ELECTRICAL HAZARDS GENERAL CONTROL MEASURES BASIC INFORMATION AND TUTORIALS

General control measures for electrical hazards

The principal control measures for electrical hazards are contained in the statutory precautionary requirements covered by the Electricity at Work Regulations. They are applicable to all electrical equipment and systems found at the workplace and impose duties on employers, employees and the self-employed.

The regulations cover the following topics:

S the design, construction and maintenance of electrical systems, work activities and protective
equipment
S the strength and capability of electrical equipment
S the protection of equipment against adverse and hazardous environments
S the insulation, protection and placing of electrical conductors
S the earthing of conductors and other suitable precautions
S the integrity of referenced conductors
S the suitability of joints and connections used in electrical systems
S means for protection from excess current
S means for cutting off the supply and for isolation
S the precautions to be taken for work on equipment made dead
S working on or near live conductors
S adequate working space, access and lighting
S the competence requirements for persons working on electrical equipment to prevent danger and injury.

Detailed safety standards for designers and installers of electrical systems and equipment are given a code of practice published by the Institution of Electrical Engineers, known as the IEE Regulations. While these regulations are not legally binding, they are recognized as a code of good practice and widely used as an industry standard.

The risk of injury and damage inherent in the use of electricity can only be controlled effectively by the introduction of employee training, safe operating procedures (safe systems of work) and guidance to cover specific tasks.

Training is required at all levels of the organization ranging from simple on-the-job instruction to apprenticeship for electrical technicians and supervisory courses for experienced electrical engineers. First aid training related to the need for cardiovascular resuscitation and treatment of electric burns should be available to all people working on electrical equipment and their supervisors.

A management system should be in place to ensure that the electrical systems are installed, operated and maintained in a safe manner. All managers should be responsible for the provision of adequate resources of people, material and advice to ensure that the safety of electrical systems under their control is satisfactory and that safe systems of work are in place for all electrical equipment

For small factories and office or shop premises where the system voltages are normally at mains voltage, it may be necessary for an external competent person to be available to offer the necessary advice.

Managers must set up a high voltage permit-to-work system for all work at and above 600 volts. The system should be appropriate to the extent of the electrical system involved. Consideration should also be given to the introduction of a permit system for voltages under 600 volts when appropriate and for all work on live conductors.

The additional control measures that should be taken when working with electrical or using electrical equipment are summarized by the following topics:
S the selection of suitable equipment
S the use of protective systems
S inspection and maintenance strategies

ELECTRICAL SAFETY HAZARDS THAT NEEDS TO BE IDENTIFIED ON ELECTRICAL SAFETY WORKS

WHAT ARE THE DIFFERENT ELECTRICAL SAFETY HAZARDS?

Identifying electrical safety hazards.



When electrical systems break down what are the primary hazards and what are the consequences to personnel? Electric shock Exposure to Arc-Flash Exposure to Arc-Blast Exposure to excessive light and sound energies

Secondary hazards may include burns, the release of toxic gases, molten metal, airborne debris and shrapnel. Unexpected events can cause startled workers to lose their balance and fall from ladders or jerk their muscles possibly causing whiplash or other injuries.

Electric Shock
When personnel come in contact with energized conductors they receive a shock with current flowing through their skin, muscles and vital organs. The severity of the shock depends on the current’s path through the body, the current intensity, and the duration of the contact.

They may only experience a mild tingling sensation or it could result in serious injury or death. As voltage levels increase, the effects of electric shock escalate. Current may also cause an erratic heartbeat known as ventricular fibrillation.

If fibrillation occurs even briefly and goes untreated, the effects are usually fatal. A clear understanding of how electric current travels through the body can help minimize injury if such contact occurs. The table below outlines the effects that various values of electrical current have on the human body.

There are three basic pathways electric current travels through the body;

1) Touch Potential (hand/hand path)

2) Step Potential (foot/foot path)

3) Touch/Step Potential (hand/foot path)



1) In a touch potential contact, current travels from one hand through the heart and out through the other hand. Because the heart and lungs are in the path of current, ventricular fibrillation, difficulty in breathing, unconsciousness, or death may occur.

2) In a step potential contact, current travels from one foot through the legs, and out of the other foot. The heart is not in the direct path of current but the leg muscles may contract, causing the victim to collapse or be momentarily paralyzed.

3) In a touch/step potential contact, current travels from one hand, through the heart, down the leg, and out of the foot. The heart and lungs are in the direct path of current so ventricular fibrillation, difficulty in breathing, collapse, unconsciousness, or death may occur.

Even though there may be no external signs from the electrical shock, internal tissue or organ damage may have occurred. Signs of internal damage may not surface immediately; and when it does, it may be too late.

Any person experiencing any kind of electrical shock should seek immediate medical attention. Using the correct personal protective equipment (PPE) and following safe work practices will minimize risk of electrical shock hazards.

