Wiring electrical question

[slbran]

On my last camping trip one night we returned and had no 12v power (dry camp). I found that there was a glass fuse in a holder under one of the seats were the power came in from the battery and it just did not have a good connection.

After looking at the wiring in detail I am not sure it is the best set-up. The wiring diagram in the documents shows a fuse near the battery and one in the trailer but does not show sizes. Why are there 2 fuses?

I was going to replace the old glass fuse with a newer mini type fuse and put it close to the battery. My old fuse is a 15A. I started thinking that the TV charge input while towing might be more than 15A and that might blow the fuse. Is that the reason for 2 fuses - one by the battery for higher current and on for in the trailer at 15A?

Thanks,
Steve

[Carol H]

Steve your profile does not show what type of trailer you have - if its a Scamp then yes under the front passenger side hatch near the front of the trailer there is a glass inline fuse and I had to replace mine about 5 years ago - no problems since. You may be aware of this already but there are (at least on mine there is) a number of fuses inside the converter box as well. As far as why two fuses I havent a clue! :-) I just know I have only ever replaced the one under the front hatch.

[Darwin Maring]

Byron should be able to answer this as he designed wiring for trucks.

[Raz]

Seeing this on the factory drawing suggests this is not an after purchase wiring job. If you are saying that the two fuses are in series then there is no electrical reason for doing so. Further if the two are different values then the smaller is the only one that should blow. That said, I put a fuse near my battery as an easy way to disconnect the battery from the trailer. Are you sure the two fuses are not in different circuits? Raz

[Hambone]

on my Scamp there was a fuse outside the trailer by the battery box, and one inside the forward closet ???, dont know if it was factory or PO. I just replaced it all with battery disconnect and 1 fuse.

[Dan Meyer]

Originally Posted by P. Raz

Seeing this on the factory drawing suggests this is not an after purchase wiring job. If you are saying that the two fuses are in series then there is no electrical reason for doing so. Further if the two are different values then the smaller is the only one that should blow. That said, I put a fuse near my battery as an easy way to disconnect the battery from the trailer. Are you sure the two fuses are not in different circuits? Raz

I too initally thought that two fuses in series was not necessary. After studing the schematic and pondering the possiblities I changed my mind. All of the fuses are needed to protect the wires against overcurrent from multiple current sources.

The three current sources are:

1) Battery suppling trailer converter / trailer loads
2) Converter charging battery
3) Tow Vehicle charging battery.

If you were to plug into your tow vehicle and also plug your trailer into shore power at the same time, you have the possiblity of pushing too many amps thru the wire going to the battery.

-- Dan Meyer

[Donna D.]

There are two fuses on my Scamp as well, but Steve has a Fiber Stream (I clicked on his Trailer Profile to discover that), so I don't have a clue if it's standard on that brand of trailer

[Raz]

Originally Posted by Dan Meyer

I too initally thought that two fuses in series was not necessary. After studing the schematic and pondering the possiblities I changed my mind. All of the fuses are needed to protect the wires against overcurrent from multiple current sources.

The three current sources are:

1) Battery suppling trailer converter / trailer loads
2) Converter charging battery
3) Tow Vehicle charging battery.

If you were to plug into your tow vehicle and also plug your trailer into shore power at the same time, you have the possiblity of pushing too many amps thru the wire going to the battery.

-- Dan Meyer

Hi Dan. Don't mean to be contrary here but if the circuit is truly a series circuit then the current is constant by definition and using two fuses is redundant. Without a schematic I have no way of knowing what the actual circuit looks like but if as you suggest there are three sources involved then it is doubtful the circuit is that simple. Perhaps Steve will give us further information. Raz

[steve dunham]

Originally Posted by P. Raz

Hi Dan. Don't mean to be contrary here but if the circuit is truly a series circuit then the current is constant by definition and using two fuses is redundant. Without a schematic I have no way of knowing what the actual circuit looks like but if as you suggest there are three sources involved then it is doubtful the circuit is that simple. Perhaps Steve will give us further information. Raz

The current is only constant if the load stays constant (a battery load will changes as it charges ) The rule is "In series the current is constant and the voltage divides , in parallel the current divides and the voltage is constant. Series connections of over current devices are common practice . If you look at Art 430 in the NEC there are many references to series over current devices. Some provide short circuit protection and some provide running over current protection and are installed in series. Residential Air conditioners can be UL listed in a manor that requires a circuit breaker and fuses .You would install a 20 amp 2 pole circuit breaker at the source (breaker panel ) and 2 -30 amp time delay fuses in the disconnect located at the AC unit . If the AC experiences an overload hopefully the breaker would trip before the fuse blows allowing you to just reset the breaker and not have to go out to the AC and change fuses.



'

[Frederick L. Simson]

Originally Posted by slbran

After looking at the wiring in detail I am not sure it is the best set-up. The wiring diagram in the documents shows a fuse near the battery and one in the trailer but does not show sizes. Why are there 2 fuses?

Originally Posted by Donna D.

There are two fuses on my Scamp as well, but Steve has a Fiber Stream (I clicked on his Trailer Profile to discover that), so I don't have a clue if it's standard on that brand of trailer

The wiring diagram in the documents is for a Scamp. It is representational enough for a generic trailer's wiring diagram, but is not totally universal across brands.

Fiber Stream only put in one Battery fuse, and it's usually located where you found yours.
The Converter Module has 2 circuit breakers for the A/C circuits...

