depending on the KVA rating.
When a short circuit forces a power source to supply more current than it was designed to deliver or when a wire is made to carry more current than the wire gage can handle. The power source is destroyed and in the case of a wire, it melts.


Grounding protects the equipment because when there is a live to ground fault in a properly grounded system, sufficient current flows back to the power source through the ground and protective devices like fuses and circuit breakers can sense this excess current as a fault. The excess current will blow the fuse or trip the breaker protecting the equipment. We can see from the image above that if the equipment was not grounded there will be no return path for the excess current to complete the circuit and operate protective devices.

System grounding Vs Equipment grounding




System grounding involves connecting neutral wires in the distribution board or on the transformer to the earth ground through a grounding electrode or grounding rod. In essence, current-carrying parts of the system is connected to earth for the purpose of equipment protection and protective relay systems.

Equipment grounding is simply providing a wire from an appliance’s non-current carrying parts (for example the outer case of an electric motor) back to the ground bus-bar in the electrical panel. This ground bus is ultimately connected to the earth ground through a ground electrode. In essence, equipment grounding involves connecting noncurrent carrying -parts of the system like the equipment body to earth with the primary purpose of protecting personnel.

Comments

Post a Comment

Popular posts from this blog

How to check continuity over long distance

Image
When I began my career as an electrical technician my first challenge was the problem of checking the continuity of a wire that spans a long distance. My problem was compounded due to the physical layout of the wire under test. Let me use a simple picture to layout the nature of my problem. A bunch of Armoured cables outgoing from the  PCC (Power Control Center) in Building 1 feeds a group of distribution panels in Building 2 over 100ft away. Ideally, the cables should be tagged but due to aging and poor maintenance, most of the tags were not visible.  The problem was that some Distribution boards in building 2 had power while some others didn't.  The DB of interest didn't have power and thorough checks showed that there was no power on the cable feeding that DB.   All outgoing cables from Building one PCC had power so I suspected that the cable was no longer continuous. Anyway in the initial design of the facility 3 spare cables was laid underground but without tag
Read more

Making sense of a transformer nameplate data

Image
The nameplate of a power transformer contains the following details as per standard, then additional information could be provided varying from manufacturer to manufacturer. Name Of Manufacturer Serial Number Year Of Manufacture Connection Symbol – This tells you the HV winding and LV winding configuration of a transformer and difference in phase angle between them. Example DYN11 where the first letter represents the HV winding and indicates that it is connected in delta, the second letter represents the LV winding and indicates that it is connected in Star(wye)  and the third letter N indicates that the LV winding has a Neutral while 11 denotes a 30 degree lead in phase angle. This article explains more on the Vector group of a transformer and why it is important.  (Transformers connected in parallel must have the same vector group i.e. same phase angle shift to avoid circulating currents. This is a situation where one source will become load to the other sourc
Read more

Stall Rotor Protection Using Magnetic Overload Relays Oil Dashpot type

Image
  A Magnetic Overload relay works by detecting the magnetic field strength generated by the current flowing to the motor.  In most cases the relay input is a representation of the  current (through C.Ts)  flowing into the motor.   The Magnetic Overload Relay is built in such a way that the magnetic flux set up by current flowing through the  coil  drags (pulls) the core upward. A typical dashpot type magnetic overload relay is shown in the image below.                                    Construction. The relay consists of two main parts which is the trip mechanism at the top and the dashpot at the bottom.  The parts of the relay are further divided into a coil, a plunger (core), an oil-filled dashpot and switching contacts (trip contacts). The plunger is attached to a piston (disc). The piston is suspended in an oil filled chamber.  The relay coil is connected in series with the motor supply circuit. During normal operation magnetic flux induced by the coil is not great enough to c
Read more