Using electricity to switch

By Philippa Martin-King

Relays are electromechanical devices. That is an area for which the experts in IEC TC (Technical Committee) 17 are responsible.

Accessory control box. Accessory control box.

Sending the signals to the heavy load circuit

When a high-power system needs to handle a large number of amps because of the current it's drawing, a traditional hand-operated on-off switch with its direct connection doesn't serve much purpose. Temperatures can rise to such a level that the wiring inside simply is not able to handle the power and will melt!

A relay is an electromechanical remote control switch that turns on or off using electricity. Inside the relay, instead of wires, there's a small electromagnet or other system that activates a contact to make or break the circuit. A relay is able to control an output circuit of higher power than the input circuit, which means that it doesn't need to be wired to support a heavy load. Instead, it acts a bit like an automated electrical amplifier, sending out its on-off signals to the heavy load circuit.

Dating back to the 1830s

The first relays were developed by Joseph Henry in his studies of electromagnetism. It is he who devised the first electromagnetic telegraph in 1836. It was later adapted by Samuel Morse and used for his Morse code. That was several years before the IEC came into being in 1906. Relays were used extensively from then on in the first telephone exchanges and early computers to perform logical operations. Henry was also responsible for developing the first electric motor. And, although today in many modern circuits, solid-state devices with semiconductors have replaced relays, these switches are still used in a high proportion of motors and heavy load applications.

IEC TC 17 established in 1927

IEC TC (Technical Committee) 17: Switchgear and controlgear, was formed in 1927. The committee prepares International Standards that specify circuit-breakers, switches, contactors, starters, disconnectors, busbars and other types of switchgear assemblies – with the exception of those destined for domestic use. TC 17 is divided into four SCs (Subcommittees) that cover both high- and low-voltage switchgear and controlgear, together with assemblies and their associated control and/or power equipment, measuring and signalling equipment.

Work continues to be important for safety and reliability today

TC 17 work is vitally important today where switchgear and controlgear are heavily dependent on standardization to ensure the safety of those people who are involved in high-voltage transmission and distribution networks, in setting up electric power systems and so on. This is particularly the case in industrializing and developing countries where often workers lack training and experience. The TC 17 publications act as guides in passing on information and providing planning outlines.

Low-voltage switchgear assemblies are taking on greater significance too as buildings and factories are automated, and complementary functionality – e.g. integrated sensors – is added to the original equipment. At the same time, the markets are changing and becoming more global and deregulated. There are also moves to increase energy efficiency. Often, in an aim to reduce operational costs and increase output, a high degree of automation is involved, with switchgear and controlgear playing a major role. The reliability and dependability of systems are all-important.

Recent publications released in 2011

In August 2011, IEC SC 17A released a consolidated version of IEC 62271-1, High-voltage switchgear and controlgear – Part 1: Common specifications. This part of the IEC 62271 series [see the separate e-tech article on some of the new publications available from the IEC Webstore], applies to a.c. switchgear and controlgear designed for both indoor and outdoor installations with frequencies of up to 60 Hz and voltages above 1 000 V.

Another publication, to be read in conjunction with IEC 62271-1, is IEC 62271-200, High-voltage switchgear and controlgear - Part 200: AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV. It is the responsibility of IEC SC 17C: High-voltage switchgear and controlgear assemblies; the second edition was released at the end of October 2011.

IEC 62271-200 specifies requirements for prefabricated metal enclosed switchgear and controlgear for alternating current of rated voltages above 1 kV and up to and including 52 kV for indoor and outdoor installation, and for service frequencies up to and including 60 Hz. Enclosures may include fixed and removable components and may be filled with fluid (liquid or gas) to provide insulation.

This second edition of IEC 62271-200 has introduced more precise definitions, classifications and test procedures. It includes specific ratings for parameters such as fault level to earth; describes optional rating "cable test voltage" type tests and gives internal arc classification test guidance regarding the test arrangement, room simulation and arc initiation.

30-contact relays in "Connector" circuits in 20th century 1XB switch and 5XB switch telephone exchanges 30-contact relays in "Connector" circuits in 20th century 1XB switch and 5XB switch telephone exchanges
Phase failure with under voltage relay Phase failure with under voltage relay
Accessory control box. Accessory control box.