Arc-Flash and Arc Blasts 

An Arc-Flash is an unexpected sudden release of heat and light energy produced by electricity traveling through air, usually caused by accidental contact between live conductors. Temperatures at the arc terminals can reach or exceed 35,000 degrees Fahrenheit (F), or four times the temperature of the sun’s surface.

The air and gases surrounding the arc are instantly heated and the conductors are vaporized causing a pressure wave called an Arc Blast. Personnel directly exposed to an Arc-Flash and Arc-Blast events are subject to third degree burns, possible blindness, shock, blast effects and hearing loss. Even relatively small arcs can cause severe injury.

The secondary effect of arcs includes toxic gases, airborne debris, and potential damage to electrical equipment, enclosures and raceways. The high temperatures of the arc and the molten and vaporized metals quickly ignite any flammable materials.

While these fires may cause extensive property damage and loss of production, the hazards to personnel are even greater. Any energized electrical conductor that makes accidental contact with another conductor or with ground will produce an Arc-Flash.

The arcing current will continue to flow until the overcurrent protective device used upstream opens the circuit or until something else causes the current to stop flowing. The arc current can vary up to the maximum available bolted fault current

SAFETY ON WORKING OVER OVERHEAD POWER LINES BASIC INFORMATION

Statistics on accidental electrocution show that quite a few of them involve work on or near overhead electric lines. Work on overhead lines is only to be done by qualified electrical lineworkers.

Many times, due to the need to maintain service continuity, the lines are kept energized while work is being performed on them. Lineworkers must be well trained to perform such tasks using safe practices, appropriate personal protective equipment, and insulated tools.

When planning for work on overhead lines, however, one should always try to make the safest choice, which is to put the lines in an electrically safe work condition. Grounding the lines to create an equipotential zone within which a lineworker can be safe is advisable while working on overhead lines.

Work on or near overhead lines requires unique safety analysis because

a) The overhead lines can change position due to wind or other disturbances.
b) A person working on the lines is not usually in the most stable position.
c) The voltages and energy levels involved with overhead lines are often large.

Working near overhead lines, or near vehicles and equipment that could contact overhead lines, requires electrical safety training even for nonelectrical personnel.

The National Electrical Safety Code¨ (NESC¨) (Accredited Standards Committee C2-1997) is a key document that gives significant detail regarding the safety rules for the installation and maintenance of overhead electric supply and communication lines. NFPA 70E-1995 also mentions safety around overhead lines in Part II.

The OSHA regulations that cover work on and near overhead electric lines are 29 CFR 1910.269 and 29 CFR 1910.333 for general industry, and 29 CFR 1926.955 for the construction industry.

TEMPORARY PERSONAL PROTECTIVE GROUNDING BASICS

What is temporary personal protective grounding?

Sometimes, additional measures are desirable to provide an extra margin of safety assurance. Temporary personal protective grounds are used when working on de-energized electrical conductors to minimize the possibility of accidental re-energization from unexpected sources. Sometimes these are called safety grounds or equipotential grounding.

Induced voltages, capacitive recharging, and accidental contact with other circuits can occur. Depending on the electrical energy available, these occurrences could cause injury or death.

More often, however, they only cause reßexive actions. For example, although most induced voltages will not normally cause serious injury themselves, they could cause a person to jump backward suddenly, possibly tripping against something or falling to the floor.

Temporary protective grounding devices should be applied where such conditions might occur. Temporary personal protective grounds should be applied at possible points of re-energization. They can also be applied in such a way as to establish a zone of equipotential around a person.

When these grounds are used, they shall be connected tightly, since they establish a deliberate fault point in the circuit. If current does somehow get onto the circuit, the grounds shall stay connected securely until a protective device clears the circuit.

It is difficult to set firm criteria for when temporary personal protective grounds are needed. Blanket requirements are usually established. Many times, it is a decision made in the field by the person performing the work.

When there is uncertainty about exposure, it is wise to add this extra protection. Many industrial facilities and utilities require temporary personal protective grounding for all aerial power line work and for all work on power systems over 600 V because of the increased exposure these systems often have due to their length and location.

Temporary personal protective grounding can also be used as the additional safety measure required when hazardous electrical energy control must be performed using a tag only. Temporary personal protective grounding devices should meet the specifications in ASTM F855-96 and should be sized for the maximum available current of any possible event.

Temporary personal protective grounds should only be installed after all other conditions of an electrically safe work condition have been established. Because the unexpected can happen at any time, however, the installation and removal of temporary grounding devices should be performed, by procedure, as the conductors are energized.

When installed inside equipment enclosures, temporary grounds should be lengthy enough to extend outside of the equipment so that they can be easily seen. If they cannot extend out, they should be made highly visible. Brightly colored tapes are helpful identifiers. Once they are installed, bare-hand work could be permitted.

