Non- Contact High Voltage Detector

The Non Contact High Voltage Indicator is a safety device which indicates with a buzzer sound and flashing light whether a conductor or an equipment is live or not. 

When working with High Voltages, it is extremely important to ensure that we know which equipment is live and which equipment is dead.

Too often, engineers and workers are exposed to severe injuries or even death when someone accidentally gets exposed to live conductors or tries to connect a test equipment to a conductor which they thought was dead.

Hence, it is absolutely essential that we ensure that equipment does not have any voltage and is discharged before we begin work.

The Non Contact High voltage indicator works by sensing the electric charge lines emanating from a point at high potential.  Since, these lines of electric charge can be detected at a distance from the conductor, contact with the conductor is not necessary.  The Non contact high voltage indicators available in the market can be used to detect a wide range of voltage from 80V to nearly 500kV.  These indicators are usually powered with alkaline batteries.

Each High voltage detector has a inbuilt test function which tests the device for functionality prior to use. 

The Non Contact High Voltage Indicator is held near the live conductor using a hot stick.  Gloves of the appropriate rating should be worn when using the equipment. 

Harmonic Mitigating Transformers

Harmonics Mitigating transformers are special kind of transformers which are used to mitigate harmonics in the connected loads. They are often confused with K Transformers. K Transformers(Refer Article) is a type of transformer which is over sized to cater to loads which generate harmonics. A Harmonics Mitigating Transformer, however, works by passively suppressing the harmonics.

Harmonics Mitigating Transformers are designed like ordinary transformers except that they have specific phase shifts (+15 deg, -15 deg, 30 degs). The primary winding is usually delta while the secondary winding is a zig zag winding. The zig zag winding of the secondary cancels a particular order of harmonics depending on the phase shift.

A Harmonics Analysis is conducted to identify the order of the harmonics generated before choosing the type of Harmonics Mitigating Transformer

The harmonics are prevented from reaching the primary and affecting transformer output. Harmonics Mitigating Transformers fall under the passive methods of harmonic suppression.

The Circular Mil

The circular mil is a unit of area which is used to specify the size of wires.  The mil is a thousandth of an inch which corresponds to 0.0254mm.  The circular mil is the area of a circle whose diameter is 1 mil.  The mil is also known as thou (thousands of an inch).

The circular mil is used as the area of the circle can be calculated without the use of Pi.

Thus the cross-sectional  Area of the circle wire = D²

     where D is the diamter of the circle in mils.

Larger wire sizes are rated in MCM or Thousand circular Mils.  If the conductor cross section is square shaped, the size will be indicated in Square mils.

Square Mils = π/4 * D²

Testing Electrical Protection Gloves

Air Leakage Test Kit

          Electrical Protection Gloves are an important part of the personnel protective equipment of people working in electric equipment.  Electrical Gloves should be used when working on High voltage equipment (See article on Electrical Gloves).

           Using damaged electrical gloves which do not have the required dielectric strength can seriously compromise safety and is extremely dangerous.  It is therefore essential to check electrical Gloves at regular intervals.  Electrical Gloves should be checked before they are issued to personnel and after every six month thereafter.  Electrical Gloves need to be checked for physical integrity and for insulating strength. 

               Physical integrity checks involve checks for cuts,abrasions or punctures.  This done by blowing air into the gloves and checking for any leakage.  There is no specific method to do this.  Just blowing air into the glove and them observing for any loss of air would be sufficient. Special Glove testers are available in the market. 

Insulation Test for Gloves

               The other test to be done on the gloves is the dielectric strength test.  Electrical gloves are rated by the voltage they should be used on.  It is necessary to test the gloves at this voltage.  The test is done by a special apparatus.

               The gloves are hung on holders and then filled with water.  The gloves are then immersed into the liquid bath.  A high voltage is applied between the water inside the glove and the water in the bath.  If current flows through the glove, the glove is rejected. 

               Glove testing should be carried out after every incident.

Ultra Isolation Transformers

          Ultra Isolation Transformers are a type of isolation transformers used in sensitive electronic systems.  They are also known as noise-cut off Transformers. 

          Isolation Transformer are transformers which are used to provide isolation between two parts of a circuit.  Isolation Transformers work by using a grounded shield between the primary and the secondary winding.  This shielding is used to prevent capacitive coupling between the primary and the secondary.

