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Disconnecting shore power = blown 15 amp fuse


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Hi Escapees,


We are in week 3 of living in our 2011 Shasta Revere (built by Forest River). We've discovered that when we disconnect shore power on occasion one of our 15 amp DC fuses will blow. After the fuse blows none of the lights, water pump, fridge or LP/CO alarm work. I insert a new fuse (which sparks) and then everything goes back to normal. It seems this behavior only occurs if the battery is not fully charged because if we've been connected to shore power for a day or more then the fuse does NOT blow when we disconnect shore power.


Any ideas?






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With all due respect, even though it's a better idea to turn off the breaker before connecting and disconnecting, not doing so should not cause a problem like the OP has noted. We've been to plenty of CGs (especially in Canada) where you can't easily switch the breaker off so you can't count on that to resolve your problems. It sounds as if a surge is being created in the circuit containing your 12V converter/charger. I'd try using a 30A surge suppressor on your main power cord to see if that damps the oscillation which is causing your problems.

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Seems to me the issue lies with your particular Converter/Charger and how its designed and configured to charge your batteries PLUS supply DC voltage to your RV's 12 VDC Distribution Panel. (one common output, or two separate output circuits for DC distribution and battery charging, no idea what you have). Very old Converter/Chargers had a transfer switch to feed the RV off the Converter OR batteries, later ones do not. I have a Smart Battery charger always, of course, connected to my battery bank and the bank directly feeds my DC distribution panel. If your Converter/Charge feeds the batteries only or has one circuit to charge the batteries and another to feed your DC distribution fuse panel SORRY I JUST CANT SAY SITTING HERE I bet some of the other good sparkies can???



1) The blown fuse must be due to a high current surge.


2) When plugged to shore power and the Converter/Charger operating properly, your house batteries are at a higher voltage, perhaps 13.2 up to 14+ volts depending on Charger and Batteries and Loads....


3) When NOT plugged to shore power, your house batteries "once settled" will be more like 12.6 or less (as batteries discharge) voltage.


4) Since I = V/R your resistive DC loads draw different current depending on battery voltage (12 to 14)


HOWEVER I don't envision voltage difference of 12+ to 14+ making such a huge current difference into a typical RV DC resistive load that a surge blows a fuse. Even if the current was a few amps different, I don't see that as a surge to blow a 15 amp fuse.


ALSO The battery bank serves as a huge electron bulk storage bank and does NOT charge its voltage that rapidly. The battery acts as a big buffer and big

capacitor such that if a charger is connected or not, voltage doesn't change all that fast UNLESS there are high current draws, NOT 15 amps.


I GUESS WHAT IM SAYING IS I DONT SEE YOUR BATERY VOLTAGE CHANGING FAST ENOUGH (when RV plugged or unplugged) AND EVEN IF IT WAS, AT 12.6 VERSUS 14 VOLTS, I DONT SEE THAT CAUSING ENOUGH CURRENT SURGE TO BLOW THE FUSE. Note motor loads cause more surges then resistive you know. If the water pump or vent fan is running when you plug or unplug that's more critical then if a light is on.


I don't see placing any AC current surge suppression for 120 VAC input to the RV as curing the problem of blowing a 15 amp DC fuse that's fed energy from your batteries. Your Battery energy storage electron bank serves as a buffer similar to a capacitor, I don't see AC line voltage surges the cause of blowing DC panel fuses


5) I don't see your DC battery voltage changing when plugged or unplugged fast enough to matter or blow a fuse REGARDLESS IF YOU UNPLUG OR THROW THE BREAKER. Those breakers are not arc extinguishing dielectric type, simple air break and regardless if you pull the plug or trip the breaker I just don't see that making a difference or causing your problem.


What am I left with????????? I see it as some sort of Converter/Charger issue. I just don't see unplugging or tripping a breaker making a difference to cause the surge,,,,,,,,,,,,,,,,, nor battery voltage changing rapidly enough regardless if converter/charger is operating or not to cause such a surge to blow the fuse.


Some sort of a Converter/Charger problem or correct application would be a likely culprit instead of plugging or unplugging or tripping the breaker making a difference in my opinion. Absent one huge current draw, when you plug or unplug or trip a breaker (so charger is on or off) battery voltage isn't gonna change very fast, which leads me back again to your Converter/Charger and it having separate output circuits for charging versus DC distribution WHICH I HAVE NO IDEA HOW ITS DESIGNED.


That's the best I have to offer, sorry its NOT an answer, just my best guesses not being there.


John T

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I like the analysis of JohnT, although I'll have to poke him just a bit for his obvious engineer approach! :P While as a tech my approach to parts of the question are different from those of JohnT, the conclusions are pretty much the same. His is more theory while mine is more a practical one but if we are both correct we should reach the same conclusions, and did so.


