How Does a SF-6 Circuit Breaker Work?

SF6 is an insulative gas can be worked in a higher voltage in a smaller cabinet. so that It can reduce size of switcher. But it has to work in a higher air pressure station t keep insulation.

Let me reminisce what I learnt 40 Yrs back- It is available in text books When a circuit breaker breaks the connection, an arc is formed. Due to the vacuum created due to separation as well as the temperature of the arc, sorrounding fluids rush in (oil in OCB, Air in ACB or SF6). This will get ionised in the arc, but in each cycle of the sinusoid, it gets a chance to break the arc (if I remember it is called the regenarative rate) SF6 has the important properties a) At a pressure of I think 5 bar - its insulation resistance is as high as distilled water b) It has a very good thermal conductivity - rare in gasses c) It is not poisonous d) Due to Flourine it is highly stable e) rate of regeneration is very high. f) Being gas it immediately flows in the space. Due to these properties SF7 as on date is the master arc breaker and it breaks the arc within a couple of cycles.

I would not agree with the statement that SF6 is a safe gas. Once portionately ionised and before any recombination it is dangerous to humans, and if actuated beyond its designed current breaking capacity it needs to be emptied and regassed. And not emptied to atmosphere. I have witnessed SF6 etching glass in protection doors as well. Rule is, dont breathe it.

Rok

I agree with sb. SF6 is a commonly used tracer gas used when diagnosing ventilation problems in buildings. It was also released into the London Subway system at one of their stations as a planned evolution, to determine the dispersion properties and tendencies of gasses. This was part of the work that was done to analyze what the impact would be if there were a terrorist attack which involved the release of toxic gas. I believe it is also used to press up the eyeball during a corneal reattachment procedure.

We used it at Yucca Mountain as a tracer gas too.

There are other applications where SF6 is used with another element as a propellant due to the expansion properties of the reaction gas.

It is worth mentioning that when used for arc supression, one of the by-products can be disulfur decaflouride S2F10, which is toxic to humans. It is also a potent greenhouse gas, estimates are up to 40,000 times as powerful as CO2.

SF6 can be detected with great accuracy at low levels and it does not exist in the earth's atmosphere at any appreciable level.

Are You sure ? SF6 recombines very fast on ionisation, and before it leaks out of the arc, it will recombine, that is the property of its arc quench. Any ionised gas , remaining ionised can not quench the arc.

SF6 is non toxic, and cause asphyxation due to displacement of O2 (being heavier than air)

MSDS of SF6 says non hazardous except in concentration.

We regularly use it in our CBs (we make all three types at our different mfr units- VCB, MOCB and SF6 CBs and for 400KV and more transmission, it is exclusively it even for 220KV lines most are these.

It usually work at above a pressure of 3 bar ,say 4.5bar or 0.45Mp. It has higher stable nature, which made it suit to high voltage insulation.

Only combust it can give out a some hurtful. it widely use for HV products. dont worry about it.

But air cargo co.dont allow to transport it, include any gas in aeroplane unless it less pressure than 0.28Mp.

In addition to that, These days the combination of VCB and SF6 Breakers are used as compact modules, For Example: ABB safe Ring (Ring main Units). Here the VCB insulated by SF6 as additional insulation to maintain the higher insulation capacity in view of the compact in construction. In any case the SF6 pressure to be maintained between 3KG/sqcm and 6KG/sqcm. Chances of flashover likely to be higher if the pressure drops. HT Cable termination and CT fixing will be very difficult in this type of breakers as these compact breakers are meant for Cu cable terminations.

Regards

Vinu

after all SF6 is a green house gas.

should we reduce it's usage