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: 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.
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
Involved with Plastic Film Dielectric
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:
voltage spikes strike the load's mains input. The current
can easily reach a level of 200 A for a few microseconds.
RFI capacitor offers very little virtual resistance to
the high voltage spikes.
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
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).
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.
in the case of fire, the capacitor remains so highly
resistive that even a mains fuse connected in series does
Breakdown channel polyester
Breakdownchannel metallized paper
of the WIMA Metallized Paper Technology
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.
||Total formula of the molecular chain
||Proportion of deposited carbon in %
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
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
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