If you have not checked voltages with a good volt/ohm meter, I suggest that you do so. I like my recording meter as it allows me to see what if any the voltage spike may have been, as I agree with JohnT that there must have been one. The statement that it sparks when the new fuse is inserted, tells me that something is turned on and you should have most all of the items that you list turned off before you disconnect, other than the refrigerator and LP/CO alarm. The alarm current draw is in the miliamp range and even the refrigerator doesn't draw much more than perhaps 1 or 2A.


My conclusion is exactly the same as that of JohnT, even with a somewhat different approach, I totally agree that this must be some sort of issue with the converter/charger. I suggest that if your converter is wired to receive 120V power via a power plug, test things one time by disconnecting that plug before you unplug shore power as a means of narrowing the possible causes one more step.

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Wow...amazing responses, and so fast! Much appreciated. Here are some more details:


1. We use a surge protector when connected to shore power.


2. I always ensure the plugs on both ends are attached before switching the breaker on and always switch the breaker off before disconnecting the plug ends. But as docj noted, this really is no different than yanking the plug out. (In any case, the blown fuse occurred yesterday with switching off power via the breaker.)


3. When inserting a new fuse I have EVERYTHING that I'm aware of switched off except for the LP/CO alarm. All lights, pumps, fridge are off. But I still get a healthy spark when reconnecting and the converter fan spins up for a brief moment and then stops.


I've only experienced the problem when the battery is low and we haven't been hooked to power long enough to provide a full charge. It seems that if we've been hooked up long enough to provide a healthy charge that the issue doesn't occur. (Unfortunately, my multimeter is not with me so I'm using idiot lights to diagnose the problem. Maybe I can find another RVer along the way who may be able to lend a multimeter.)




(currently in West Yellowstone, headed to a hardware store for a basket of 15 amp fuses!)

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My assumption is that Andy's converter is either malfunctioning or has a relatively poor design and is exhibiting classic voltage overshoot and ringing when it turns on. If the overshoot is large enough and long enough in duration it's possible that a fuse will blow. I had suggested an AC surge suppressor to reduce voltage transients in case they were being passed through the converter but since Andy says he's using one that can't be the problem.


It might be possible to fix the problem by using a "slow blow" fuse as a replacement for the standard one. If transients are the culprit this should cure it.

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So I just called the manufacturer (WFCO) and got connected to a tech support person in under 60 seconds! Amazing. Here's a summary of our conversation after I described the problem:


"Oh, we get calls about that quite frequently. The DC circuit #6 (which is the one blowing) can take up to a 30 AMP fuse as long as you have 10 gauge wire running from the battery to the converter. A lot of manufacturers put in a 15 amp fuse and then this problem occurs. Go ahead and confirm the wire size and replace with a 30A fuse."


He then posited that when the battery is low and voltage drops during disconnect that a higher amperage is required, thus blowing the fuse. Strange that the problem wouldn't also occur when CONNECTING the shore power, too.


docj....I like the sound of the "slow blow" fuse. Maybe I'll try that in a 15 amp and then if I still have the problem, upping to a 30 AMP.


(currently in West Yellowstone, headed to a hardware store for one slow blow 15 amp fuse)



UPDATE: I don't think a slow blow blade type fuse exists (at least not one that would fit in my converter). Looks like I'll be trying the 30 AMP route first.

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Thanks for the feedback, that's always good to hear to see how far off (or on maybe) we were were...........


I don't recall slow blow fuses if yours are the type I think they are.


Fuses are sized to protect the feeder conductors (you compute the load, then size the feeders, then size the overcurrent protection device to protect the feeders), so if the circuit in question uses 10 Gauge wire, I would first try maybe a 20 then 25 then 30 amp fuse as necessary, NOT using any bigger then absolutely necessary and NEVER bigger then the wires ampacity. If no slow blows are available, pick up a few 20 and 25 and 30 amp fuses is what I would do.


The fact you get a healthy spark when inserting the fuse leads me to believe there's a decent current draw taking place MORE then any LP detector, which may be part of the problem (short or phantom load or an added load scabbed to an existing circuit)


Anytime Kirk agrees with me makes me feel good, if a theoretical engineer type and an experienced tech agree ANSWER MUST BE RIGHT MAYBE???? LOL


Other then perhaps a higher initial inrush current when switching on an incandescent light, pure resistive loads don't cause surges (especially high enough to blow a fuse) to the extent a motor like a water pump or vent fan would when switched on or off..............


Nuff said


John T

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Since the circuitry was designed and installed by the RV builder, I'd wonder just how the guy at WFCO can possibly know what that circuit supplies???

