Creating Reliable Electrical Connections

Creating Reliable Electrical Connections
Cleaning, filing, and abrading surfaces are among the options.
One of the important ingredients for making and keeping a reliable electrical connection is clean contact surfaces. The other important ingredient—force—was covered in “The Trouble with Torque in Electrical Connections."



RESISTANCE VS. FORCE Fig. 1. Contact theory says that contact points are actually cold welds. If the high contact force is decreased, the contact resistance remains constant and does not increase until a much lower force is reached.

When initial contact is made between electrical contact surfaces, no matter how smooth and level the surfaces, only a few high points touch. As the contact force increases, more points make contact until at optimum force most of the metal-to-metal contact has been accomplished. Contact theory tells us that these points are actually cold welds.
This sounds logical because if the high contact force is decreased, the contact resistance remains constant and does not increase until a much lower force is reached. See Fig. 1. This would be expected with welding, not with a spring-like mechanism. For cold welding to occur, clean surfaces and massive distortion are required. This is accomplished by proper contact preparation, abrasion from relative motion, and volume reduction during the high forces.
As this discussion implies, the true contact area is where we have the welds. For example, a 4 in. wide bus bar with a 4 in. connection overlap does NOT produce a contact area of 16 sq in. (4 in. x 4 in.). If the connection is made with a single bolt in the center, the contact area is under the head of the bolt. If a large diameter, thick washer is added, the contact area then is increased to the washer area. Note that NEMA Standard CC1, Electric Power Connection for Substations, specifies four bolt holes for a 4 in. wide bus.
Get surfaces clean, level
From a practical standpoint, try to make the contact surfaces as clean, smooth, and level as possible. Dirty or oily surfaces should be cleaned with a solvent. Rough surfaces should be filed and/or sanded. For most surfaces, abrading with a wire brush, sandpaper, or steel wool and removing loose particles is sufficient. With stranded conductor, try to use fresh portions and wire brush the outer strands. When pressure is applied to the conductor, the strands abrade each other.
With copper conductor, it is usually easy to see how the cleaning is going. With aluminum this is more difficult since the oxide coating is colorless and starts to reform after cleaning. Therefore, extra care must be exercised with aluminum. The oxide film is initially weak and thin, so right after cleaning, apply a joint compound to bus bar contact surfaces or, if cable, to the outer strands. Then immediately tighten bolts or compress the connector. Tests have shown that this procedure results in a good connection.
If the contact surface is plated, try not to remove the coating. Solvent cleaning is usually sufficient.
Years ago a common recommendation for aluminum was to apply the joint compound prior to cleaning and to abrade the surface through the joint compound. This messy procedure is not required if the compound is applied immediately after cleaning. In a cable connection, the compound will be forced between the strands when pressure is applied. The joint compound should not be relied upon to clean the contact surfaces; its purpose is to surround the cold welds to prevent the ingress of harmful matter such as air, moisture, and contaminants.
Check contact resistance
Checking an electrical connection is difficult. Ideally, a contact resistance reading is best. But with high current connections we are dealing with micro-ohms, hard to read in the field and masked by nearby bulk resistance. This makes the contact resistance hard to determine. If possible, make trial connections in the shop and check as follows:
• Bus bar connections. Place a die penetrant or a pressure sensor film in the contact area prior to tightening the connection. Contact area and/or contact pressure is determined after disassembly.
• Cable connections. Cut through the crimped or tightened section of the connection and then prepare like a metallurgical specimen. All strands should be distorted and no air spaces should be evident. This technique also is useful for failure studies.
Analysis of these results can assist in specifying the connection procedure and in improving the proposed connection. MT

Source : http://www.mt-online.com/component/content/article/202-may2003/1037-creating-reliable-electrical-connections.html?directory=90

Electrical Safety Basics

Following these safety principles can provide a safer work environment.

There are three basic electrical hazards that cause injury and death: shock, arc-flash, and arc-blast.

