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66Overlander 02-08-2005 05:27 AM

First, I can pretty much assure you that prewired vehicles will have a fuse or circuit breaker from the factory (Ford, Chevy, Dodge, etc.) because while a wire "will act as it's own fuse" if it shorts out, it may do so in an inconvenient location, start a fire, and burn the vehicle down in the process. Therefore, any wiring done by the tow vehicle manufacturer will have provisions for a fuse or circuit breaker so that the fuse "open" occurs in such a way as to protect the rest of the vehicle.

Second, every set of trailer brake installation instructions I have seen indicates that the makers of the brake controllers also see this as a requirement and specify a fuse or circuit breaker in the power feed wire.

To do anything else is taking unnecessary chances. In most situations with a loss of trailer brakes you should be able to eventually get the rig stopped (but it'll take longer), as long as your dash or other part of your tow vehicle is not "on fire" or smoking heavily.

And in case you're wondering why I'm disagreeing with Inland Andy on this one, I've been an engineer in the Auto Industry for many years and am familiar with their circuit protection practices. Andy is a valuable resource to this Forum and he may be right on many issues, but there is definitely room to disagree on this one. If you add an unfused brake control circuit, you are (as Andy might say) asking to become a statistic. Luckily shorted wires are probably much rarer events than trailer sway, and most people may never experience one. But why take your chances when peace of mind is as close as the addition of a fuse or circuit breaker?

Inland RV Center, In 02-08-2005 09:56 AM


Come down a 7000 foot mountain with a 31 foot trailer without brakes.

Then tell us how foolish we are to not have fuse in a brake line. Tell us how your going to stop this rig on a narrow two lane winding down hill highway, and fix the problem.

Things being things, when in an emergency, "ANY" help is good.
Having a blown fuse, is not a good excuse, for not having brakes.

Engineers or not, coming down a mountain, such as we have here, is no fun, even when everything is OK.

Peace of mind? What really is peace of mind when it comes to brakes? Know that you have something instead of perhaps nothing, describes a peaceful mind.

There has never been a known fire from a trailer brake line being shorted.

Bottom line is everyone can make their choices.

The issue of what is ideal becomes useless, when we must have the brakes, and we don't.

Fusing any electrical circuit is a very wise thing to do. "BUT" there are a very few exceptions.

Each owner can decide for themselves, which way they want to go. But when they are posed the question of no brakes to some brakes, seldom would anyone say "I will go with the no brakes."

It's not a matter of whats right or who is right, it's much more of a matter of
what do you do, if your faced with a "got to."

Some people drive over the edge. Fortunately, most use good common sense, and have prepared their equipment and themselves accordingly.


Chuck 02-08-2005 10:50 AM


Originally Posted by Over59
Seems like something could be built into the controller to indicate there is a short on the system.....

There is on mine. something I need to investigate before next season. My brake controller is indicating a short in the system somewhere, when the trailer is NOT attached, and I step on the brake. (light flashes rapidly). However, when the trailer is attached, the indicator shows that everything is normal, with my foot on and off the brake. ??

(tekonsha voyager, plugged into a factory trailer-package equipped Dodge.)

Over59 02-08-2005 04:37 PM

Chuck, I think it's telling you you forgot the trailer. :p

66Overlander 02-08-2005 05:40 PM

This will be my last post on this issue, so you need not debate the issue with me further. You cannot prove that a Trailer Brake wire has not cause a fire anymore than I can prove it has. In the aftermath of a fire it is often hard to pinout the cause. By the way, an unfused wire represents a risk 24/7/365, not just whenever you have a trailer hooked up.

In any case, I chose to follow proper circuit protection practices to minimize the chance of such events. Each owner can chose to do as they wish and take any chances they wish. I was just trying to provide the information necessary for them to make an informed decision.

Anon 02-12-2005 12:53 PM


Originally Posted by markdoane
I'm just repeating what I see in the installation instructions for brake controllers. Most specify a 20a circuit breaker.

I don't follow your logic about being able to stop, rather than worry about smoking wires. If the brake circuit is grounded enough to trip a 20 amp breaker, then there's probably nothing getting through to the magnets anyway. And if you have a short on one side only, feeding full amps to the other side could cause the trailer to trip sideways.

I think you should have a fuse or breaker in front of the breakaway switch too.

Just my personal opinion, not a theory.

The breaker that the instructions are referring to is an automatic-reset breaker. They are small and metal-encased. This is absolutely essential to have to isolate the brake controller from being shorted. It has to be a fast-acting automatic reset type. This way you will have brakes no matter what. Sometimes the breaker will trip and reset without you even knowing that it happened. DO NOT use a fuse or a manual-rest breaker. As Andy said, you will have no brakes then.
Do not put a fuse in front of the break-away switch. If the break-away is triggered it is because of a MAJOR problem, like the trailer and tow vehicle are no longer travelling together. When that happens and your break-away trips it sends power from your trailer battery to the brakes. It takes EVERYTHING in the way of supplied power and sends it to your brakes. Guaranteed that if you have a fuse in that path it will surely blow and release the trailer brakes.