It should be quite obvious that all personal protective grounds must be removed prior to reenergization. Identification and accountability controls may be necessary on large construction or maintenance jobs. The installation and removal of these grounding devices can be controlled by permit in order to avoid re-energizing equipment into a faulted condition.

The integrity of personal protective grounds should be maintained through the use of periodic inspection and testing. It is a good idea to document this inspection and testing.

POOR HAZARDOUS ELECTRICAL ENERGY CONTROL PRACTICES

What are the examples of poor hazardous energy control practices.

The following items discuss some practices that were used in the past for safety control. These practices are not truly safe practices and should not be used today.

a) Locking out a push-button, control switch, or other pilot device does not ensure that the circuit will remain de-energized. A short circuit or ground in the control circuit can bypass the pilot device.

Another employee might even engage the contactor or starter by hand. Unless the disconnecting means is opened and locked out, an employee should not place himself in a position where unexpected equipment startup or energization might cause injury.

b) Turning the handle of a disconnect switch to the "off' position does not ensure safety. The switch linkage might be broken, leaving the switchblades engaged.

Switchblades in the open position should be confirmed by visual inspection. The load side of the switch should also be checked with a voltage tester to ensure that the outgoing circuit is de-energized, and that there is no backfeed.

c) Removing and tagging fuses does not constitute a lockout/tagout. A lockout/tagout device should be attached to the fuse clips in a manner such that no fuses can be inserted without removing the device.

If fuses are contained in a drawout fuse block,  the tag should be attached to the fuse panel, not to the drawout block. Special precautions shall be taken to prevent shock whenever energized fuse clips that are accessible to the touch must be tagged.

d) Simply opening a power circuit breaker does not ensure safety. Even if the control fuses are removed, the breaker can still be engaged with the manual operating mechanism.

The switchgear must be racked away from the bus contacts and into the "fully disconnected" position, and the racking mechanism shall be locked and tagged.

ELECTRICAL SAFE PRACTICES PROCEDURE OUTLINE BASIC INFORMATION

Typical outline of an electrical safe practices procedure

-Title. The title identifies the specific equipment where the procedure applies.

-Purpose. The purpose is to identify the task to be performed.

-Qualification. The training and knowledge that qualified personnel shall possess in order to perform particular tasks are identified.

-Hazard identification. The hazards that were identified during development of the procedure are highlighted. These are the hazards that may not appear obvious to personnel performing work on or near the energized equipment.

-Hazard classification. The degree of risk, as deÞned by the hazard/risk analysis, is identified for the particular task to be performed.

-Limits of approach. The approach distances and restrictions are identified for personnel access around energized electrical equipment.

-Safe work practices. The controls that shall be in place prior to, and during the performance of, work on or near energized equipment are emphasized.

-Personnel protective clothing and equipment. The minimum types and amounts of protective clothing and equipment that are required by personnel to perform the tasks described in the procedures are listed. Personnel performing the work shall wear the protective clothing at all times while performing the tasks identified in the procedure.

-Test equipment and tools. All the test equipment and tools that are required to perform the work described in this procedure are listed. The test equipment and tools shall be maintained and operated in accordance with the manufacturer's instructions.

-Reference data. The reference material used in the development of the procedure is listed. It includes the appropriate electrical single-line diagrams, equipment rating (voltage level), and manufacturer's operating instructions.

-Procedure steps. The steps required by qualified personnel wearing personal protective clothing and using the approved test equipment to perform specific tasks in a specified manner are identified.

-Sketches. Sketches are used, where necessary, to properly illustrate and elaborate specific tasks.

WORK AUTHORIZATION - WORK PERMITS FOR ELECTRICAL WORKS

Before beginning any work, particularly in an existing operating facility, a person should receive a request to do the work from the custodian, thoroughly plan the job, review the job plan with the custodian, and obtain permission from the facility manager to proceed with the work.

Some kind of work authorization document is advisable to ensure that everyone who may be affected is aware of what is going on. In addition to approvals, this document could contain a checklist of safety items that should be considered before proceeding with the work.

The work authorization document forces people to think about the safety aspects of the job. This concept applies to all kinds of work, not just electrical.

Again, when performing electrical work in a country other than the U.S., make sure that any laws of the country that may be applicable to the job being performed are known.

SWITCHING TRANSIENT LOADING EFFECTS ON THE SYSTEM BASIC INFORMATION AND TUTORIALS

One of the primary uses of electricity is for general lighting and the local DNO must ensure that its supply is suitable for this purpose. Repeated sudden changes in voltage of a few per cent are noticeable and are likely to cause annoyance.

The local DNO must ensure that these sudden variations are kept within acceptable levels and this means placing limits on consumers’ apparatus which demands surges of current large enough to cause lighting to flicker.

In order to evaluate flicker in measurable terms, two levels have been selected: the threshold of visibility and the threshold of annoyance. 

