Date: Tue, 29 Sep 2009 21:37:55 -0400
Reply-To: Mike <mbucchino@CHARTER.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Mike <mbucchino@CHARTER.NET>
Subject: Re: No High Beams
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It's not as much about in-rush, start-up current or even continuous current draw in a normal circuit. You could install a slow-blow fuse of the original rating to counter start-up in-rush currents.
But, now factor in corroded contacts in the bulb sockets, poor grounding (both common complaints on our older Vanagons), and corrosion that 'wicks' up into the wire strands on the ends (invisible under the insulation), and it all adds up to more total, continuous current draw. This gets the entire circuit warm (even hot!) and will break down insulation and plastic parts over time (remember how our headlight switches melt down even with an all-stock circuit?).
You need to do an actual ammeter measurement on the van, not a theoretical mathematical calculation based on book and diagrams. Your circuit is far from new and is now modified from stock.
When a conductor shorts to ground, accidentally, anywhere along it's entire length, you now have a wire that'll blow (and WILL glow!) before the higher-rated fuse will blow. As I said, it's not a good scenario, no matter how you like to ratonalize it to yourself. You MUST upgrade the conductor to a larger cross-section to prevent this possibility. Alternatively, you could also add another conductor, in parallel, alongside the existing wire, but I don't like that idea much for a couple of reasons (what if it come apart somewhere?).
Also, your wires are not running in "free air", but are cabled in a harness alongside conductors, and thusly, the circuit needs to be 'de-rated' accordingly. This means that your calculations are wrong, so, an engineer, you're not! (neither am I, but I use my 'book-knowledge' and real-world experience to guide me in making decisions to 're-engineer' things sometimes....) Hope I've explained it properly.
Mike B.
----- Original Message -----
From: Rocket J Squirrel
To: vanagon@GERRY.VANAGON.COM
Sent: Tuesday, September 29, 2009 8:58 PM
Subject: Re: No High Beams
I don't dispute your knowledge and experience.
Let's see how closely VW cut things.
97.58 (Bentleys) shows the brake light switches fed by a single 18 gauge
wire through a 8A fuse (S8). 18 gauge wire is rated to carry 13 amps,
continuous, in free air, forever, if it has insulation rated for 60C.
There are insulations rated for higher-temperature operation, and the wire
can be run up to 18A or more with better insulation. But I don't know what
kind of insulation is used in the Vanagon, so I'll use this rating, the
most conservative I could find.
The two brake lights on the Vanagon are 21W, so they each draw 1.8A,
steady-state, although the turn-on surge of a tungsten filament can exceed
the steady-state value by 5 to 10 times because a cold filament has lower
DC resistance than a hot one.
So the initial turn-on current of the two bulbs together can be anywhere
between 18 to 36 amperes, but that surge will be brief - somewhere in the
tens to hundreds of milliseconds. Less than a second, anyway.
Clearly the steady-state current draw of the two bulbs (3.6A) is well
below the 13A current rating of the wire and the 8A fuse. The fuse can
handle the brief turn-on surge.
When I doubled the load by adding a pair of 21W trailer lights, the fuse
blew the moment I stepped on the brakes for the light test. Maybe because
the steady-state current (7.2W) exceeded the 8A rating of the fuse --
that's darn close (anyone know the tolerances on these cheap fuses?). More
likely, doubling the inrush current to somewhere in the range of 36 to 72
amperes popped the fuse.
But doubling the brake light loading doesn't exceed the current carrying
capacity of the wire. "Noxious smoke and fumes" are not likely to result
from passing 7.2A through wiring rated to 13A @ 60C insulation. The inrush
current of the bulbs is too brief to heat a length of wire 12 or more feet
long by more than a few degrees. It certainly won't "glow like a lightbulb
filament" even if I was passing 16A through it steady-state due to a
wiring fault. It would get darn warm and eventually the insulation would
fail somewhere.
Going from 3.6A to 7.2A in wiring very conservatively rated for 13A is
safe enough for me, especially since I only drive with the trailer a few
times a year. I reckon the engine will give out before the wiring will.
BUT -- Mike B. is right: DO NOT try this at home. Just because I jump off
a bridge doesn't mean you should. I do not need a phone call from your
widow's lawyer. Never modify any wiring or electrical accessories on your
van unless you know what you are doing and are willing to accept the risks
of a miscalculation.
--
Mike "Rocket J Squirrel" Elliott
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Westrailia: (Ladybug Trailer company, San Juan Capistrano, Calif.)
