Air Termination Rods

Air Termination Rods are used to provide lightning protection to parts of a building which protrude from the superstructure.  Air termination rods can fixed to terraces, beams, etc.  Air termination rods consist of a rod or any pointed surface which is connected to the lightning protection system of the building. 

Air Termination Rods are also used for equipment which are kept in exposed flat surfaces such as solar panels and pipelines. 

Air Termination Rods may need to be supported against winds by means of suitable support structures.

Bonded Connections-An Overview

Bonded Connections are used to connect a conductor to another body such as a pipe, gate, door or a railing.  All metallic objects in a building or a facility need to be connected to the earth.  This is necessary to prevent accidental voltage getting induced in them due to contact

Bonded Connectors are usually galvanized to prevent corrosion.  They have flat surfaces which increase the contact area with the object to be bonded.  They are made of a material similar to the conductor material such as copper or aluminium. 

Voltage Limiting Devices for Electric Traction

Voltage limiting devices are used in Electric Traction systems, to prevent dangerous voltages from appearing the insulated tracks and the earthed components of the installation. 

Overvoltages can occur due to lightning or due to short circuits.  Voltage limiting devices typically use a MOV (metal oxide varistor) and and air gap mechanism to conduct the high voltage impulse to the ground.

If case of minor overvoltages, the MOV operates and diverts the surge.  It then returns to its normal non-conducting state.  In case of severe overvoltages, a permanent short circuit occurs between the protective electrodes and the device has to be replaced.

Recurrent Surge Oscillation Test

Recurrent Surge Oscillation is done on the windings of large generators such as turbo alternators.  The Recurrent Surge Oscillation Test (RSO) helps identify shorts in the winding.

Shorts in the winding occur as the insulation between turns deteriorates and fails.  Shorts can cause localised heating and arcing which can further damage the alternators.  Shorts can also become earth faults in course of time.  Multi-turn shorts can also result in a drop in the voltage.

The Recurrent surge oscillation tests is done by sending voltages of low voltage and high frequency through the winding and checking the waveform at the other terminal.  If the waveform has suffered any distortion, it may indicate an abnormality.  The waveform can give information such as the location of the fault and its severity.

Some short circuits may not be obvious when the rotor is at rest.  The conductors will come in contact with each other only during the running condition, due to the centrifugal force.  To identify such faults, the rotor is made to rotate and the test is conducted.

Half Wave Rectifier

The Half Wave Rectifier functions using a single diode.  It rectifies only half of the sine wave. 

During the positive half cycle, the diode D1 is in forward bias and current flows to the load.  In the negative half cycle, when the voltage is applied in the opposite direction, the diode is in reverse bias and no current flows.

The Half wave rectifier is a simple device which requires only one diode to be put in series with the load.

The half wave rectifier has a low power output. 

Besides, the ripple content of the rectified DC supply is very high.  This can damage the loads.

It has very few applications and is only used in emergencies as a temporary measure.

Full Wave Rectifier with Centre Tapped Transformer

This Full Wave rectifier has only two diodes.  Each diode conducts during one half cycle.  In the first half cycle, diode D1 is forward bias and the current flows into the load and returns through the centre tap of the transformer. 

During the negative half cycle, diode D2 is in conduction.  The current flows through the load in the same direction and returns through the centre tap.  Thus the current is in the same direction through the load.

This means of rectifier has generally been replaced by the bridge rectifier as the centre tap is not always available.

Controlled Rectifiers

Controlled Rectifiers are rectifiers which have thyristors in place of a diode.  A thyristor is a three terminal device which can be switched on by applying a suitable gate voltage. 

In a Controlled Rectifier, generally two of the diodes are replaced with a thyristor.  The thyristor enables the enables switching on the output of the rectifier at the desired time.  This allows the output voltage and current to be controlled.

A rectifier with two thyristors is called a half controlled rectifier while a rectifier where all the diodes have been replaced with thyristors is called a fully controlled rectifier.

Controlled Rectifiers are also known as converters.