What this technique is used for ....
Arcs and sparks occur at a great many electromechanical contacts, such as switches, relays, contactors and the commutators of electric motors. They generate random electrical noise over the entire RF spectrum from 0Hz to 400GHz.
Any all of the frequency channels set aside for radio and TV broadcast and for radiocommunications can be affected by the emissions from arcs and sparks.
The emissions from arcs and sparks can be dealt with by shielding and filtering, but these are expensive techniques. If the RF energy in the arc or spark can be reduced, the cost of filtering and shielding required can be reduced or even eliminated.
So arc or spark suppression techniques (also called contact suppression) are used to reduce the amount of RF energy generated by the arc or spark. This can have the side-benefit of increasing the operational life of the electromechanical contacts.
How this technique is used
Arcing and sparking at electromechanical contacts is caused by the ‘flyback’ voltage generated by the inductance in the circuit when the electromechanical
contact tries to interrupt the circuit’s current abruptly. The flyback voltage is so high that it causes the air in the contact gap to break down and an electrical discharge (an arc or a spark) to occur.
contact tries to interrupt the circuit’s current abruptly. The flyback voltage is so high that it causes the air in the contact gap to break down and an electrical discharge (an arc or a spark) to occur.
All arc and spark suppression techniques use circuit components and techniques to slow the rate of change of the current and thereby reduce the magnitude of the flyback voltage.
These are sometimes called ‘snubbers’, and the technique called ‘snubbing’.
Snubbing involves connecting capacitors, resistor-capacitor networks, rectifiers, zeners or voltage-dependant resistors (e.g. metal-oxide varistors) to the electromechanical contacts so that they provide an alternative path for the current allowing it to decay more slowly when the contacts open. The more slowly decaying current will generate smaller flyback voltages, and the technique involves choosing components and circuits which will make the flyback voltage small enough that arcs or sparks do not occur.
Key issues in employing this technique
Where is the inductance?
It is usually the load inductance in the circuit that ‘flies back’ when the current is interrupted and so creates the arc or spark at the opening electrical contacts. Identifying where the inductance is helps to apply the mitigation technique.
For example, in a d.c. motor the rotor 
inductance causes most/all of the arcing at the commutator, so the best place to fit the snubbers is on the rotor itself. It is possible to buy ‘varistor discs’ that are assembled with the rotor and suppress most/all of the sparking at the commutator.
inductance causes most/all of the arcing at the commutator, so the best place to fit the snubbers is on the rotor itself. It is possible to buy ‘varistor discs’ that are assembled with the rotor and suppress most/all of the sparking at the commutator.
Other inductive loads which tend to cause arcing and sparking include relay and contactor coils, and the coils of electric bells. It is generally better for EMC to suppress an inductive load at the load itself, so that the current path for the high-frequency components of the flyback waveform covers the smallest possible area and so radiates least.
Mains-powered applications have a considerable amount of stored energy in the inductance of the mains supply cables themselves, and this can als
Source : Emcuk