Bend, OR
KG6RCR
On 9/29/2009 5:16 PM Mike wrote:
> You should NEVER swap up to the next size fuse, when any type of circuit
> fault causes the correct size fuse to keep blowing.
> The fuse rating is engineered to blow at nearly the maximum
> current-carrying capacity of the wire. If you add a greater load to an
> existing circuit, and that causes the original size fuse to blow, that
> means that the wiring is pulling more amps than the wire size is able to
> safely handle. If you proceed to install the next-size-larger fuse, you
> now have created a fire hazard that could cause a total loss of your
> treasured vehicle. When a wire draws too much current, it gets hot,
> melts insulation, the copper can even glow like lightbulb filament along
> its entire length! Not good! At this point the wire becomes the
> weakest link and 'blows' instead of the fuse! Ii's also capable of
> filling the vehicle with noxious smoke and fumes. Do you want to keep
> your family in this vehicle? I can't recommend it.
>
> Mike B. (licensed FAA aircraft mechanic, and licensed electrician in 2
> states)
>
> ----- Original Message -----
> *From:* Al Knoll <mailto:anasasi@GMAIL.COM>
> *To:* vanagon@GERRY.VANAGON.COM <mailto:vanagon@GERRY.VANAGON.COM>
> *Sent:* Tuesday, September 29, 2009 7:09 PM
> *Subject:* Re: No High Beams
>
> Actually, Rocket J. you can discern the wire size to the brake lights by
> gazing at the current track in the bently. With that in mind you can
> estimate the current carrying capacity, then see if the total draw
> exceeds
> the reasonable capacity of the wire. If it does't then just fuse to the
> next higher value. If it does exceed the RCOFTW by some you might
> have to
> run a second circuit. Works for all accessories of course.
>
> Pensionerd.
>
> On Tue, Sep 29, 2009 at 7:20 AM, Rocket J Squirrel <
> camping.elliott@gmail.com <mailto:camping.elliott@gmail.com>> wrote:
>
> > Along those lines, I instantly blew the fuse for the brake light
> circuit
> > when I connected up my little trailer and tested the lighting. Not a
> > miswiring, just the additional load of the trailer's two little brake
> > lamps was sufficient to pop the fuse. VW must rate the fuses
> pretty close
> > to the expected load current so even a non-shorting fault is
> sufficient to
> > open the fuse. Had to swap up to the next-sized fuse there.
> >
> > --
> > Mike "Rocket J Squirrel" Elliott
> > 84 Westfalia: Mellow Yellow ("The Electrical Banana")
> > 74 Westrailia: (Ladybug Trailer company, San Juan Capistrano, Calif.)
> > Bend, OR
> > KG6RCR
> >
> >
> >
> > On 9/29/2009 4:15 AM Frank Condelli wrote:
> >
> > In a message dated 28/09/2009 3:14:30 P.M. Eastern Daylight Time,
> >> LISTSERV@GERRY.VANAGON.COM <mailto:LISTSERV@GERRY.VANAGON.COM>
> writes:
> >>
> >> If you were to read my _Headlamp Relay Upgrade_
> >> (http://frankcondelli.com/hdltrela.htm) webpage. It
> specifically says
> >> that it is necessary
> >> to change the 10 amp fuses to 15 amp when installing the higher
> wattage
> >> bulbs.
> >>
> >>
> >>
> >> Mark Drillock got it, right on the button. Both fuses 9 and
> 10 were
> >>>>>>>
> >>>>>> blown.
> >> I guess those little 10 amp fuses don't like the bigger lamps I
> have in
> >> both
> >> units. And since I drive at night very seldom they could have
> been out
> >> for
> >> some time.<<<<<<<<
> >>
> >>
> >>
> >>
> >>
> >> Cheers,
> >>
> >> Frank Condelli
> >> Almonte, Ontario, Canada
> >> '87 Westy & Lionel Trains (_Collection for sale_
> >> (http://frankcondelli.com/trainsal.htm) )
> >> _Frank Condelli & Associates_
> (http://frankcondelli.com/busindex.html)
> >> -
> >> Vanagon/Vanagon Westfalia Service in the Ottawa Valley
> >> _Vanagon Stainless Steel Exhaust Systems_
> >> (http://frankcondelli.com/exhaust.htm)
> >> _BusFusion_ (http://www.busfusion.com/) a VW Camper camping event,
> >> Almonte, ON, June 11 ~ 14, 2009
> >>
> >>
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