Electrical Fire Hazard? - Trailer Brake Wiring

[Luv2Ski]

<< Note: Edited to reflect the results of further investigation. >>
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Normally I would post this question in the Momentum Electrical forum but it may have applicability across all GD trailer lines so I'm posting it here. Mods can move it if this isn't OK.

I found a potential fire hazard in my Momentum when tracing out the towing harness. Let me set this up:

A red heavy gauge battery cable runs from the 12v positive terminal of the house battery bank to a copper strap that is tied to one side of a block of Bussmann Shortstop self-resetting circuit breakers. They are located behind a plastic shield in the forward battery compartment (see attached picture). This is not unusual and the cable is relatively short so that's not an issue. The issue is that a black 10 gauge wire that goes to the breakaway switch is also connected directly to the copper strap and is therefore unprotected if it ever shorts out or feeds something that is shorted. This black wire goes into the tow harness which runs through the curbside propane compartment then up and under the bedroom floor on its way to the junction box above the pin box then onward to the high side of the breakaway switch. See the fire hazard issue? If not, consider this:

If a short occurs along the path from the Bussmann breaker block to the high side of the breakaway switch, the black wire in the tow harness could superheat from very high current coming from the house battery bank. The high current could end up "welding" at the short so the current continues until something on the path back to the house battery bank burns up. There are several ways a short along this path could happen:

• The breakaway switch is physically damaged (from an impact for example) and the guts short to the pin box it's mounted to (or to the mounting screw or mounting plate).
• On our unit, leads from the breakaway switch are too short to reach the junction box so they have pigtails spliced on. The pigtail splice could come apart and the hot wire could contact the frame wing, the pin box, a pin box mounting bolt, etc.
• There are additional crimps inside the junction box that could come apart and short to the metallic body of the box.

The other wire from the breakaway switch is tied to the blue wire in the jacketed pair that goes to the brake electromagnets in the wheel hubs. If the umbilical cable is attached to the tow vehicle, this circuit also goes through the umbilical to the tow vehicle and through its electrical harness up to the brake controller's protective breaker. Pulling the breakaway cable connects the unfused house battery bank in the trailer to the above which could in theory cause a fire in the tow vehicle or along the route of the jacketed pair going back to the axles. If any load on the blue wire is shorted or if the wires inside the jacket short to each other, high current will flow in up to four wires inside the tow harness that runs between the battery compartment and the j-box at the pinbox. A worst case breakaway example would be a shorted brake electromagnet in a breakaway event. The battery bank's excessive current output flows over the following path that connects the positive to the negative terminals:

Starting at the positive house battery terminal, current flows through the heavy gauge red wire to the Bussmann breaker block copper bus bar where the 10 AWG black wire ties on. It flows over that wire through the tow harness to the junction box and onward to the breakaway switch. It flows through the breakaway switch and back into the junction box and onto the blue wire of the jacketed pair going the opposite direction through the tow harness back to the battery compartment and then below the basement all the way back to the axles and on to the electric brakes (wired in parallel??). Since one of the electromagnets is shorted in our scenario, almost all the current flows through it and onto the white return wire in the jacketed pair. It flows back to the battery compartment and through the tow harness back to the junction box. There this white pair member is bonded with other white return wires - one of which carries the flow back through the tow harness to the battery compartment where it is bonded to the chassis. Sharing that chassis bond is the black heavy gauge wire that goes back to the negative battery bank terminal. That folks is four trips through the tow harness and four very hot wires running through the curbside propane compartment and under the bedroom floor. Not cool!!

So, hopefully I've made a convincing argument for providing overcurrent protection to the high side feed of the breakaway switch.

Do you know how the 12v feed to the breakaway switch is wired on your trailer?

[offtohavasu]

Good point!

[Luv2Ski]

I bought a 40A Cooper/Bussmann Type 1 self resetting circuit breaker at AutoZone for $3.59 and installed it with copper strapping. The current to the breakaway switch is now limited well above the maximum draw of the four electric brakes. The new breaker is pictured below before I leveled it and attached mounting screws.

[FT4NOW]

Good write up, the pictures help. With the way you wired the new breaker, and if it was to trip and you needed the breakaway switch to be activated for whatever reason, wouldn't you not have power to the breakaway switch? Obviously you would only need it to activate if you became disconnected from the trailer, but im just trying to understand the new wiring.

Sent from my SM-G920V using Tapatalk

[LCBoyer]

My break away switch is wired to my hydraulic actuator which is fuse. Previously it was not fused

[Luv2Ski]

Good write up, the pictures help. With the way you wired the new breaker, and if it was to trip and you needed the breakaway switch to be activated for whatever reason, wouldn't you not have power to the breakaway switch? Obviously you would only need it to activate if you became disconnected from the trailer, but im just trying to understand the new wiring.

Sent from my SM-G920V using Tapatalk

The self resetting breaker I added is dedicated to the breakaway circuit only so no other loads would trip it. I tried to oversize it at 40A which is probably twice what the four electromagnets would draw on the tandem axle electric brakes. Operating ampacity of 10 AWG wiring is typically 30A @ 75 degrees C. so I wouldn't want to risk going higher than 40A on the amperage limit. Cold electromagnets can have a short lived inrush current spike that shouldn't trip a thermally activated breaker like this one.

According to Lippert, the electromagnets are 3.2 ohms each and I assume they're wired in parallel which would be an 0.8 ohm load. If the house battery bank is running at 13.5 volts, then it would drive 13.5/0.8 = 16.8 Amps to lock up the brakes (assuming they're not greased). So I'm of the opinion it should be very unlikely for the 40A breaker to trip in a genuine breakaway scenario - at least until the unguided trailer swerves, rolls and self destructs in the highway median.

One thing that kinda bothers me is the thinner wiring on the breakaway switch and the jacketed pair going to the brakes. If it's important to use the dedicated 10 AWG wire to feed the switch, why use higher gauge (thinner) wiring on the switch and downstream of it in the jacketed pair that runs to the axles? Doesn't make a whole lot of sense to me.

[Smooth Sailing]

Has anyone contacted Grand Design Service about the break-away switch possible short causing a hot wire fire. How many models lack overload protection i.e. circuit breaker or fuse to avoid hot wire problem? What is the amp draw for the break-away switch? Is the break-away switch connected directly to the battery in all models? Sounds like the break-away switch should be moved to a protected area where it can't be crushed.

[Luv2Ski]

I PM'ed Bill at GD about it and asked him to forward it to engineering. Haven't heard anything back yet.

[gbkims]

My breakaway is wired without a fuse or circuit breaker also.
I wonder if GD is required to wire it this way b/c it's an Emergency Shutdown Device (ESD), in this case emergency brakes.

My breakaway wire is 16 awg. It runs from the pinbox junction box, hangs loose with the other wires and runs over to street side and draps over outside frame and then down to the propane compartment.

I've accindentally wired a circuit board on my workbench so my supply wires shorted across the battery.
It was something to watch the wire go incandescent and then melt in two.

I'll be wiring this breakaway wire through a 40A CB also.
Brown wire on top left is the breakaway to battery.

[Luv2Ski]

I still haven't heard back from GD on this. Maybe more of us need to ask. There's been enough time for engineering to come back with an official response to whether this is intentional or an oversight that requires correction.