Both are functions of frequency of occurrence as well as voltage change.

Since both these thresholds are subjective it has been necessary to carry out experiments with various forms of lighting and panels of observers to ascertain consensus relationships between frequency of occurrence and percentage voltage change for the two thresholds.

The DNOs have used this information in setting the planning levels for flicker contained in Engineering Recommendation P28, which govern motor starting currents, etc.

The network impedance from the source to the point of common coupling between the lighting and the offending load is of paramount importance and thus the local office of the DNO should be consulted in cases where the possibility of creating an annoyance arises.

Intermittently loaded or frequently started motors, such as those on lifts, car crushers, etc., together with instantaneous water heaters, arc welders and furnaces, are all potential sources of disturbance.

Large electric furnaces present a particular problem and it is frequently necessary to connect them to a higher voltage system than is necessary to meet their load in order to achieve a lower source impedance.

Fluctuations occurring about ten times a second exhibit the maximum annoyance to most people, but even those as intermittent as one or two an hour will annoy if the step change is of sufficient magnitude.

CENELEC Standard EN61000-3-3, limits voltage fluctuation emissions from equipment rated less than or equal to 16 A and EN61000-3-11 limits emissions from equipment rated from 16 A to 75A.

UNBALANCED LOADS AND POWER FACTOR LOADING EFFECTS ON THE SYSTEM BASIC INFORMATION AND TUTORIALS

Any normal load causes a voltage drop throughout the system. This is allowed for in the design, and the cost associated with the losses incurred is recovered in the related unit sales.

Unbalanced loads

Unequal loading between the phases of the network causes an unequal displacement of the voltages. Extreme inequality causes motors and other polyphase equipment to take unequal current and perhaps become overloaded on one phase.

For this reason DNOs impose limits on the extent to which they accept unbalanced loads at any particular location in order to ensure that other consumers are not adversely affected. Installation designers need to ensure that the same problem does not arise due to an unbalanced voltage drop within the consumer’s installation itself.

While most voltage unbalance is caused by single-phase loading, the effect on a three-phase motor can best be assessed in terms of the negative phase sequence component of the voltage thereby created.

Providing that this is less than 2% the inequality of current between phases should not be more than the motor has been designed to withstand. Engineering Recommendation P29 aims to limit continuous levels of voltage unbalance to 1%.

Power factor

Many types of apparatus such as motors and fluorescent lighting also require reactive power and thereby take a higher current than is necessary to supply the true power alone. This extra current is not recorded by the kWh meter but nevertheless has to be carried by the distribution system and uses up its capacity thereby.

It also increases the losses on the system. 

A power factor of 0.7 means that the current is 1/0.7 = 1.43 times as great as absolutely necessary and thus doubles the losses (I2 R). If all the loads in the UK were permitted to have as low a power factor as this, the additional cost of the losses (if the system could stand the burden) would be in the order of £200 million per annum.

TOP 10 HAZARDOUS TASKS IN ELECTRICAL WORKS

Typical hazardous tasks in electrical work

The following tasks are some examples of possible exposure to energized conductors:

a) Measuring, testing, and probing electrical system components;

b) Working near battery banks;

c) Opening electrical equipment enclosure doors or removing covers;

d) Inserting or pulling fuses;

e) Drilling, or otherwise penetrating, earth, walls, or ßoors;

f) Pulling conductors in raceways, cable trays, or enclosures;

g) Lifting leads or applying jumpers in control circuits;

h) Installing or removing temporary grounds;

i) Operating switches or circuit breakers;

j) Working inside electronic and communications equipment enclosures.

THE DANGERS OF ASBESTOS - BASIC INFORMATION AND TUTORIALS

What are the dangers of inhaling asbestos in construction?

Inhaling asbestos dust has been shown to cause the following diseases:

• asbestosis
• lung cancer
• mesothelioma (cancer of the lining of the chest and/or abdomen).

Asbestosis is a disease of the lungs caused by scar tissue forming around ve ry small asbestos fibres deposited deep in the lungs. As the amount of scar tissue increases, the ability of the lungs to expand and contract decreases, causing shortness of breath and a heavier wo rkload on the heart.

Ultimately, asbestosis can be fatal.

Lung cancer appears quite frequently in people exposed to asbestos dust.While science and medicine have not yet been able to explain precisely why or how asbestos causes lung cancer to develop, it is clear that exposure to asbestos dust can increase the risk of contracting this disease.

Studies of asbestos wo rkers have shown that the risk is roughly five times greater than for people who are not exposed to asbestos.

Cigarette smoking, another cause of lung cancer, multiplies this risk . Research has shown that the risk of developing cancer is fifty times higher for asbestos workers who smoke than for workers who neither smoke nor work with asbestos.

Mesothelioma is a relatively rare cancer of the lining of the chest and/or abdomen.While this disease is seldom observed in the general population, it appears frequently in groups exposed to asbestos.

