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 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

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

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.