The DC circuit #6 (which is the one blowing) can take up to a 30 AMP fuse as long as you have 10 gauge wire running from the battery to the converter.

That seems to be saying that the fuse is between the converter's 12V output and the battery. If you have one of the WFCO power centers, he should know what he is talking about as those units come as a converter and distribution panel, all in one. Our KZ-Sportsman has one of them in the 35A version.(WF-8735) I'm not completely thrilled with mine but have learned to live with it. I just looked at mine and it has 6/120V breakers (1 main & 5 branch) and it has 7 fuse holders with the top one empty and specs say it has 6(1 battery circuit). The bottom fuse would seem to be the battery circuit and it has a 40A fuse, the one just above that has a 20A fuse and the other 4 are 15A fuses.


if a theoretical engineer type and an experienced tech agree ANSWER MUST BE RIGHT MAYBE???? LOL

That's pretty much my feeling also! Different approaches with the same answer must be a good thing! :P


I do not like self resetting breakers in this sort of situation because if there is a real problem which blows that fuse, I'd not want it to happen over and over. In this case we seem to be talking about the main 12V power fuse and it should take a pretty good jolt to open it.

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This is basically in response to Kirk's analysis immediately above, which makes good sense to me NOT armed with the Converter/Charger wiring diagram.


I missed or overlooked that part about "The DC circuit #6 (which is the one blowing) can take up to a 30 AMP fuse as long as you have 10 gauge wire running from the battery to the converter." and Kirks response (good catch, I overlooked that) "That seems to be saying that the fuse is between the converter's 12V output and the battery.


That makes sense because if the Converter/Charger is a 30 Amp rated unit (typical Converter sizes may be 30 or even up to 50 or more amps) 10 Gauge Wire and a 30 Amp Fuse IS EXACTLY WHAT ID EXPECT THERE.


If the house batteries are in a fairly low discharge state, say down to 12.08 volts, when that Converter/Charger is initially turned on, in the circuit FROM Converter TO Battery bank, I CAN CERTAINLY ENVISION AN IMMIEDIATE SHORT TERM CURRENT SURGE AS HIGH AS 30+ AMPS AND PERHAPS EVEN MORE, WHICH COULD POP A 15 AMP BREAKER IF THE CHARGER IS TURNED ON.


NOTE My discussion above about how the battery bank serves as a buffer (acts like capacitor) and that battery voltage isn't gonna change drastically over a short time, REMAINS TRUE AND VALID, but the issue Kirk describes IS DIFFERENT. If were talking about a circuit and fuse FROM Converter/Charger TO Battery Bank, a short term current surge can take place there if a 30 Amp Charger is suddenly turned on and connected to a battery bank. If its a big depleted electron bank and is short on electrons, you can bet when coupled to a Charger shes gonna suck electrons big time (above rating) for a short period but plenty long enough to pop a 15 amp fuse.............


MY UPDATED ADVICE AFTER KIRKS ANALYSIS ABOVE, If that's indeed in the circuit FROM Converter/Charger TO Battery Bank, and if it uses 10 gauge Wire, Id try a 25 or 30 amp fuse there n see what happens. Maybe previous owner needed a fuse and all he had was 15 amp, so thats what got stuck in there????????????? Earlier above I was looking at this as a Branch Circuit Fuse NOT the main fuse from battery bank to Charger SILLY ME


Nuff Said


John T

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Maybe a bad battery. Volts go down amps go up. Blown fuse. As when power is lost and voltage drops

f just had a customer travel with class C and it was not charging going down the road. When he activated the disconnect switch it took out the fuse on the disconnect switch. That fuse was on the side of the disconnect switch. So the converter would not charge the batteries as it was disconnected from the 12vdc panel




Also had a customer with 6 8d batteries in a provost conversion that were acting weird. Discharging quicker than normal under load. Seven year old batteries so installed 6 new batteries and back to normal.

I have the batteries at my shop and two of them have dropped two volts just sitting there for a week not connected to anything.



Safe Travels, Vern

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This is basically in response to Kirk's analysis immediately above, which makes good sense to me NOT armed with the Converter/Charger wiring diagram. ~ ~ ~ John T

Let's just say that I have a little edge on you since I happen to have an RV with the WFCO converter/distribution panel. They typically come as a single unit and the entire thing is one device, with the bottom fuse being between the converter and the battery. The loads share a common point on that fuse panel, rather than at the battery post.


Electrically, the supply line to the battery is also the line from the battery to the load center. As a result, what may be happening is that with the shore power connected the converter supplies power to a common point that has a wire to fuse #6 and from that to the battery and that same common point also supplies all branch fuses as well. When he disconnects the shore power cable it means that the current flow through fuse #6 reverses, having been charging it suddenly reverses to supply all branch circuits and that reversal is causing the fuse to blow. To the fuse it would be very similar to the surge in an ac circuit when shifting sources that are out of phase.