Other illnesses—There is also some evidence of an increased risk of cancer of the stomach, rectum, and larynx. However, the link between asbestos exposure and the development of these illnesses is not as clear as with lung cancer or mesothelioma.

The diseases described above do not respond well to current medical treatment and, as a result, are often fatal.

SAFETY SWITCHES FOR ELECTRONIC EQUIPMENT BASIC INFORMATION

Fuses are typically installed in safety switches. Separately mounted fused safety switches are typically categorized as general-duty and heavy-duty types.

The general-duty type safety switch is rated at 240 V maximum and is typically used in residential and light commercial and industrial applications. The heavy-duty type safety switch is rated at 600 V maximum and is typically used in commercial and industrial applications.

Safety switches can typically be ordered with neutral assemblies and equipment grounding assemblies. There is currently no listing for safety switches that are to be used specifically with nonlinear loads.

It is recommended that the manufacturer be contacted to determine if oversized neutral assemblies can be installed in safety switches serving nonlinear electronic load equipment without voiding any listing requirements. In addition, the manufacturer should be contacted to determine if an isolated equipment grounding bus can be installed in the safety switch enclosure for those applications that require this grounding configuration.

Whenever fuses are utilized, there is a risk of a single-phasing condition if one fuse on a three phase system blows. Safety switches are generally not stored energy devices, and may not contain auxiliary functions such as undervoltage release or shunt trip attachments that help protect against a single-phasing condition.

This is an important consideration because some three phase electronic load equipment may be susceptible to damage if a single-phase condition persists. Other devices may need to be installed to provide proper single-phasing protection.

Blown fuse indicators
Recommended practice is to use blown fuse indicators for the quick and safe determination of the source of power outage affecting downstream electronic load equipment. Some safety switches and fused circuit breakers contain indicating devices located on the front enclosure that indicate a blown fuse condition. Some fuses contain an indicator light, providing visual indication that a fuse is blown.

Interrupting ratings
Interrupting ratings of new fuses or existing fuses should be evaluated to determine if proper interrupting ratings are applied. Interrupting ratings need to be reevaluated if there are any changes to the power system, such as installing K-factor transformers.

These transformers are typically specified or manufactured with a low impedance (%Z) resulting in a higher available short-circuit current on the secondary. This condition can be a problem especially where low interrupting capacity fuses, such as Class H fuses, are installed (Class H fuses have an interrupting rating of only 10 000 A).

BEST LOCATIONS FOR PANEL BOARDS AND SWITCH BOARDS OF ELECTRONIC EQUIPMENT BASIC TUTORIALS

Where should panel boards be located?

Switchboards and panelboards that support electronic load equipment and related loads should be properly designed and installed. Recommended practice is to use panelboards specifically listed for nonlinear loads if they serve electronic load equipment.

As a minimum, panelboards should be rated for power or lighting applications, and should not be a lighterduty type. Special attention should be given to the location and installation methods used when installing panelboards.

In addition, protective devices shall adequately protect system components, neutral buses should be sized to accommodate increased neutral currents due to harmonic currents from nonlinear electronic load equipment, and equipment ground buses should be sized to accommodate increased numbers of equipment grounding conductors due to the recommended practices of using insulated equipment grounding conductors and dedicated circuits for electronic load equipment.

Surge protective devices may also be installed external to, or internal to, the switchboards or panelboards.

Location
Panelboards that serve electronic load equipment should be placed as near to the electronic load equipment as practicable, and should be bonded to the same ground reference as the electronic load equipment.

Other panelboards located in the same area as the electronic load equipment that serve other loads such as lighting, heating, ventilation, air conditioning, and process cooling equipment should also be bonded to the same ground reference as the electronic load equipment.

Panelboards should be directly mounted to any building steel member in the immediate area of the installation. Isolation of a panelboard from the metallic building structure by an electrically insulating material, as an attempt to prevent flow of high frequency current through the panelboard, is not recommended practice.

The panelboard and metallic building structure, separated by a dielectric material, become capacitively coupled. The capacitive coupling presents a low impedance at high frequency defeating the original purpose.

NFPA 780-1997 requires effective grounding and bonding between objects such as structural building steel and a panelboard located within side-flash distance (approximately 1.8 m (6 ft), horizontally) of each other. Insulation materials, commonly used in an attempt to separate a panelboard from building steel, are rarely capable of withstanding lightningi nduced arcing conditions.

K - RATED TRANSFORMERS BASIC INFORMATION AND TUTORIALS

What are K-Rated Transformers?

UL and transformer manufacturers have established a K-factor rating for dry-type power transformers to indicate their suitability for supplying nonsinusoidal load currents. The K-factor relates a transformer’s capability to serve varying degrees of nonlinear load without exceeding the rated temperature-rise limits.

