Re: [VAC] Battery Switch

["Dr. Gerald N. Johnson" <geraldj@xxxxxxxx>]

I think there's a compromise practical, but it requires more attention
to wire protection AT the battery, or very close to the battery. I'd
prefer fuses or circuit breakers to be located away from the battery
just because battery fumes are a bit corrosive and cause breakers and
fuses to fail. Fundamentally, electrically protecting the wiring in a RV
is just like protecting the wiring in a house or factory, each circuit
needs to have a fuse or circuit breaker (and in the RV rated for DC) to
limit the current to something safe for the wire diameter and
insulation. Disconnecting the battery while traveling does protect the
wires from being overloaded by a short from vibration wear while
traveling, but does stop charging and functioning of the breakaway
switch. Even if the other wires aren't protected, it would be better to
have the charging wire protected at both ends (engine charging isolator
and at the battery because that wire is exposed to two power sources)
with something like a 100 amp fuse, IF the wire can handle that. Then
the break away switch circuit probably a 20 amp fuse, if the wire can
handle that. Then I'd go for nothing bigger than 40 or 50 amps for
EVERYTHING else preferably with branch circuit fuses suited for the wire
size in the branch. 

The disconnect switch is still not a bad idea for long term idle times.
No use having the clock and furnace thermostat run the battery down. A
problem with using the disconnect switch is that while it keeps the
battery from fueling a fire in the wiring while on the road, when most
of the wiring damage occurs, it merely postpones that fire if the wiring
in the RV isn't fully protected by main and branch circuit fuses or
breakers.

My old trucks had maybe 8 or ten fuses, this newest ('98 F150) must have
65 fuses. An RV could do as well. Fuse blocks for the modern automotive
blade type fuses are available in farm stores and should be in
automotive stores, might need to be a hot rod store where they cater to
the building of racing cars. The fuses are commonly available, but I'd
not use brands other than Bussman, Littlefuse or Chase-Shawmut, the
imports may not perform as sensitively. Fuse performance is very
dependent on material and alloying and something that looks the right
size may not open when needed.

When one has an RV opened up for restoration, its important to
physically protect the wires anywhere a conductor might be chaffed.
Sleeve it with insulation stripped from a larger wire, put a grommet in
the hole, (Alligator nylon grommet strip fits in holes with the wires
already in place, I think I saw some last night in the catalog from
Marline P. Jones, www.mpja.com), tie the wires down. They don't chafe if
they don't move to rub. But they still can cold flow, so tying too tight
can be a detriment too. Nylon cable ties are nearly universally
available (check with a friendly police officer for the largest ones
often used for disposable hand cuffs) and work well.

Cable ties, fuses, breakers and many other interesting things are
available from McMaster-Carr, www.mcmaster.com. They have no minimum
order, take plastic, charge a minimal amount for shipping and deliver
rapidly. Automotive and aircraft style circuit breakers are available
from Fair Radio http://www.fairradio.com/ and Hosfelt Electronics
(800-524-6464 if I remember right) and many other electronics
distributors. Being a long ways from the sea, I'm not familiar with
marine suppliers but would expect them to have a fine collection of
suitable materials, as some RV stores might.

And the wiring needs regular inspections, probably every few thousand
miles to catch failures before they happen, both the 12 volt and the 120
volt wiring. When the GFI on the campground pedestal trips, its time to
find the problem in the RV wiring before it turns into a shock disaster
or a fire disaster.

Crimp lug connections to stranded wires survive vibration far better
than soldered connections and getting stranded wires under fixture or
switch screws is a real pain. Crimp with a ratchet crimper for best
results, otherwise crimp the lug at least twice, once over the bared
wire and once over the insulating to anchor the wire better. A crimp
isn't strong if you can pull the wire out, and is overdone if the
crimper cuts the lug into pieces. Between those extremes, you are
working towards a cold weld and that takes quite a bit of pressure.

My business is electrical engineering, and has involved a lot of
building wiring and most recently the design of wiring for a campground.
The National Electrical Code has gained a lot of new sections on
campgrounds and RVs in the past couple editions where earlier editions
totally ignored the RV and RV makers generally totally ignored good
wiring design. One time 20 or 25 years ago I went to a camping show,
looking in the closets and cabinets of all the RV and MH on display and
if I'd been an electrical inspector, would have red tagged every last
one of them for using solid instead of stranded wire, using physically
unprotected wires, no anchors on the wires, no DC circuit protection and
the like. Small wires to a lamp on the front of an upper cupboard that
passed diagonally through the storage space without support or
protection from sliding contents are a fire or wiring failure waiting to
happen. Not a matter of IF, a matter of which stop.

Gerald J.