Anon 02-12-2005 12:59 PM

This link will show you that Draw-Tite requires a automatic-reset breaker. It is an Adobe Acrobat file so you can save it to your PC for reference.

Over59 02-12-2005 02:00 PM

Thanks Lou.

You managed to agree with everyone. I think that's the setup the dealer did on mine. The DIYer needs to get the right type resetting breaker. Would that be a Type I or III? I think Type II is manual? Type I is autoreset with power on and Type III is auto reset when the power is turned off. I may have them backwards.

Anon 02-12-2005 04:34 PM

Two types of switches...

There are two types of automatic resetting breakers: Type I (cycling) and Type II (non-cycling).
More information on these breakers can be found here:

The particular breaker I used, (Type I) can be found here:

PS I usually hang around a discussion until I see the general consensus and then agree with the masses!

TomW 02-12-2005 05:43 PM


Originally Posted by pattersontoo
...PS I usually hang around a discussion until I see the general consensus and then agree with the masses!

Perhaps you should make include that sentiment in your signature so that everyone has the proper perspective on your advice.


markdoane 02-12-2005 10:16 PM


Originally Posted by pattersontoo
It takes EVERYTHING in the way of supplied power and sends it to your brakes. Guaranteed that if you have a fuse in that path it will surely blow and release the trailer brakes.

That's just plain wrong. Each brake magnet has a set resistance (around 3 ohms, I think), so it doesn't matter at all how big the battery is, or how many amps it will put out, each magnet can only draw about 4 amps at 13 volts. I=V/R. So if you have four magnets, a 20 circuit breaker would be sufficient to handle every possible ounce of power that the magnets can use. It won't blow the fuse or circuit breaker unless there is also a short in the system.

And if you have a short in the brake line, you won't be getting any juice to the brakes. Electricity will follow the path of least resistance, direct to ground. Believing that running without a fuse or circuit breaker will allow more juice to flow around the short, and reach the magnets anyway, is not correct.

Anon 02-13-2005 12:36 AM

Plain wrong....

I had originally typed out a long dissertation on Electrically Safe Working Practices and how it applies to not fusing safety circuits. The link above will show you that even the manufacturers of the breakaway switch do not show a fuseable link between their switch and the power source.
I was also going to explain how Ohm's Law is affected by heat.
I stopped myself because I figured that it is not worth it.
This is SUPPOSED to be a place where people help people.
Suffice it to say that in my 20 years designing, building and repairing industrial automation that you never fuse a safety device. The NEC states that you never fuse a safety device. If you would like the chapter and sub-section of the 2002 NEC Handbook I will go get it for you. I am a member of the IAEI for a number of years. I bet I can find it quickly. Does anyone else out here have the Handbook so you can check me on it? I do not want to be called a liar.
In retrospect I feel that maybe I should have left it at saying that no trailer manufacturer has fused their breakaway switches.
I will not say anything that I cannot back up without third party documentation.

PS Sorry I could not agree with the masses on this.

Anon 02-13-2005 12:41 AM


Originally Posted by TomW
Perhaps you should make include that sentiment in your signature so that everyone has the proper perspective on your advice.


Why is it that everyone here wants to exchange barbs? Are we suffering from cabin fever?
That was considered a joke. Maybe even a pun.
I am just talking about the things I have documentation on. If you read my posts you will see that I usually have a useful link that is helpful and/or insightful.
At least I am trying to be helpful.
Relax, man.

john hd 02-13-2005 05:43 AM

don etal,

if there was a short in a brake wire it would be in parallel with the magnets.

there would still be some current going to the magnets no matter what the resistance of the short. ohm's law.

i think that is what andy was trying to get at all along.

some brakes no matter how little, are better than no brakes. as i pointed out earlier my truck's brake line is fused. but, since it is "slugged" with a 60 amp mega fuse i would expect the wire to burn in the clear before it would blow.

even though everyone here need not agree on this subject, remember, andy adjusted insurance claims on airstreams for many years and has seen the results "doing things the other way" first hand. just ask him.

and if you think about it, anyone in his position would naturally opt for the safest advice.

other than the current crop of ford f150s that burst into flames from electrical problems caused by faulty antilock wiring, when was the last time you heard of tow vehicles spontainiously igniting?

i think the possibility of losing your brakes at the worst possible moment is a larger risk.


74Argosy24MH 02-13-2005 06:30 AM


Originally Posted by john hd
if there was a short in a brake wire it would be in parallel with the magnets.

there would still be some current going to the magnets no matter what the resistance of the short. ohm's law.

It will only be in parallel if the short is in the wire to an individual magnet. If it is in the main wire that feeds all the magnets it will be in series. This will very, very quickly burn in half and there will still be no brakes. But you do have an uprotected relatively light wire that is part of a harness burning end to end.

Andy might have 39 years of experience, been an insurance adjuster, et al, but he hasn't thought this through. He needs to take a piece of 10 ga. wire 30 ft. long and lay it across a set of battery terminals and report the results.