The K-factor is the ratio of stray losses in the transformer winding for a given nonsinusoidal load current to the stray losses in the transformer winding produced by a sinusoidal load current of the same magnitude. These transformers are typically specially designed to handle the increased heating effects and neutral currents produced by nonlinear electronic load equipment. The following are some of the design features:

a) The neutral bus is rated at 200% of the secondary full load ampere rating to accommodate the large neutral currents that principally result from triplen harmonics and phase imbalance. The transformer neutral bus rated at 200% is capable of accommodating oversized or multiple neutral conductors.

b) The winding conductors are specially configured and sized to minimize heating due to harmonic load currents. Special configurations and sizing such as multiple, parallel conductors can reduce the skin effect of the higher frequency harmonics and accommodate the balanced triplen harmonics that circulate in the transformer primary (delta) windings.

c) Cores are specially designed to maintain flux core density below saturation due to distorted voltage waveforms or high line voltage. Standard K-factor ratings are 4, 9, 13, 20, 30, 40, and 50. The K-factor for a linear load is 1.

For any given nonlinear load, if the harmonic current components are known, the K-factor can be calculated and compared to the transformer’s nameplate K-factor (refer to

As long as the load K-factor is equal to or less than the rated K-factor of the transformer, the transformer is suitably rated and is considered safe to operate at rated load without overheating. Typical load K-factors for facilities containing large numbers of computers appear to range between 4 and 13.

Measured K-factor on the secondary of step-down transformers that serve almost exclusively nonlinear loads, such as personal computers, have been observed to range as high as 20, but this is extremely rare. In most cases, a transformer with a K-factor rating of 13 can be sufficient to handle typical nonlinear electronic load equipment.

ANALYSES OF HARMONIC CURRENTS AND VOLTAGE OF ELECTRONIC EQUIPMENT BASIC INFORMATION AND TUTORIALS

Refer to IEEE Std 519-1992 for a general discussion of harmonic currents. Recommended practice is for all power distribution systems intended for use with electronic load equipment comply with IEEE Std 519-1992 and IEEE Std 399-1997 guidelines. Calculation or estimation of load harmonic profiles is a necessary requirement when installing power factor correction equipment, selecting K-factor rated transformers or derating existing conventional transformers.

Improvements in power factor may be desired for financial reasons (to lower utility costs associated with power factor penalties) or operational reasons (to lower system losses, increase system reserve capacity, or improve voltage conditions). Extreme caution should be used when applying capacitors.

The manner in which they are applied can cause resonance conditions that can magnify harmonic levels and cause excessive voltage distortion. Power factor correction equipment may be applied directly at or close to the facility service entrance, or as close as practicable to the load equipment.

The location of the power factor equipment will depend on economic reasons, as well as operational and design considerations.

Thorough analysis of distribution system characteristics and load characteristics should be made prior to applying power factor correction capacitors to determine what effect harmonic currents will have on the system, and to determine proper harmonic mitigation techniques. Refer to IEEE Std 141-1993 for further discussion on application of power factor correction capacitors.

It is recommended practice to measure and record the harmonic profile of load currents at the transformers serving the load. When the harmonic profiles of individual loads at downstream locations are measured, there is a tendency to calculate a higher than necessary K-factor.

This is also the case in new installations where the current harmonic profile is estimated from typical
individual pieces of electronic load equipment based upon experience or data supplied by the OEM. Due to cancellation, the combined contribution to K-factor of several loads is always less than the sum of individual loads.

This reduction may be substantial when there is a large number and a diversity of nonlinear load types. Figure below shows an example of how harmonic levels vary in a typical electrical distribution system. Note that the level of harmonic current distortion decreases from the individual electronic load equipment to the branch circuit panelboards, through delta-wye stepdown transformers, and upstream to the power source.

However, when loads are removed from the electrical distribution system, the cancellation benefit produced by these loads is also removed. In many cases, this will not be a problem for a transformer that is conservatively loaded or is K-factor rated. It may be a problem if the load or K-factor rating is marginal.

Cancellation results when harmonics produced by different loads are phase-shifted relative to each other. Impedance in branch circuit wiring, as well as isolation transformers or series inductors and shunt capacitors that may be incorporated in the loads, shift harmonic currents.

A delta-wye transformer serving single-phase nonlinear loads randomly distributed among the three phases will trap the balanced triplen load harmonics in the primary winding. This may substantially reduce the triplen harmonic currents and the related current and voltage distortion that would otherwise appear on the primary side.

It is difficult to predict a harmonic diversity factor without modeling the nonlinear loads and the electrical distribution system. Computer programs and methods that allow modeling and simulation are becoming available.

With more experience, these computer analysis tools are expected to provide diversity factors for typical loads in industrial and commercial power systems. For new installations, where such diversity factors are not available, recommended practice is to monitor the load current distortion and diversity relative to the load mix in a comparable facility.

