The Polarity of current transformers is extremely important. Just like a battery, a current transformer too has a polarity. The polarity determines the direction of the secondary current in relation to the primary current.

Wrong connection of the current transformers can cause false operation of the protection relays. Hence, it is vital to ensure that the current transformers are connected with the correct polarity.

The figure shows a setup to test the polarity of a current transformers.

A DC source is connected with the positive terminal to P1 and the negative terminal to P2. An analog voltmeter is connected to the secondary terminal of the CT. The positive terminal of the meter is connected to terminal S1 of the CT while the negative is connected to terminal S2.

A contact is momentarily made through the switch. The contact is made for a second and broken. This is important as continuous contact can short-circuit the battery. The momentary make-break contact causes a deflection in the analog multimeter in the positive direction, if the polarity is correct.

If the deflection is negative, it indicates that the polarity of the current transformer is reversed. The terminals S1 and S2 need to be reversed and the test can be carried out.

The Reverse power relay is used to protect a synchronous generator, running in parallel, from motoring. Motoring occurs due to the failure of the prime mover such as a turbine or an engine driving a generator that is connected to the grid. The generator which is running at the synchronous speed will continue to run at the same speed. However, the power required to keep the generator running along with the prime mover will be drawn from the mains. Hence, power flows in the reverse direction i.e. bus to generator. This condition is called reverse power.

Reverse power operation may cause damage to the prime mover. Hence, reverse power protection is a vital part of the generator protection scheme.

The reverse power relay operates by measuring the active component of the load current, I x cos φ. When the generator is supplying power, the I x cos φ is positive, in a reverse power situation it turns negative. If the negative value exceeds the set point of the relay, the relay trips the generator breaker after the preset time delay.

The typical setting for reverse power is 4% in case of turbines and 8% in case of diesel engines. The time delay can be set from 2 to 20 seconds.

The vector surge relay is used to decouple synchronous generators from the grid utility in case of grid failure.

Synchronous generators are generally operated in parallel with the grid utility. This ensures greater reliability and enables the generator to export power to the grid. In this condition, there is a chance, of a momentary interruption of the grid supply which may result for a few milliseconds. Such temporary interruptions can be caused to mal-operation of the circuit breakers on the grid transformer side.

For a synchronous generator, running in parallel with the grid utility, such a temporary interruption and restoration of the supply can be dangerous. As the restoration of the supply can be asynchronous i.e. the generator and the grid are now not in a synchronised condition. The can lead to the consequences of wrong synchronization such as damage to the generator or the prime mover.

The vector surge relay prevents this condition by decoupling the generator from the grid as soon as the grid supply fails. This is an extremely fast acting relay with an operating time of less than 300ms from relay operation to breaker opening.


The vector surge relay functions by monitoring the rate of change of the rotor displacement angle of the generator. During parallel operation there is an angular difference between the terminal phase voltage (Up) and the internal synchronous voltage of the generator (Ui). This is due to the fact that the generator rotor is magnetically coupled to the generator stator and is forced to rotate at the grid frequency. The angle between the vector of the mains voltage Up and synchronous electro-motive force is known as the rotor displacement angle.

This angle is constantly varying and is dependent on the torque produced by the generator rotor. In the case of the grid failure, there is sudden change in the rotor displacement angle.

This causes a surge in the generator voltage shown in the figure. The relay works by monitoring the time taken between the zero-crossings in the waveform. Under normal operation, the time interval between two consecutive zero-crossings is almost constant. During the grid failure, the vector surge which occurs causes a delay in the zero-crossing. This delay is detected by a highly sensitive timer inside the relay and the relay operates.

The relays are usually set to operate for a change in the rotor displacement angle of 0 to 20 degrees

The DIN rail is a popular connector used widely in the industry for mounting components such as relays , meters, circuit breakers, etc. It is also known as the top-hat rail due to its shape which resembles an inverted hat. DIN is the acronym for Deutsches Institut für Normung e.V, the German national organization for standardization.

The standard DIN rail used is 35mm wide. However, there are types of the rail having lesser widths such as 15mm or 7.5 mm.

The European Standard EN 50022 and the IEC standard 60715 specify the DIN rail for mounting low voltage switchgear and controlgear.

The ampere hour ratings of a battery indicate the rate at which the battery can be loaded. For Example a 20Ah battery indicates that indicates that the battery can supply a current of 1 ampere for 20 hours.
Smaller batteries have their discharge rates indicated in mAh (milli ampere hour)

When batteries of different ampere hour ratings are connected in series, it is necessary to ensure that none of the batteries is discharged beyond its capacity. This is because if a battery connected in series in a battery bank is discharged to its maximum, the voltage of this battery will become zero. This will cause a voltage from the other batteries to be applied across the battery in the wrong direction. This can cause damage to the battery

Computer hackers have embedded software on the United States’ electricity grid that could disrupt service or cause other damage, two former federal officials told CNN, with one of those officials saying that this type of hacking could become “the 21st century version of cold war spying.”