Over59 02-13-2005 06:30 AM

Thanks Lou. I had them all mixed up. Type III is manual.

I a 100 amp fuse a good size for the battery main.

66Overlander 02-13-2005 08:42 AM

I said I wouldn't post again on this topic, but I need to re-emphasize one thing I stated in my last post. Then I'll shut up and let everyone do whatever they want, because I don't want to get into a nasty debate.

Anyway, an unfused wire in your tow vehicle represents a risk of a short circuits 24/7/365 - i.e. all the time. This fact alone would make me want this wire fused. It does not matter whether a trailer is hooked up. Since most times that most vehicles are driven, a trailer is not attached, does it seem surprising that people haven't seen too many cases of a tow vehile burning? Most such shorts probably do occur when a trailer is not attached. And if the circuit is fused (by fused I mean with a fuse or circuit breaker), as I suspect most are, the short will eventually be found and repaired and the vehicle will not haven insurance claim for burning down.

When towing a trailer you should be checking that your trailer brakes actually work frequently and monitoring the display on the controller even more frequently for indications of a problem. Thus, with due care, your "un-fused" circuit should only make any difference if that short circuit were to actually occur when pulling down that mountain. Of course a dead short in parallel with two or four or six brake magnets (4 ohms - 3 Amps each I think), will mean that most current will still go thru the short, with little to no measurable braking performance. But you still would be "letting the smoke" out of a wire somewhere.

And, I guess I'll agree with some of the other thoughts mentioned in this thread. The fuse (again fuse or circuit breaker as you prefer) is to protect the wire from the Battery to the Brake Controller. A short after the controller will likely do one of two things. It'll either cause the controller to shut down it's output to the brakes and report a fault or I suppose if the Brake Controller is more basic, it could damage the brake controller. Either way, this is likely to happen faster than a fuse or circuit breaker would open.

I will not discuss the Breakaway Switch wire in trailer, because I have no idea of how trailer manufacturers handle this (fused or not?), but I will say that if it were up to me, I'd consider that fact that that wire respresents an opportunity for a short 24/7/365, not just when the breakaway switch is used.

john hd 02-13-2005 09:42 AM

we ain't talking christmas lights here!
1 Attachment(s)

with much respect i do not agree about the series explanation.

it is clearly a parallel circuit. otherwise it would not operate if one magnet were to fail.

i have included a very crude drawing to explain my point. as you can see any fault to ground along the entire braking system would be in parallel with the brake magnet coils.

the thing everyone needs to remember about parallel circuits is that the voltage drop across a resistance is the same. only the current varies. that is why with even a low resistance (read BAD) short the brake magnets will still receive some current.

hence, the some is better than none point of view. again this is what andy is getting at.

perhaps the best solution would be a way of deenergizing the braking circuit when not in use! but then that could lead to human error.

here is my attachment for what it is worth.


Anon 02-13-2005 10:56 AM

As I stated earlier:
Fuses are a no-no.
Automatic-reset breakers are required by the brake controller manufacturer. They reset so quickly you would be hard-pressed to even see the change even on a DMM, (maybe on a VOM). These breakers open and close in milliseconds. The breaker manufacturer supplies the reset times in the literature that comes with the breaker.
If you have a factory-installed brake controller, (like in the 2005 Ford F-250), or a factory-installed pre-wired option then it has this built into the fuse panel. This is why they are sometimes confused with fuses. Mine is installed on the firewall and then ran to the battery.
As stated before, this is considered a safety circuit by automakers. Fuses are not present in safety circuits. This is the reason electronics manufacturers have been manufacturing automatic-reset breakers for quite a long time now.
As for the breakaway switch, there should never be a fuse between it and the power source. A fuse is considered the "weak link" in an electrical circuit. It is designed to protect equipment from damage or personnel from a dangerous electrcial shock. The NFPA and NEC considers anything over 50V, AC or DC, to be a hazardous voltage. I do not see how your system would have hazardous voltage at the levels we are dealing with in a 12VDC system. This rules out using a fuse to protect personnel.
Now, to use a fuse to protect the switch. These are just plain switches. When the insulator, (the part that is hooked to the tether), is pulled out, (when the trailer goes walk-about), then the two conductors touch. Two simple little pieces of metal hooked to two simple little pieces of wire. The alternative path to ground is completed and power is passed from the trailer's battery to the trailer brake circuit. What would you need to protect from damage in this circuit at that time?
Remember, there are only two reasons you use a fuse in a circuit:
1. To protect personnel from injury or death.
2. To protect equipment from damage.
I teach these in the Electrician Apprenticeship program I do for a number of automotive plants.

74Argosy24MH 02-13-2005 11:24 AM


Originally Posted by john hd

with much respect i do not agree about the series explanation.

it is clearly a parallel circuit. otherwise it would not operate if one magnet were to fail.

Ok, I see your point, all that training wasted. But I still think with E=IR when R=0 that you are asking for trouble with no breaker. My only hope would be the controller failed open. Either way I don't think you will have braking for more than a second or two and the problems it could cause aren't worth the little time it will work.


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