THREE PHASE VS SINGLE PHASE SYSTEM ELECTRONIC EQUIPMENT- WHAT'S THE DIFFERENCE, WHICH IS BETTER? BASIC INFORMATION

When to use single phase, and three phase system?

Some power conditioning and electronic load equipment are operable only from a three phase power source. Often single-phase equipment can be operated directly from a single phase component of a three-phase system. However, these alternatives should be carefully determined before selecting an electrical system design.

The acceptable voltage limits of all equipment must be determined and carefully evaluated to ensure proper operation on the electrical system into which it is installed. Some equipment may have features such as internal taps or other adjustments that will allow it to accept common utilization voltages.

When evaluating the choice between three-phase and single-phase systems, consideration should always be given to the fact that three-phase systems may generally support larger loads with greater efficiency. In addition, the source impedance of three-phase systems is generally lower than single-phase systems, which is important to minimize voltage waveform distortion due to nonlinear load currents. Three-phase power may also be derived from single- phase systems.

However, the derivation of three-phase power from a single-phase system is not always practical and is not recommended. Certain methods of converting a single-phase circuit to supply three-phase loads such as capacitor phase shifters are considered inappropriate for electronic load equipment and may damage these loads per IEEE Std 141-1993.

Still other methods, such as utilizing single-phase motors to drive three-phase generators, may be used to convert single-phase to three-phase. Even so, special precautions should be observed such as balancing the load among the three phases.

Most three-phase electronic load equipment cannot tolerate the application of single-phase power to its input. The resulting downtime and equipment damage can be extensive.

Because fuses and circuit breakers generally cannot prevent all types of single-phasing conditions, recommended practice is that electronic phase-failure or voltage-unbalance relays be installed where necessary to mitigate single-phasing events.

SELECTION OF SYSTEM VOLTAGE FOR ELECTRONIC EQUIPMENT SAFETY BASIC INFORMATION AND TUTORIALS

The selection of the ac supply system voltage typically begins at the service entrance of the facility. In most commercial environments in the U. S., the utility supplies three-phase power at 480 Y/277 V (or 600 Y/347 V) or 208 Y/120 V.

In industrial environments, the utility may supply three-phase power at even higher voltages such as 4160 V, 13 800 V and higher. The magnitude of the voltage will typically depend on the size of the facility, the load conditions, and the voltage ratings of the utilization equipment in the facility.

In some cases, the facility owners may design, install, and maintain their own medium-voltage electrical distribution system.

Recommended practice is to provide distribution power in most facilities at 480 Y/277 V (or 600 Y/347 V) rather than at the actual utilization equipment level of most electronic load equipment (208 Y/120 V). Electrical distribution systems operating at 480 Y/277 V (or 600 Y/ 347 V) have the following benefits over 208 Y/120 V systems:

a) The source impedance of 480 Y/277 V systems are typically less than 208 Y/120 V systems. This characteristic provides a more stable source with better voltage regulation, and minimizes voltage distortion due to the nonlinear load currents.

b) 480 Y/277 V systems are less susceptible to on-premises generated disturbances. Step-down transformers (and other power enhancement devices) for 208 Y/120 V utilization equipment help attenuate disturbances originating on the 480 V system.

c) 480 Y/277 V systems distribute power at lower currents, which result in lower heat losses in feeders. 480 Y/277 V systems may also decrease material and labor costs associated with installing long feeder circuits.

Step-down transformers (and other power enhancement devices) may be located physically close to the electronic load equipment to minimize the buildup of common-mode voltage.

Delta-connected transformer primaries trap balanced triplen harmonic currents generated on the secondary side by nonlinear electronic load equipment. This action serves to reduce distortion of the voltage waveform at the 480 Y/277 V level.

It is not recommended practice to step-up the voDue to the generally lower impedance of 480 Y/277 V distribution systems, higher short-circuit currents may be available throughout the system. Overcurrent protective devices with higher interrupting capabilities and equipment with higher withstand ratings may be required.

In some situations, electrical distribution at 208 Y/120 V is unavoidable. This may be due to limitations of the utility or facility to provide higher voltages. As previously noted, nonlinear electronic load equipment may cause undesirable voltage distortion that can adversely affect the entire premises.

In these situations, a system analysis may be performed to determine proper mitigation techniques such as the installation of isolation transformers, and other power conditioning or filtering equipment located close to the electronic load equipment tage from the service entrance by means of a locally installed transformer in order to obtain a higher power system voltage for the electrical distribution system serving electronic load equipment.

Although this can be done in certain cases, it is also possible that less satisfactory results can occur than if the system voltage at the service entrance was used.

TYPES OF AC GENERATOR ROTORS BASIC INFORMATION AND TUTORIALS

Synchronous AC generators are fitted with one of two different rotor designs depending on their intended rotational speeds.