Shock Current can pass through the human body's nervous or vascular systems, and across the surface of the body. The current required to light a 71/2 W, 120 V lamp, passing through the chest, can cause death. Of those killed while working on voltages below 600 V, half were intentionally working on "hot" energized equipment. Most electrocutions can be avoided with proper training, planning, job preparation, procedures, and equipment.

Arc-flash (extremely high temperature conductive plasma and gases resulting from an arc fault incident). As many as 80 percent of all electrical injuries are burns resulting from an arc-flash contact and ignition of flammable clothing. Arc temperatures can reach 35,000 F four times hotter than the sun s surface. Arc-flash can cause second and third degree burns.

Arc-blast (pressure wave caused by the rapid expansion of gases and conducting material with high flying molten materials and shrapnel). An arc-blast may result in a violent explosion of circuit components and thrown shrapnel. The blast can destroy structures, and knock workers from ladders or across a room. The blast can rupture eardrums and collapse lungs.

Training, planning, and writing procedures
Provide training. Obviously, an important aspect of electrical safety is training. To be qualified, workers need training on the tasks and procedures (such as lockout/tagout procedures) that are essential to conducting their work in a safe manner.

Plan every job. Take the time to prepare a work plan that considers all possible eventualities. Before starting the job, think about each step and try to visualize the potential for hazards.

Anticipate unexpected results. When thinking about a job, break each task into small steps. Understand that plans can change, so be ready to modify the plan. Make sure that everyone involved in the job is working according to the same plan. Whenever work is required near an electrical hazard, a written plan is needed to outline the scope of the job.

Use procedures as tools. Procedures are the best way to help you prepare, execute, and complete a job. Like any tools, make sure procedures are maintained.

Identify the hazard. After your work plan is complete, review each step. Consider that the equipment might be perfectly safe under normal conditions and very unsafe when systems are not working properly. Also consider potential hazards that may be unrelated to electrical energy.

Assess people's abilities. Any person assigned to tasks associated with electrical energy must be qualified and trained for the job at hand. He or she must be able to identify electrical hazards, avoid exposure to those hazards, and understand the potential results of all action taken.

Providing an electrically safe work condition
Use the right tool for the job. Use the appropriate tools for the job at hand, keep them accessible and in good working condition. Using a screwdriver for a job that requires a fuse puller is an invitation to an accident.

Isolate the equipment. The best way to avoid an accident is to reduce exposure to the hazards present. Keep doors closed. Keep barricades in place. Install temporary voltage-rated blankets covering exposed live parts. Put the equipment in a safe working condition prior to performing maintenance. Lock out the disconnect.

Protect the person. Use the proper personal protection equipment for the job. This may include safety glasses or goggles, head protection, voltage-rated gloves, safety belts and harnesses, or flame-resistant clothing.

Minimize the hazard. If it is impossible to establish an electrically safe work environment, be sure to shut down every possible energy source. Understand that sometimes a de-energized circuit can become re-energized and do something to lessen the risk.

Audit these principles. A principle is something you believe in enough to be willing to do. Review these principles often; add to them as needed.

Lockout/Tagout
The Lockout/Tagout Standard was created to help reduce the death and injury rate caused by the unexpected energization or start-up of machines or the release of stored energy. Normal production operations, cords and plugs under exclusive control, and hot tap operations are not covered. This standard applies to energy sources such as electrical, mechanical, hydraulic, chemical, nuclear, and thermal.

Procedures for applying the lock/tag:

1. Before the shutdown, know the unit and power sources

2. Power down the equipment

3. Isolate the power source(s)

4. Apply the lock and/or tag

5. Use proper techniques, personal protective equipment, and test measuring devices to verify that the electrical circuit is de-energized

6. Release residual energy

7. Try to power up

Procedure for removing the lock/tag:

1. Inspect machine and/or equipment

2. Give notification to personnel

3. Remove the lockout/tagout device

National Fire Protection Association Standard NFPA 70E
There are a number of important electrical industry consensus standards that have indirect or direct impact on human safety. NFPA 70E, the "Standard for Electrical Requirements for Employee Workplaces," is one of the most important. This standard focuses on protecting people and identifies requirements that are considered necessary to provide a workplace that is generally free from electrical hazards.

