wrote:

I came across that in a 110 volt 3 phase to DC transformer many years ago.
The fast acting fuses were failing frequently costing about £3 to replace.
The diodes they were protecting were about 80p each.

After replacing the fuses with cheaper ones fast enough to protect for
overload, neither the fuses or the diodes failed.

On a rather larger scale, the substantial semiconductors used in
the Traction Convertor in APT-P had pretty large (1) and
expensive fuses which appeared to be blowing spuriously.

Upon opening them up for investigation, the internal structure
was of multiple parallel thin copper strips, with sections
punched out to create lots of necking points in series. Close
examination showed that the necking points which had shared the
arcing when it blew all had little blobs of fused sand (with
which the fuse cartridge was filled). However, there were also a
fair number of clean breaks, and it was eventually determined
that they were failing mechanically due to vibration, and thus as
the number of parallel paths reduced, the current capacity slowly
degraded.

(1) In the region of 1000 V, several hundred amps.

Chris
--
Chris J Dixon Nottingham UK


Have dancing shoes, will ceilidh.