SF6 Gas Properties

Introduction

SF6 is a combination of sulfur and fluorine its first synthesis was realized in 1900 by French researchers of the Pharmaceutical Faculty of Paris.

It was used for the first time as insulating material, In the United States about 1935. In 1953, the Americans discovered its properties for extinguishing the electric arc. This aptitude is quite remarkable.

 

Physical properties

It is about five times heavier than air, and has a density of 6.1 4kg / m3. It is colorless, odorless and non-toxic.

 

Tests have been carried out replacing the nitrogen content of air by SF6 (the gaseous mixture consisted of 79 % SF6 and 24 % oxygen): five mice were then immersed in this atmosphere for 24 hours, without feeling any ill effects.

 

It is a gas which the speed of sound propagation is about three times less than in air, at atmospheric pressure. The interruption of the arc will therefore be less loud in SF6 than in air.

The dielectric strength of SF6 in on average 2.5 times that of air, and, by increasing pressure, it can be seen that the dielectric strength also increases and than around 3.5 bar of relative pressure, SF6 has the same strength as fresh oil.

 The principal characteristics of the gas are as follows:

Molar mass                                   146.078

Critical temperature                     45.55°C

Critical pressure                           37.59 bars

In short, SF6 at atmospheric pressure is a heavier gas than air, it becomes liquid at - 63.2°C and in which noise propagates badly.

SF6 on the market

SF6 which is delivered in cylinders in liquid phase, contains impurities (within limits imposed by IEC standards No. 376)

 Carbon tetra fluoride             (CF4) 0.03 %

 Oxygen + nitrogen                   (air) 0.03 %

 Water                                            15 ppm

 C02                                                 traces

 HF                                                  0.3 ppm

 SF6 is therefore                        99.99 % pur.

 

SF6 Safety precautions:

Today there is no known dielectric and breaking agent combined better than SF6 gas.

 Initial state

In its initial state, before it has undergone thermal stress (usually the electric arc); SF6 is perfectly safe in normal conditions:

- It is non-toxic,

- It is uninflammable,

- It will not explode.

This does not mean that no precautions need to be taken: because of its lack of oxygen, this gas will not support life.

However, the concentration of SF6 would have to be high, since the International electro technical Commission (IEC) has shown that five mice left for 24 hours in an atmosphere of 79 % SF6 and 21 % oxygen will not only remain alive but will show no signs of abnormal behavior.

Man dies when the oxygen level of the gas he is breathing falls below 12 %.