NFPA 70E suggests that:

• Electrical hazards include shock and arc-flash.
• The best way to avoid injury or incident is to establish an electrically safe work condition prior to beginning work.
• Procedures and training are extremely important if injury is to be avoided.
• Shock and flash protection boundaries determined and adhered to.

NFPA 70E identifies the requirements for enhanced personal safety. It is an extremely important national consensus standard and must be considered to define the requirements for an overall electrical safety program.

IP20 (Finger Safe) ratings
The NFPA 70E requires that a guard be used to prevent access to voltages above 50 V. Guarding and the installation of insulating barriers must be completed if work is to be performed while the equipment is energized.

IP20 is often referred to as the "Finger-Safe" rating for electrical components such as a disconnect fuse holder. It refers to the fact that a probe, the approximate size of a finger, must not be able to access or make contact with hazardous, energized parts. Live parts that could easily be touched while resetting, adjusting, or replacing nearby components must be provided with protection against direct contact to at least the IP20 rating.

Electrical hazard heat facts
The heat generated from electrical hazards kills and maims. Consider the following:

• No material on earth can withstand temperatures of 35,000 F without vaporizing.
• Copper expands to 67,000 times its original volume when vaporized and can be inhaled into human lungs with debilitating or fatal results.
• Skin temperature of 200 F for more than one-tenth of a second will cause incurable, third degree burns—consider the damage to human skin exposed to the heat resulting from an arc-flash.
• Serious or fatal burns can occur at distances of more than 10 feet from the arc-flash source.

Designing an electrical system--for safety
It is not enough to focus just on work practices and the worker. The first step in providing a safer work place is designing and retrofitting the electrical system with specific safeguards. There are a number of things that can be implemented including the following:

Isolate the circuit. Electrical systems must be designed to support preventive maintenance, with easy access to the equipment. Designers need to make it easy to isolate equipment for repair with a disconnecting means that provides for proper implementation of Lockout/Tagout procedures. Install disconnects with permanent lockout provisions within sight of all motors and driven machinery.

Cover exposed components. Equipment must be "Finger-Safe" IP20 where possible to avoid potential contact with energized conductors. It is not always possible to de-energize the equipment before working on it. Specify electrical components that are IP20 "Finger-Safe."

Limit the energy. Circuits should also be designed to limit the available arc-flash energy. The use of current limiting overcurrent devices can help to reduce the flash energy. This will provide for improved protection for both equipment and employees. Use current-limiting fuses or current-limiting circuit breakers.

Retrofit the fuses. If the electrical system is an existing fusible system, upgrade the fuses to the most current-limiting fuse types. This measure provides a greater degree of current-limitation if an arc-flash incident occurs.

Electrical hazard victim first aid
As part of your safety program know the name, location, and phone number of the nearest burn center medical facility.

• Have a victim in burning clothing drop and roll to extinguish flames.
• Cool the burn with water or saline for a few minutes or until the skin returns to normal temperature.
• Remove constricting items such as shoes, belts, jewelry, and tight collars.
• Elevate burned limbs to reduce swelling.
• Handle the victim with care as he may have broken bones or spinal injuries.
• Treat for shock. Maintain body temperature; do not give anything by mouth. Administer high concentrations of oxygen, if available.
• Keep the victim warm and as comfortable as possible during transport to the medical facility. Cover victims with clean, dry sheets or blankets. Wounds should be covered with sterile dressings or clean sheets.
• Have someone familiar with the incident immediately inform the medical staff of vital details of the incident. This helps the medical staff diagnose injuries more quickly and more accurately.