One government official said that although he is unaware of specific threats, he assumes that these types of attacks from other countries happen frequently.

“Their foreign intelligence service has been probing our computers, our defense computers, our defense contractors, our power grids, our telephone system. ... I just came from a speech at the national defense university and they were hit by the Chinese trying to get into their systems,” Robert Baer, a former CIA operative, told CNN. “They are testing and have gotten in portals. It’s a serious threat.”

Although the government would take such an attack seriously, the Department of Homeland Security Director Janet Napolitano would not confirm this specific breach to CNN.

“There have been, to my knowledge, no disruptions of power on any grid caused by a deliberate cyberattack on our infrastructure—on the grid,” Napolitano said. “Nonetheless, we remain in constant protection, prevention, education, resiliency mode and we work with the utility sector particularly on that.”

But if the software is breached, why haven’t we seen any problems? Baer told CNN that if the software has been put in place he doubts they would use it right away. Instead, they would likely keep the bugs in place in a case of future conflict.

Security experts say that the hacking could be the work of either Russia or China. Any country able to breach the U.S power grid would hold an obvious advantage in any kind of future confrontation.

Although finding the coding can be destroyed when found, experts say that is very difficult to do, and tracing it is next to impossible.

“If you have somebody who knows what they’re doing writing that code and embedding it in a clever way, you can look right at it and not recognize it,” Scott Borg, director and chief economist at the U.S. Cyber Consequences Unit, said to CNN.

Although the power grid is owned by individual companies, not the government, the Department of Homeland Security plans to continue working with power companies to help prevent an attack.

“Can we continue to work to enhance efforts within critical infrastructure like the utility grid? Yes,” Napolitano told CNN. “Are we continuously looking for ways to enhance and educate for the prevention and protection of the cyberworld? Absolutely.”


The International Electrotechnical Commission (IEC) is an organization with the objective of standardizing methods, phenomena procedures, techniques, etc related to the field of Electrical Technology.

Founded in 1906, the IEC today counts nearly 130 countries as its members. All the standards of the IEC are accepted as national standards by the member countries. The standards are laid down by numerous experts from academia, industry and government.

The institute was originally located in London. However, in 1948, it was relocated to Geneva, Switzerland. The Commission now has regional centers in Singapore, Sao Paulo and Boston.

The IEC publishes in two languages, English and French, although Russian Editions and Spanish Translations are also available.

The IEC is responsible for a wide range of publications such as International Standards, Technical Reports, Technical Specifications, Technology Trend assessment etc. It is estimated that nearly 10000 experts work for the IEC.

The standards of the IEC are prepared by numerous Technical Committees and Sub-committees. These committees are peopled by experts from government, industry, test laboratories and academia. The standards prepared by the committees are studied by the National Commitees, which represent the member countries are then voted to become international standards. The documentation of the standards is exclusively electronic which makes distribution efficient and environment-friendly.

The standards of the IEC follow a definite numbering pattern. The numbers range from 60000 to 79999. For instance, IEC 60076 deals with Power Transformers.

The IEC standards can be purchased online at

Useful links The online dictionary for electro-technical terms Home page of the IEC

MY40 is a digital insulation tester from Yokogawa. Powered by 4 AA(RGP) batteries, the tester can measure both insulation as well as conductor resistance. The unit comes with a 3 ½ display which is backlit with a logarithmic bar graph of the indicated values. It can also measure AC voltage up to 600 V

The memory can store 20 measurements. The tester has a “live line” indication for user safety which indicates live parts. It also has an automatic discharge feature after insulation resistance measurements.

The tester’s dimensions are 125(W) x 103(H) x 53(D) and weighs 420 g

Power Cables can be classified into Earthed and Unearthed cables. The terms "earthed" and "unearthed" refer to the application for the which the cables are to be used.

By Earthed system, we mean a three phase system whose star point is grounded directly. In this case, in the event of the earth fault, the voltage in the faulty phase will become zero. The voltage between the healthy phases and the ground will be the same, 11kV/1.732 or 6.6/1.732.

However, in case of an earth fault in an ungrounded system, the voltage between the healthy phases and the ground will be equal to the phase to phase voltage. i.e. 11kV or 6.6 kV.

Hence, the insulation level of these cables needs to be higher.

Thus a cable suitable for a 11kV earthed system will be suitable for a 6.6kV unearthd system.