Date:         Mon, 9 Sep 2002 13:39:16 -0700
Reply-To:     Jack <john.cook58@VERIZON.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Jack <john.cook58@VERIZON.NET>
Subject:      Re: Headlight upgrade questions
Comments: To: Chris Mills <scmills@TNTECH.EDU>
In-Reply-To:  <5.1.0.14.2.20020909140640.00aba600@gemini.tntech.edu>
Content-type: text/plain; charset=US-ASCII

On 9 Sep 2002, at 14:09, Chris Mills wrote:

> I upgraded my '78 Westy headlights using the ever popular relays feeding
> directly from the battery via an aux. fuse panel.
>
> Now that my headlight switch is only driving the headlight relays and the
> toggle relay (high/low beam) - - should I reduce the fuse for the original
> circuits? The new headlight circuits are fused at a new fuse panel out back.
>
> I'm thinking a pretty major short to ground would have to occur to blow the
> OEM fuses at 8 amps - enough to melt the wiring before the fuse blew.
>
> Chris M. <"Busbodger" of "TEAM SLOWPOKE">
> Cookeville, Tennessee
>
> ICQ# 5944649
> scm9985@tntech.edu
>
> '78 VW Westfalia (67 HP -> that is...67 Hamster Power)
> '65 Beetle - Type IV powered
> '99 CR-V AWD station wagon
> '81 CB900 Custom moto-chickle
> 2.5 Corvair engines for my Trans-vair Conversion


Chris,

You can certainly drop the size of the fuse as long as you accommodate
anything else which might be on the same circuit plus the relay coils plus
a healthy margin.

Fuses are not usually precision devices and they can fail from repeated
temperature cycling if they are rated too close, so I'd guess (roughly)
you should at least double the actual (worst-case) current for the fuse
rating for a reasonable trade-off between circuit reliability & safety.
(There may be a better rule for how much margin to provide available
somewhere but it would depend upon a lot of factors & IMHO not worth the
effort to pursue it.)

The existing 8 amp fuse should blow long before there's any risk of
melting wire (or its insulation) in the case of a hard short, but it's
still a good idea to reduce it.  Hard shorts (low resistance) will pop
almost any good fuse quickly and safely.  It's the higher resistance
"shorts" which are dangerous (e.g., arcing, carbonized connection areas).
This probably isn't intuitively obvious unless you already have a good
understanding of electricity.  But this is why fires in home wiring can
occur even with proper & functional breakers/fuses.

//Jack
'85 GL