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  Source : http://www.mt-online.com/component/content/article/125-may2002/877-electrical-safety-basics.html?directory=90
  

Electrical Engineering

Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical power supply. It now covers a range of subtopics including power, electronics, control systems, signal processing and telecommunications.

Electrical engineering may include electronic engineering. Where a distinction is made, usually outside of the United States, electrical engineering is considered to deal with the problems associated with large-scale electrical systems such as power transmission and motor control, whereas electronic engineering deals with the study of small-scale electronic systems including computers and integrated circuits. Alternatively, electrical engineers are usually concerned with using electricity to transmit energy, while electronic engineers are concerned with using electricity to process information. More recently, the distinction has become blurred by the growth of power electronics.

Sub-disciplines

Electrical engineering has many sub-disciplines, the most popular of which are listed below. Although there are electrical engineers who focus exclusively on one of these sub-disciplines, many deal with a combination of them. Sometimes certain fields, such as electronic engineering and computer engineering, are considered separate disciplines in their own right.

Power

Power engineering deals with the generation, transmission and distribution of electricity as well as the design of a range of related devices. These include transformers, electric generators, electric motors, high voltage engineering and power electronics. In many regions of the world, governments maintain an electrical network called a power grid that connects a variety of generators together with users of their energy. Users purchase electrical energy from the grid, avoiding the costly exercise of having to generate their own. Power engineers may work on the design and maintenance of the power grid as well as the power systems that connect to it. Such systems are called on-grid power systems and may supply the grid with additional power, draw power from the grid or do both. Power engineers may also work on systems that do not connect to the grid, called off-grid power systems, which in some cases are preferable to on-grid systems. The future includes Satellite controlled power systems, with feedback in real time to prevent power surges and prevent blackouts.

Control

Control engineering focuses on the modeling of a diverse range of dynamic systems and the design of controllers that will cause these systems to behave in the desired manner. To implement such controllers electrical engineers may use electrical circuits, digital signal processors, microcontrollers and PLCs (Programmable Logic Controllers). Control engineering has a wide range of applications from the flight and propulsion systems of commercial airliners to the cruise control present in many modern automobiles. It also plays an important role in industrial automation.

Control engineers often utilize feedback when designing control systems. For example, in an automobile with cruise control the vehicle's speed is continuously monitored and fed back to the system which adjusts the motor's power output accordingly. Where there is regular feedback, control theory can be used to determine how the system responds to such feedback.

Electronics

Electronic engineering involves the design and testing of electronic circuits that use the properties of components such as resistors, capacitors, inductors, diodes and transistors to achieve a particular functionality. The tuned circuit, which allows the user of a radio to filter out all but a single station, is just one example of such a circuit. Another example (of a pneumatic signal conditioner) is shown in the adjacent photograph.

Prior to the second world war, the subject was commonly known as radio engineering and basically was restricted to aspects of communications and radar, commercial radio and early television. Later, in post war years, as consumer devices began to be developed, the field grew to include modern television, audio systems, computers and microprocessors. In the mid-to-late 1950s, the term radio engineering gradually gave way to the name electronic engineering.

Before the invention of the integrated circuit in 1959, electronic circuits were constructed from discrete components that could be manipulated by humans. These discrete circuits consumed much space and power and were limited in speed, although they are still common in some applications. By contrast, integrated circuits packed a large number—often millions—of tiny electrical components, mainly transistors, into a small chip around the size of a coin. This allowed for the powerful computers and other electronic devices we see today.

Microelectronics

Microelectronics engineering deals with the design and microfabrication of very small electronic circuit components for use in an integrated circuit or sometimes for use on their own as a general electronic component. The most common microelectronic components are semiconductor transistors, although all main electronic components (resistors, capacitors, inductors) can be created at a microscopic level. Nanoelectronics is the further scaling of devices down to nanometer levels.

