Various Factors Influencing the Breakdown In Gases
Q. Discuss the various factors which affects the breakdown of gases.
- Distance between electrode
- Shape of electrode
- Size of electrode
- Material of electrode
- Material in the gap
- Temperature
- Pressure
- Applied voltage
- Polarity of the small electrode
- Humidity
- Dust
- Irradiation
- Nearby earthed objects
- Miscellaneous
1. Distance between Electrodes
If distance between two electrodes (d) is increased, breakdown voltage increases. As distance is increased each and every electron has to travel more distance and hence transit time for charges to cross over the gap and energy needed is more and hence breakdown voltage increases.
2. Shape of Electrode
- If shape of electrode gives non uniform field, breakdown voltage reduces due to increase in electric field intensity near the small electrode.
- If the shape of electrode gives rise to uniform field then there is increase in breakdown voltage value (because strong bonding).
3. Size of Electrode
As diameter of electrode is increased, it reduces sap distance and at the same time reduces electric field intensity on electrode surface and hence effect on breakdown voltage is high breakdown voltage will depend on ratio S/D.
d = S = Spacing between
D – diameter of electrodes
d‘ = changed spacing between electrodes
4. Material of Electrode (Cu, Brass, Steel)
- Breakdown voltage for different materials is different as ionization potential (IP) for different metals is different.
- For Cu electrons are loosely bound with nucleus, they can be easily detached from the nucleus. Hence positive ions can be bombarded on Cu cathode which produces more number of secondary electrons and hence breakdown voltage will be less.
- But for steel, number of secondary electrons are less due to high IP and hence breakdown voltage is high.
5. Material between the Electrodes (Solid, Liquid, Gas)
Breakdown voltage depends upon nature of materials. Moreover it also depends on the impurities present in all the 3 types. With increase in the amount of impurities, the breakdown voltage reduces.
6. Temperature
With increase in temperature, breakdown voltage reduces due to increase in thermal energy. Thermal energy causes easy electron liberation and easy collision due to which transfer of energy and hence breakdown voltage will reduce.
7. Pressure
As per Paschen’s law, as pressure increases, breakdown voltage (BDV) decreases initially but after certain value it goes on increasing.
V = f (p x d)….. V = Breakdown voltage
p = pas pressure
d = Electrode gap distance
8. Applied Voltage
The breakdown voltage of a sphere pup will change depending on the type of applied voltage viz AC/DC or impulse.
9. Polarity of Small Electrode
In a non uniform electrode configuration (sphere- rod or plate-rod or sphere-plate) the polarity of electrode with smaller dimensions decides the breakdown voltage. e.g. in rod-plate electrode configuration if the rod is negative, the breakdown voltage will be lesser as compared to when rod is positive. This difference is low.
10. Humidity
If humidity is increased, spark over voltage increases. Humidity consists of water vapours which are electronegative in nature i.e. water vapours will have affinity for electrons. Thus free electrons in gap will get attached to these vapour and ionization process will be delayed which in turn will increase the breakdown voltage.
11. Dust
Dust particles acquires the same charge of the electrode in its vicinity and is drifted towards the oppositely charged electrode. Thus breakdown process is accelerated and hence breakdown voltage decreases.
12. Irradiation
Irradiation helps easy ionization and therefore breakdown voltage reduces.
13. Nearby Earthed Objects
If earthed objects are in the vicinity, the breakdown voltage decreases.
14. Miscellaneous
All other factors which are not included above as well as human error are to be accounted under miscellaneous.