          Nevertheless, ordinary isolation transformers may not be enough for sensitive electronic systems such as those in hospitals, digital communication systems and sophisticated telemetry systems where the measured signals can be distorted due to noise and other disturbances.  Noise, spikes and transients can cause damage to sensitive electronic equipment.

          Ultra Isolation Transformers filter out noise other disturbances caused by switching transients. While they are constructed similar to isolation transformers, an electrostatic shield is used to reduce the capacitive coupling between the primary and the secondary.  The filtering effect is bi-directional i.e. any noise produced by the equipment connected to the ultra isolation transformer will also not be transmitted to the system.  Noise generating loads can thus be contained.

Ultra Isolation Transformers can also be used in a cascade.  The cascade greatly increases the filtering effect produced.

Control Transformers

Control Transformers are transformers which are used in control circuits of motor control circuits, auxilliary relays, etc.  These transformers step down the main power supply to a lower value which can be fed to the relays and contactors in the circuit. 

For instance, a crane whose motors require 440 VAc will have a control circuit which operates at 230V. The 230V power supply can be drawn from a step-down transformer which is connected to the mains power supply.

Control transformers make it simpler by drawing the power for the control circuit from the main power supply instead of having a separate power source.  The voltage for the control circuit can be 230 V or 110 V.  Lower voltages are also possible.  Low voltages may be necessary in hazardous environments. 

Another advantage of the control transformer is the isolation is the excellent transient response it provides.  When heavy contactors or electromagnetically operated valves are switched on, a high inrush current results.  The control transformer is able to withstand such high inrush currents and prevent any instability to the main power circuit. 

The control transformer should be designed to provide the steady state kVA demand of the control circuit.  In addition, it should also be capable of providing the inrush kVA demand which is the initial kVA when all the contactors and other coils may be switched on at the same time.  

Lighting Transformer

Lighting Transformers are usually used in residences, swimming pools, gardens to specifically feed lighting loads.  These transformers step the low voltage of the distribution system, 230 V to a still lower voltage (Extra Low Voltage) 12V or 24 V.  

The use of such low voltage, enables the design of smaller lights.  In applications such as lighting in swimming pool or gardens, lighting transformers producing voltages less than 30 V are used for safety reasons.

Lighting transformers are usually hidden in the ceiling or in the ground on in a recess in a wall.  Since the output voltage of the lighting transformer is quite low, they need to be placed very near the lighting load to avoid any drop in the voltage. 

A single lighting transformers can power more than one lighting load.  Lighting transformers for outdoor applications are designed with weatherproof enclosures. Lighting transformers can be classified into two major types, the electromagnetic type transformers and the electronic transformer type. 

 

The electromagnetic type transformer is designed like a conventional step down transformer.  A toroidal core is usually used to avoid the humming noise from the transformer.  

Lighting transformers also provide protection against surges, spikes and other transient phenomena

Frameless Motors

Frameless motors as the name suggests, are motors which do not have a frame.  The stator and the rotor are delivered separately and are assembled to the load.  Thus, the motor does not have bearings.  The rotor is directly coupled to the load while the stator is coupled to the frame of the machine.  These motors are custom designed to meet the speed requirement.

Thus this arrangement eliminates the need for the coupling.  This increases the stiffness of the power train of the machine.  It also eliminates torsional application.  The size of the motor is also considerably reduced to around one seventh. This reduced weight for a given power output reduces inertial in the machine and enables quick movements and direction reversals.  

A hand tool with Frameless Motor

Frameless motors are widely used in Hand tools, Medical devices, in satellite technology and in semiconductor equipment. 

Vibration Motors

Vibration motor used in cellphones

Vibration motors are motors which deliberately generate vibration.  These motors are used in mobile phones to create vibration alerting the user to a call or a message.

They are also used widely in the industries such as in construction industry to vibrate the concrete so that air pockets are not formed which the concrete is solidifying.  They are also used in mixers to prevent material from being left behind after the mixing process is over.

Vibration motors are also used in flour mills, pharmaceutical, food industries, etc to facilitate the smooth flow of materials in conveyors, hoppers and other mixing equipment. 

These motors are constructed just like normal motors.  However, they have a mechanically unbalanced weights attached to the output shaft.  These unbalanced weights create vibration.  The frequency and the magnitude of the vibration can be changed by modifying the shape and weight of the counterweights.