Round rotors are solid steel cylinders with the field winding inserted in slots milled into the surface or the rotor. They usually have two or four poles. Round rotors can withstand the stresses of high-speed rotation.


Salient-pole rotors have multiple pole pieces (typically six) mounted to the rotor structure, and the field winding is wound around the pole pieces. Because of their more complex construction and larger diameter-to-length ratios, salient-pole rotors cannot withstand the stresses of high-speed rotation.


Electric utility steam-turbine–driven generators designed for 50- or 60-Hz AC output voltage have round rotors with two poles because they can withstand the stresses of speeds of 3000 and 3600 rpm.

Hydroelectric, diesel, and natural-gas engines have far lower shaft speeds than steam turbines, so the generators they drive usually have six or more pole rotors, requirements usually met with more complex salient-pole rotors.

Three-phase AC generators have a winding that is made up of three separate stator windings, each displaced from the other two by 120 electrical degrees. The three windings can either be wye- or delta-connected. The wye connection is more common because it is better suited for direct high-voltage generation.

TRANSFORMER TERMS GLOSSARY BASIC INFORMATION AND TUTORIALS

The following technical terms apply to transformers.

BIL: An abbreviation for basic impulse level, a dielectric strength test. Transformer BIL is determined by applying a high-frequency square-wave voltage with a steep leading edge between the windings and between the windings and ground.

The BIL rating provides the maximum input kV rating that a transformer can withstand without causing insulation breakdown. The transformer must also be protected against natural or man-made electrical surges. The NEMA standard BIL rating is 10 kV.

Exciting current: In transformers, the current in amperes required for excitation. This current consists of two components: (1) real in the form of losses (no load watts) and (2) reactive power in kvar. Exciting current varies inversely with kVA rating from approximately 10 percent at 1 kVA to as low as 0.5 percent at 750 kVA.

Eddy-current losses: Contiguous energy losses caused when a varying magnetic flux sets up undesired eddy currents circulating in a ferromagnetic transformer core.

Hysteresis losses: Continuous energy losses in a ferromagnetic transformer core when it is taken through the complete magnetization cycle at the input frequency.

Insulating transformer: A term synonymous with isolating transformer, to describe the insulation or isolation between the primary and secondary windings. The only transformers that are not insulating or isolating are autotransformers.

Insulation system temperature: The maximum temperature in degrees Celsius at the hottest point in the winding.

Isolating transformer: See insulating transformer.

Shielded-winding transformer: A transformer with a conductive metal shield between the primary and secondary windings to attenuate transient noise.

Taps: Connections made to transformer windings other than at its terminals. They are provided on the input side of some high-voltage transformers to correct for high or low voltages so that the secondary terminals can deliver their full rated output voltages.

Temperature rise: The incremental temperature rise of the windings and insulation above the ambient
temperature.

Transformer impedance: The current-limiting characteristic of a transformer expressed as a percentage. It is used in determining the interrupting capacity of a circuit breaker or fuse that will protect the transformer primary.

Transformer voltage regulation: The difference between the no-load and full-load voltages expressed as a percentage. A transformer that delivers 200 V at no load and 190 V at full load has a regulation of 5 percent.

BUCK BOOST AUTOTRANSFORMERS BASIC INFORMATION AND TUTORIALS

The buck-boost transformer is a simple and economical means for raising a voltage that is too low or decreasing a voltage that is too high. This transformer can raise or lower voltage being supplied to the load more than ±5 percent, to improve the efficiency of the device or system.

Buck-boost transformers are small single-phase transformers designed to reduce (buck) or raise (boost) line voltage from 5 to 20 percent. A common application is boosting 208 V to 230 or 240 V AC.

For example, there might be a requirement to power the motor in an air conditioner with a 230- or 240-V AC motor from the 208-V AC supply line. This can be done with a buckboost transformer.

Buck-boost transformers are standard distribution transformers with ratings ranging from 50 VA to 10 kVA. Commercial units are made with primary voltages of 120, 240, or 480 V AC.

They can also power low-voltage circuits for control or lighting applications requiring 12, 16, 24, 32, or 48 V AC. Schematics of buck-boost transformers that can transform 120 and 240 V AC to 12 and 24 V AC are shown in the figure below.

When the primary and secondary lead wires of buck-boost transformers are connected together electrically in a recommended bucking or boosting connection, they become autotransformers. Some typical connection diagrams for these transformers in autotransformer arrangements for single-phase systems are shown below.

Buck-boost transformers have four windings for versatility. Their two primary and two secondary windings can be connected eight different ways to provide many different voltage and kVA outputs.

Because their output voltage is a function of input voltage, they cannot be used as voltage stabilizers. Output voltage will vary by the same percentage as the input voltage.

These transformers can also function in three-phase systems. Two or three units can be used to buck or boost three-phase voltage. The number of units needed in a three-phase installation depends on the number of wires in the supply line.