Sparks Electrical News

by Terry-Mackenzie Hoy

I started in engineering when I was 26-years-old and I’m now 62. In this time I have seen many developments in 11 000 V circuit breaker technology and I thought I would share some of them with you.

Before my time circuit breakers there were so called air circuit breakers. When the circuit breaker tripped two contacts would separate, drawing an arc between them. A mechanically driven ‘puffer’, a small bellows, would blow the arc into an arc chute which would split up the arc and cause it to extinguish. The resulting heating of the air would cause the circuit breaker to emit a loud bang which is terrifying if you’re not expecting it. Such circuit breakers are fairly durable and for many years were used to supply arc furnaces where over current conditions are very common.

The air circuit breaker evolved into the oil immersed circuit breaker. This had all the contacts inside a tank which was filled with insulating oil. When the contacts separated, the arc was cooled to extinction by the oil. The system worked well but the circuit breaker could only trip a limited number of times since the oil became contaminated and had to be changed. This was no a bad thing since it was often standard maintenance procedure to change the oil every two years which meant that the circuit breaker would be tested at least every two years.

The circuit breakers were shipped from the factory without the first filling of oil and the circuit breaker tank had to be filled on site. It was possible to forget to do this. If this happened then the circuit breaker would blow up when switched on, injuring the operator. These circuit breakers were widely installed throughout RSA and there are many of them still in service – I think the oldest ones were installed in about 1925.

The various circuit breaker suppliers all searchedfor alternatives to the oil filled circuit breakers, ones which would require less maintenance. Two competing technologies emerged – the vacuum circuit breaker and the SF6 circuit breaker. The vacuum circuit breaker was designed on the principal that an arc cannot exist in a vacuum. Thus if the circuit breaker contacts were enclosed in a vacuum (called the vacuum bottle) then the arc could not exist. The vacuum circuit breakers worked (and still work) well. To guard against loss of vacuum they are designed to open if the vacuum falls below a certain level. The alternative technology was the SF6 gas filled circuit breaker. These worked on the principal that the contacts of the circuit breaker were in a chamber which was filled with high pressure sulphur hexafluoride gas, known as SF6. This gas is a very good insulator. The down side of these circuit breakers is that SF6 is the most potent greenhouse gas that the Intergovernmental Panel on Climate Change has evaluated. It has a global warming potential that is 23 900 times worse than CO2. Some governments have implemented systems to monitor and control the emission of SF6 to the atmosphere. This aspect has not stopped the manufacture of SF6 circuit breakers since, unlike vacuum, they can be used at very high voltages apart from at 11 000 V.

The best aspect of the more modern circuit breakers is their increasingly compact construction. Earlier circuit breakers were normally closed using a spring which had to be charged and then released to close the circuit breaker. It was very heavy work to move the circuit breaker into position and charge the spring. But we were much fitter then.

A danger is the latest circuit breakers, which can interrupt a current of up to 35 000 A as opposed to the 20 000 A of the older circuit breakers. These new circuit breakers are large and won’t fit into existing sub-stations. In RSA we hardly need 35 000 A circuit breakers but European manufacturers are making little else. It won’t be long before
an engineer who doesn’t know too much will ‘play it safe’ and specify 35 000 A circuit breakers as replacements for existing and will cause the substation to be rewired. Oh well!

BANNER 2

Contact Sparks Electrical News

Title: Editor
Name: Gregg Cocking
Email: sparks@crown.co.za
Phone: +27 11 622-4770
Fax: +27 11 615-6108

Title: Advertising Manager
Name: Carin Hannay
Email: carinh@crown.co.za
Phone: +27 11 622-4770
Fax: +27 11 615-6108

 
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