Wound Primary Current Transformers

Current Transformers can be classified into two types depending on the construction of the primary winding.  In bar type current transformers which are more common.  The conductor over which the current transformer is mounted also serves as the primary winding.  Thus the bar type current transformer has only the secondary winding.

In the Wound Primary current Transformer, the primary winding is also constructed inside the current transformer body.  The primary winding is usually of a single turn.  Wound Primary Current transformers are used in applications which require small current transformation ratios.  They are more accurate and have a higher burden capacity.  The wound primary is usually designed with toroidal cores for high efficiency. 

These current transformer are used in High Voltage applications

Resistors and Power Rating

Resistors find wide application in electrical and electronic circuits.  Resistors are used  in a wide range of applications such as to induce voltage drop, in filters, for energy dissipation and to limit current.  

While choosing a resistor it is necessary to ensure that the resistor has the proper wattage rating.  The wattage rating determines the amount of heat energy the resistors can absorb or dissipate.  When a resistor is selected, its wattage should also be checked for suitability to the application.  Selecting a resistors with the right ohmic value but the wrong wattage can cause serious damage to the resistor due to overheating.

When current flows through a resistor, heat is generated in accordance with Joule's Law.  H=I² x R. The resistor should be designed to withstand the heat generated.  The heat generated in a resistor is proportional the current which flows through it.  The current, in turn, is proportional to the voltage across the resistor. 

Power = V x I

Now I=V/R

Therefore Power = V²/R 

The Power rating for the resistor can thus be calculated using the above formula. 

High Power Resistors  with ratings of around 50 Watts are designed to be large in size (the large surface area dissipates heat).  The resistors are located away from other components to provide for ventilation. 

Wear in Brushes and Replacement

Carbon brushes find wide application in electric machines such as motors, generators, etc.  The function of a carbon brush is to transfer power from a stationary component to a rotating component or vice versa (See Article on Brushes).  

Over time, brushes can get worn off due to age, friction, heat and arcing.  Usually, the wear is all the brushes is uniform.  However, there are situations where the wear on one brush is more than the others.  Uneven wear can be caused by eccentricity in the commutator, uneven spring tensions,etc.

Carbon brushes need to be replaced all at the same time.  This ensures that the current is uniform.  Mixing old and new brushes can cause uneven wear to the commutator and damage the motor.

Flash Over - Effects, Causes and Prevention

Every year, many people die or are severely burned in Flash Over incidents.

Arc Flash  or Flash Overs occur during a low impedance fault between two conductors or one conductors and the ground.  The high current causes an arc flash which results in a high energy plasma.  This high energy arc carries very high energy as it is limited only by the impedance of the arc.

The temperature reaches 35000 deg. C (three times hotter than the surface of the sun).   Metal conductors vaporise at such high temperatures.    The immense thermal energy released causes all combustible materials to burn.

Arc Blast

The high temperature causes the air to expand causing pressure waves.  This is known as arc blast.  The impact of the arc blast can be felt within a radius of several metres.  These pressure waves can cause barotrauma (injuries caused due to the pressure waves on the brain, lungs, etc).  Molten metals, and broken parts of equipments such as insulators can be projected at speeds of 700 miles/h.

The high temperature causes severe burns and the vaporised metal can damage the lungs and other internal organs.  The bright lights can cause blindness.   The noise from the blast can cause hearing loss.

Flash Overs can occur at all voltages over 120V AC.

Preventing Flash Over incidents

• Identify all possible sources of supply.  Electric equipment such as breakers, capacitors, and bus bars can be fed from more than one supply.
• All switching equipment should be opened and visually verified if possible
• Lock out/Tag out devices should be applied.
• Voltage testers with hot rods should be used to check that the devices are dead.  All devices should be considered live if not earthed.
• Grounding rods should be used to discharge energy stored device (High voltage devices will retain voltage even after the supply has been switched off due to capacitance).  The equipment on which work is conducted should be kept grounded throughout the duration of the maintenance activity(as a precaution against induced voltage or accidentally coming in contact with live parts)

A proper Flash over Hazard Analysis should be carried out at all device locations.  A Flash over Hazard Analysis calculates the maximum current, the maximum energy which will be dissipated in the event of a Flash over and the Personnel Protection Equipment which is to be used. 

Refer link for more information on Arc Flash Hazard Analysis

http://www.avotraining.com/common/documents/John%20Lane%20ARC-FLASH%20PAPER.pdf