Flammability of Radio Interference Suppression Capacitors

Radio interference suppression capacitors serve to reduce or suppress the HF voltage interference in electronic equipment. The RFI capacitors remain on the mains for an uninterrupted period of 10, 20 or more years and have to both protect the appliance against line-side surge voltages/transients and suppress reactions of the appliance on the mains supply.
Transients are voltage spikes to which the mains voltage is subjected and which can easily occur several times a day in low voltage mains supplies with amplitudes of 2000 V and above. Peak values can be as high as 6 kV (figure 1).

Figure 1: Mains transients

A = Atmospheric disturbances (lightning flashes, faults in the high voltage system).
B = Faults in the mains or in nearby equipment (e.g. faulty fuses, response of power switches).
C = Switching on/off of electric equipment (motors, welding sets, household equipment etc.).
D = Voltage spikes from equipment like power supplies, inverters, TV-sets etc.

Radio interference suppression capacitors are used to block and attenuate these voltage spikes and are defined in X and Y classes according to the demands they have to satisfy.
Class X capacitors have unlimited capacitance and are connected between phase to neutral or phase to phase conductors. Class Y capacitors have increased electrical and mechanical safety and are installed between phase conductors and the shock protected earthed casing, thus bridging the insulation of the appliance.

Risks Involved with Plastic Film Dielectric

Polyester and polypropylene capacitors are used for radio interference suppression although there is a possibility of these components catching fire. The physical process which finally leads to the self-ignition of the capacitor is approximately as follows:

• Transient voltage spikes strike the load's mains input. The current can easily reach a level of 200 A for a few microseconds.
• The RFI capacitor offers very little virtual resistance to the high voltage spikes.
• At the weakest point of the dielectric there is a breakdown and temperatures of several thousand °C can occur in the surrounding area. A metal-free insulated area is created around the breakdown channel. This process is referred to as self-healing.
  
• As this process continues, up to 41% of the previously bound carbon is deposited in the form of conductive graphite sediment in the insulated area of plastic film capacitors and forms highly resistive carbon bridges (figure 2).
  
• With the accumulation of such damaged areas throughout the life span of the equipment, or because of high energy self-healing processes of the capacitor, the insulation resistance is considerably reduced. This inevitably leads to an increase in the capacitor current, which in turn causes overheating of the component. The gas pressure which is produced in the interior of the capacitor causes the casing to tear open and the gas mixture to ignite and burn for several minutes with a flame similar to that of a welding torch.
  
• Even in the case of fire, the capacitor remains so highly resistive that even a mains fuse connected in series does not respond.

Figure 2: Breakdown channel polyester		Figure 3: Breakdownchannel metallized paper

Advantages of the WIMA Metallized Paper Technology

Metallized paper capacitors are subject to the same physical factors, too. However, in the case of WIMA metallized paper capacitors, the amount of carbon deposited in the form of graphite sediment is 20 times smaller, thanks to the good oxidation balance of the paper dielectric.

The insulation area created during the self-healing process is free of carbon bridges, so that minimal short circuits cannot occur (figure 3). An inadmissible rise in temperature because of a decreasing insulating resistance is avoided. The capacitor has regenerated completely.

In addition WIMA MP capacitors are fully impregnated under vacuum and encapsulated with self-extinguishing material. There are no air pockets and contact of the capacitor paper with oxygen is not possible.

Extensive tests have shown that, even when high energy pulses are applied, WIMA metallized paper capacitors are not actively flammable because of the high breakdown strength and significantly better regeneration behaviour of the metallized paper.

Model of a reproduceable flammability test