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Old 14th Sep 2012, 2:30 pm   #7
dinkydi
Retired Dormant Member
 
Join Date: Feb 2011
Location: Sydney, Australia
Posts: 277
Default Re: Failing Schaffner IEC Inlet Filters

Hi Sean,

My post is written from a designer's viewpoint. A fuse fitted in a plug is not an "equipment fuse".

A designer must cover all contingencies. Even if equipment is destined for Britain, it cannot be assumed that equipment will always be used with some given fused plug. The equipment has an IEC input which allows the user to choose any of a considerable number of different power cords. Also, you cannot say with absolute certainty that equipment will forever stay in a given country.

This means that, to be safe, equipment should be designed on the assumption that there is no plug fuse.

I am aware that substantial numbers of Schaffner mains filters have and will continue to fail in the UK.



Hi Dominic,

My experience of Rifa mains capacitors is that damage is by no means consistent, although your experience may well be different. How is this seeming contradiction resolved?

Cellulose materials such as paper are hygroscopic. They have a continuing propensity to absorb water. As a result, the paper dielectric of a Rifa mains capacitor gradually absorbs moisture and swells, cracking the epoxy coat which has poor tensile strength. If equipment is used on a regular basis, the amount of moisture that can have been absorbed by a Rifa since last use is small and may only affect a small part of its dielectric. If the capacitor fails in this situation, detonation is contained and the short-circuit current may be limited.

On the other hand, if equipment is poorly stored (in a garage with climatic extremes, for example) and is left there unused for say 5 years, the moisture continues building up to a high level throughout the dielectric. If the equipment is suddenly brought out of storage and powered up, and the Rifa fails in this situation, the detonation is extreme. The capacitor fails as a whole and the severe short circuit allows very substantial current flow.

This explanation fits the observations of many posters and my own experience.

Peter
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