That is consistent with my reading. The Healey fuses are 17.5 amp slow-blow
/ 25 amp fast-blow and 35 amp slow-blow / 50 amp fast blow devices. I'm
speculating now--I expect all glass capsule fuses could be dual rated, but
the Healey fuses were for their specific application. A fuse will fast-blow
if the current is high enough to vaporize the fusible link. A fuse will
slow-blow if the competing effects of resistance heating and
application-specific radiant and air cooling cause the fusible link to melt.
The application-specific nature of the cooling may explain why the Lucas
fuses used in the Healey were not dual marked. A modern ATC plastic body
fuse has little cooling capacity so the slow-blow and fast-blow ratings
would be very close.
Does the Healey circuit take advantage of the slow-blow and fast-blow
characteristics? I think yes, to provide protection against high resistance
shorts and dead, low resistance shorts respectively. A high resistance short
could include some types of device failures.
From: Bob Haskell [mailto:email@example.com]
Sent: Friday, February 04, 2011 2:33 AM
To: Hap Polk
Cc: 'Richard Ewald'; 'Oudesluys'; firstname.lastname@example.org
Subject: Re: [Healeys] BN2 80/100 W bulbs on the PL700 tripod headlights....
One other issue that I haven't seen mentioned is how the fuses are rated. I
believe the 25 and 50 amp fuses found in the 6 cylinder healeys would be
considered to be 17.5 and 35 amp fuses in US cars.
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