I'm not sure why that reversal would blow the fuse as the actual current through it would not be higher than in other circumstances but it would see a shift from a +12V or so to the batteries to the same load from the batteries. :huh:

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Reading and reviewing all of the discussion above it is my conclusion that the only problem you have is the incorrect fuse in fuse holder (branch) #6. The converter manufacturer probably ships the converter with a 15A fuse as a safety precaution because they don't know how exactly the converter will be used and what wire the end user will implement on the the output of branch #6.


I've attached a simple diagram of the converter and DC circuitry typical of RVs that use this type of simple converter/charger. Notice that branch #6 feeds current to the battery(s) when 120V is supplied to the input of the AC/DC converter. In that situation the fuse in branch 6 only needs to carry the battery charging current. Fuses 1 thru 5 carry the current flowing from the AC/DC converter to the respective loads. However, once you disconnect the AC input to the AC/DC converter the current flowing through fuse #6 reverses direction because then the battery starts supplying current to branches 1 thru 5. In this situation fuse #6 needs to carry all the current flowing to branches 1 thru 5. Fuse #6 should be sized to carry the maximum of: (1) the maximum expected charging current, (2) the maximum expected total current through branches 1 thru 5, (3) The maximum current for which the wiring in branch 6 is rated.


Using only a 15A fuse in branch 6 limits our TOTAL 12Vdc current in branches 1 thru 5 to 15A when you are not connected to post power.


The reason you see the #6 fuse fail when you disconnect from AC power is likely because you are getting a momentary surge of current from the battery into the reactive output circuitry of the AC/DC converter. That is also likely the reason you are seeing the "healthy spark" when you insert a new fuse even though you have eliminated most of the loading on branches 1 thru 5. Also, the reason you hear the converter/charger fan spin when you insert a new fuse is because it uses a 12Vdc fan. As soon as you insert the fuse the battery(s) probably supply current to the converter/charger overtemp and fan circuitry. The air inside the converter/charger is probably still warm from prior use and the fan moves a little air around until the sensor circuit detects that cooling is no longer needed.


All of the above is deduced as a best-guess from the above discussion. If it were me I'd put a 25A or 30A fuse in branch 6. I suspect this will solve all your problems.




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Ron and Elena, you suggested "If it were me I'd put a 25A or 30A fuse in branch 6"


Above I suggested "Id try a 25 or 30 amp fuse there n see what happens"


Might it be said great minds think alike ??? Or might it be said "The blind leading the blind" lol


There's no sort of a make before break relay mechanism and a sufficient current surge could blow a 15 amp fuse. Maybe the manufacturer installed a 15 amp fuse there or a previous owner installed one???


As far as the theory involved when suddenly switching/connecting the 12 VDC Battery Bank to an:


"Inductor", at time T = 0+, AN INDUCTOR BEHAVES LIKE A HIGH RESISTANCE NEAR OPEN CIRCUIT as an Inductor resists any sudden change in current.


"Capacitor", at time T = 0+, A CAPACITOR BEHAVES LIKE A LOW RESISTANCE NEAR SHORT CIRCUIT as a Capacitor resists any sudden change in voltage.


A Capacitor, which resists sudden voltage changes, can serve as a buffer and ripple filter and if a discharged capacitor is suddenly switched/connected to a Voltage Source, it will initially draw high current until such time the capacitor becomes charged.


Us sparkies have heard or studied or used all this before, but this may help some newcomers.


John T

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I am pretty sure that Andy has one of the models I suggested in post #12 and that it is as I described, from looking at mine. All of the WFCO units in recent years are pretty nearly the same layout and design. If you examine the installation instructions from WFCO, you will find that it ships with no fuses except reverse polarity and the fuses are installed by the RV builder to match wire sizes used.


Andy, if you don't have them, here is a link to the WFCO installation instructions......


and also to the WFCO owner's manual...............

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Kirk and oldjohnT said what i was suspecting while reading your initial post detailing the problem-keys "which sparks" "low battery". High current draw is melting the fuse at the initial surge. Simply turning off the main breaker at incoming power will not eliminate that surge when the breaker is switched on again. oldJohnT's recommendation of stepping up in fuse size is a common sense, practical approach to fuse sizing.

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UPDATE: I'm an idiot. :(


The fuse was blowing when I was CONNECTING to shore power and we only noticed and resolved the issue when DISCONNECTING because then the power would go out. The battery charger goes through DC #6 and as it's the 35 AMP converter mentioned earlier in the thread so it easily blew the 15 AMP fuse if the battery was too low. I've replaced with a 25 AMP fuse and all is well.

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