SUMMARY
Inadequate wiring to the bridge can cause a momentary significant drop in
voltage to bridge instruments and electronics.  In my case the current draw
of a new 12v twin horn  set  momentarily dropped voltage to less than 8
volts at the bridge busbar.

QUESTION
The question to those knowledgable in electronics tolerance to voltage blips
is to what degree, if any, could  this be harmful to the operation of any
device that might be taking power off the bridge?........such as computers,
plotters, GPS, etc.

BACKGROUND
A few months back we purchased a new 12v  twin horn set, without air
compressor, to replace the inoperable single 12v horn  that was on our 1978
Prairie 29  when purchased in 1995.

The horn was installed using  12v power  from the busbar on the bridge.
I installed a new relay near the horn to take the signals from both the
bridge and lower helm horn buttons.

The new horns worked perfectly on three or four afternoon cruises.

I should interject we normally do not toot our horns....but because they
were new and sounded like steamship horns we were given to a little toot
just to make sure they were working.  It made our 29 footer stretch out a
little....maybe picked up a little speed???

To our disappointment the horns  soon would work only intermittently, and
then stopped working altogether. The relay would click but nothing from the
horns.  I initially assumed that because the horns worked perfectly upon
initial installation  the wiring to the bridge busbar was adequate.  The
wiring, however,  was only 14 gauge duplex at a length of about 25 feet from
the main panel circuit breaker up to the bridge busbar, IE, a round trip of
50 feet.  The wiring first goes down into the engine room and then up to the
bridge.....a tortuous path. So I did a voltage check.

THE PROBLEM & THE FIX
Voltage  at the busbar measured 13.1 volts if the horn buttons were not
depressed.  Pressing a horn button dropped the voltage to only 7.48 volts,
enough to probably heat the horns on an icy day but not enough to make
sound.

I replaced the 14 gauge duplex wire with 8 gauge duplex from the main panel
to the bridge busbar.  I used 8 gauge to get as close to a 3% max voltage
drop as possible considering the 10 to 15 amp estimated current draw of the
horns. The horns now work perfectly.  I have not as yet checked the voltage
drop with the horns blowing, but I will at the first opportunity.

THE ERRONEOUS ASSUMPTION
I initially assumed  that the original wiring to the bridge had been
configured to accommodate all typical  power requirements one might have on
the bridge.  I was wrong.  Luckily my sounder and VHF, currently the only
electronics on the bridge, were not impacted by the brief voltage blips.

MY LESSON
Do not make  assumptions that existing wiring will accommodate a retrofit.
It takes more energy than I had imagined to blow ones own horn.....no pun
intended.

Regards,
Al Rice
AMBLIN
Prairie 29

Generally, switching transients, which Al referred to as blips, are not a
problem in a well-wired boat.  If you hook something inductive (i.e., a coil
or motor or some such) to your system especially if that item draws a lot of
current, then as you make and break the circuit "spikes" of voltage enter
your boats electrical system due to the collapsing electric field in the
inductive device.  Most quality electronic equipment can tolerate fairly big
transients, but there is a limit.  It is impractical to try to measure these
transients, as specialized equipment is needed, and frankly it is usually a
waste of time (unless you are chasing specific problems).

However, my reason for this post is not the transients; it is the IR drop
(voltage drop) that Al mentioned.  I am also concerned when something works
for a time (like his horns) and then stops.  I understand that he has
replaced the wires and things are now working; still, I urge caution and
urge others contemplating rewiring something (with or without relays) to
consider this post (or hire a professional electrician).

I am concerned about the IR drop because when you have such a drop you have
an opportunity for overheating the wires, possibly leading to a fire.  The
supply voltage usually will not drop appreciably (unless the batteries are
crapped out).  The voltage across the device in question (in this case the
horns) is not the IR drop; the "missing" voltage is. The rest of the voltage
is not just lost; it is applied across the wires between the battery and the
load device (horn).  Because the horn (this one anyway) is a high current
device, this "dropped" voltage across the wires times the current through
the circuit will give you the number of watts of power that are dissipated
in the wire as heat. This is potentially a dangerous situation.

The fact that this horn only worked a few times then quit tells me that it
damaged the system.  This damage can include melted insulation, melted wire
or wire strands (in extreme cases), wire fatigue or weakening, and
compromise of joint integrity (usually where the joint was not really good
in the first place).  If you run into such a situation, look for any damaged
installation then have a real good look at all connections and crimped
joints.  Take your voltmeter and put it across each joint and connection,
and have someone energize a high current device or devices.  Any reading is
a bad sign.

About relays:
Relays do two jobs.  They are used where you want to switch a high current
device with low current contacts and/or switching wires and to avoid routing
the high current wire from the source (battery) to the load (horn) via a
circuitous path to the switch (horn button).  Al's installation allowed the
use of low current switching, but it failed to provide the high current
circuit, due to the inadequacy of the so-called buss to the bridge.  I say
so-called because the term buss implies a high current capable circuit, when
boat builders often simply use the smallest wire that they can get away with
for the devices installed at the factory (i.e., there Is little reserve
capacity for adding devices and certainly no capacity for high current
devices).

When wiring in a relay for horns, spotlights, winches and other high current
devices, generally, one does so directly from the battery or a real buss bar
(not a wire unless it is very big), using a heavy gauge wire suitable for
the current involved, through the relay and to the device, using the
shortest path possible.  Then, the switching wire (much lighter) can go
wherever you like over any reasonable length.

I just could not let this alone, as electrical wiring is something I know a
little about and I have seen so many boats where persons unknown have
rewired things, usually incorrectly and often dangerously.  We think that
12-Volts is somehow safe, because one can grab onto the wires and not
usually be hurt.  But, high current devices, even 12-Volt ones, are not
something to play with.

Anyway, enough rambling.  I'll get off the soapbox.

Best,
Maurice
AKAMA

-----Original Message <snipped bits>-----
From: owner-trawler-world-list@samurai.com
[mailto:owner-trawler-world-list@samurai.com]On Behalf Of Al & Pat Rice

QUESTION
The question to those knowledgable in electronics tolerance to voltage blips
is to what degree, if any, could  this be harmful to the operation of any
device that might be taking power off the bridge?........such as computers,
plotters, GPS, etc.

The horn was installed using 12v power  from the busbar on the bridge. I
installed a new relay near the horn to take the signals from both the bridge
and lower helm horn buttons.

The new horns worked perfectly on three or four afternoon cruises. Voltage
at the busbar measured 13.1 volts if the horn buttons were not depressed.
Pressing a horn button dropped the voltage to only 7.48 volts, enough to
probably heat the horns on an icy day but not enough to make sound.