1. Left side of the trailer
2. Refrigerator and Roof Air Conditioner

... and there are 4 fuses in the converter for individual 12 volt circuits

1. Lights in front half
2. Lights in back half
3. Water Pump
4. Furnace Blower

If you lost ALL 12 volt power, then the fuse near the battery blew.
If you lost only SOME 12 volt operation, while other 12 volt stuff works, then check the fuses inside the converter module.

[slbran]

Thanks guys for the reply's.

I should have been a little more clear. Yes I do have a Fiber Stream. The reason I questioned the wiring, besides no 12v power that one time, is that the battery was moved and the wiring to it changed.

Fred, I think your description tells me that even with the battery move my wiring is basically the same as original. I am not sure I like the way it was wired but will probably leave it and just update the glass fuse since it all works. Well, all except the side marker lights, but they have no bulbs and corroded sockets.
Always something else to fix.

[Raz]

Originally Posted by steve dunham

The current is only constant if the load stays constant (a battery load will changes as it charges ) The rule is "In series the current is constant and the voltage divides , in parallel the current divides and the voltage is constant. Series connections of over current devices are common practice . If you look at Art 430 in the NEC there are many references to series over current devices. Some provide short circuit protection and some provide running over current protection and are installed in series. Residential Air conditioners can be UL listed in a manor that requires a circuit breaker and fuses .You would install a 20 amp 2 pole circuit breaker at the source (breaker panel ) and 2 -30 amp time delay fuses in the disconnect located at the AC unit . If the AC experiences an overload hopefully the breaker would trip before the fuse blows allowing you to just reset the breaker and not have to go out to the AC and change fuses.




'

Hello Steven. With all due respect to Mr. Kirchhoff's laws and the National Electrical Code, the original post described a single wire, most likely less than 5 ft. long, with two glass fuses of different values. I can't think of a practical reason for doing that so I suspect there is something missing. Not to put you on the spot but can you think of a reason to put two glass fuses of different values that close together in a series circuit in a trailer? Can you think of a reason to put two glass fuses of the same value that close together in a series circuit in a trailer? I suspect there is a branch circuit missing between the fuses or as I indicated in my original post the fuse at the battery is to provide an easy disconnect for the battery. Raz

[Raz]

If the fiber stream is wired like the scamp then the two fuses are not in series. They have the tow vehicle charging line between them. Two different branches, two different fuses. Mystery solved. Thanks for pointing out the schematic. Raz

[Eddie Longest]

I can't find my wiring digram but my guess is the fuse under the seat is to protect the trailer. Unlike most vehicles a trailer has more than one source of 12VDC power. If your trailer is connected to the tow vehicle or the converter is plugged in the 12VDC battery lead is hot. So if you decide to remove the battery and the positive lead touches the frame then the positive lead can become a welding lead. So the battery fuse is there to protect the TV, converter or the suprised person who is trying to remove the battery while the trailer is powered by another source.
Eddie

I found the Scamp wiring diagram. The fuse is also there to protect the the TV and its wiring harness if you were to pinch or short out your tow vehicle wiring harness. The trailer battery backfeeds voltage into your TV when the trailer is connected to TV.
http://www.fiberglassrv.com/forums/d...?do=file&id=75

[steve dunham]

Originally Posted by P. Raz

Hello Steven. With all due respect to Mr. Kirchhoff's laws and the National Electrical Code, the original post described a single wire, most likely less than 5 ft. long, with two glass fuses of different values. I can't think of a practical reason for doing that so I suspect there is something missing. Not to put you on the spot but can you think of a reason to put two glass fuses of different values that close together in a series circuit in a trailer? Can you think of a reason to put two glass fuses of the same value that close together in a series circuit in a trailer? I suspect there is a branch circuit missing between the fuses or as I indicated in my original post the fuse at the battery is to provide an easy disconnect for the battery. Raz

I wasn't sure of the exact situation but your description made it clear ,So NO not in this situation. Sometimes in equipment they will put a larger fast acting over current device to provide short circuit protection and a time delay device for running overcurrent protection in series. I am assuming in this case the fuses are the same size and have the same trip characteristics .My point was that not all series connected over current devices are redundant and in some cases are required and necessary . I taught electrical theory , machine control and code at the Vo Tech College for many years (33) and sometimes I see a complex answer to a simple question. I need to work on not doing that

[Raz]

Originally Posted by steve dunham

I . I taught electrical theory , machine control and code at the Vo Tech College for many years (33) and sometimes I see a complex answer to a simple question. I need to work on not doing that

Me too. Twenty five years at the college level. Mostly Electronics, DC and AC. I usually avoid the electrical posts as the terminology tends to get in the way but this one had me curious. Take care, Raz

[Art VanDelay]

The Battery Fuse:

Should there be a SHORT in the cable that connects to the tow vehicle, and/or a short in the tow vehicle wiring, the fuse will blow at the camper battery to protect the wiring of the camper and tow vehicle.

Converter Fuse:

1. Should there be a short in the camper wiring beyond the inline fuse in the camper, the inline fuse will blow to protect the converter and outbound camper wiring.

2. Should the camper be connected to the tow vehicle while the converter is powered up and Should there be a SHORT in the cable that connects to the tow vehicle, and/or a short in the tow vehicle wiring, the fuse will blow on the inline fuse that goes to the converter to protect the wiring of the camper and tow vehicle. Both fuses should blow in this situation.