Microelectronic components are created by chemically fabricating wafers of semiconductors such as silicon (at higher frequencies, compound semiconductors like gallium arsenide and indium phosphide) to obtain the desired transport of electronic charge and control of current. The field of microelectronics involves a significant amount of chemistry and material science and requires the electronic engineer working in the field to have a very good working knowledge of the effects of quantum mechanics.

Signal processing

Signal processing deals with the analysis and manipulation of signals. Signals can be either analog, in which case the signal varies continuously according to the information, or digital, in which case the signal varies according to a series of discrete values representing the information. For analog signals, signal processing may involve the amplification and filtering of audio signals for audio equipment or the modulation and demodulation of signals for telecommunications. For digital signals, signal processing may involve the compression, error detection and error correction of digitally sampled signals.

Signal Processing is a very mathematically oriented and intensive area forming the core of digital signal processing and it is rapidly expanding with new applications in every field of electrical engineering such as communications, control, radar, TV/Audio/Video engineering, power electronics and bio-medical engineering as many already existing analog systems are replaced with their digital counterparts.

Although in the classical era, analog signal processing only provided a mathematical description of a system to be designed, which is actually implemented by the analog hardware engineers, Digital Signal Processing both provides a mathematical description of the systems to be designed and also actually implements them (either by software programming or by hardware embedding) without much dependency on hardware issues, which exponentiates the importance and success of DSP engineering.

The deep and strong relations between signals and the information they carry makes signal processing equivalent of information processing. Which is the reason why the field finds so many diversified applications. DSP processor ICs are found in every type of modern electronic systems and products including, SDTV | HDTV sets, radios and mobile communication devices, Hi-Fi audio equipments, Dolby noise reduction algorithms, GSM mobile phones, mp3 multimedia players, camcorders and digital cameras, automobile control systems, noise cancelling headphones, digital spectrum analyzers, intelligent missile guidance, radar, GPS based cruise control systems and all kinds of image processing, video processing, audio processing and speech processing systems.

Telecommunications

Telecommunications engineering focuses on the transmission of information across a channel such as a coax cable, optical fiber or free space. Transmissions across free space require information to be encoded in a carrier wave in order to shift the information to a carrier frequency suitable for transmission, this is known as modulation. Popular analog modulation techniques include amplitude modulation and frequency modulation. The choice of modulation affects the cost and performance of a system and these two factors must be balanced carefully by the engineer.

Once the transmission characteristics of a system are determined, telecommunication engineers design the transmitters and receivers needed for such systems. These two are sometimes combined to form a two-way communication device known as a transceiver. A key consideration in the design of transmitters is their power consumption as this is closely related to their signal strength. If the signal strength of a transmitter is insufficient the signal's information will be corrupted by noise.

Instrumentation

Instrumentation engineering deals with the design of devices to measure physical quantities such as pressure, flow and temperature. The design of such instrumentation requires a good understanding of physics that often extends beyond electromagnetic theory. For example, flight instruments measure variables such as wind speed and altitude to enable pilots the control of aircraft analytically. Similarly, thermocouples use the Peltier-Seebeck effect to measure the temperature difference between two points.

Often instrumentation is not used by itself, but instead as the sensors of larger electrical systems. For example, a thermocouple might be used to help ensure a furnace's temperature remains constant. For this reason, instrumentation engineering is often viewed as the counterpart of control engineering.

Computers

Computer engineering deals with the design of computers and computer systems. This may involve the design of new hardware, the design of PDAs and supercomputers or the use of computers to control an industrial plant. Computer engineers may also work on a system's software. However, the design of complex software systems is often the domain of software engineering, which is usually considered a separate discipline. Desktop computers represent a tiny fraction of the devices a computer engineer might work on, as computer-like architectures are now found in a range of devices including video game consoles and DVD players.

Source : http://en.wikipedia.org/wiki/Electrical_engineering