Blowing fuses on shore power

[fotochop]

There are two fuses in the 12v fuse panel that repeatedly, and intermittently, keep blowing when I'm on shore power at home.

I've had them blow at strange times like when I raise the bathroom mirror (?), the latest is by simply connecting the two furnace thermostat wires together at the thermostat (which didn't seem to be working correctly).

I'm not too swift with electrical, but I do understand how current flows.

I'd like to know where to begin. I have a VOM. The 12V battery is new and fully charged.

I'll take a wild guess and suspect that one or more wires are causing a short. Is my first voltage/continuity etc check across the fuse terminals in the fuse box? thanks

[Inland RV Center, In]

Originally Posted by fotochop

There are two fuses in the 12v fuse panel that repeatedly, and intermittently, keep blowing when I'm on shore power at home.

I've had them blow at strange times like when I raise the bathroom mirror (?), the latest is by simply connecting the two furnace thermostat wires together at the thermostat (which didn't seem to be working correctly).

I'm not too swift with electrical, but I do understand how current flows.

I'd like to know where to begin. I have a VOM. The 12V battery is new and fully charged.

I'll take a wild guess and suspect that one or more wires are causing a short. Is my first voltage/continuity etc check across the fuse terminals in the fuse box? thanks

Rick.

Why are 2 fuses blowing?

What are the rating of the fuses.

Do you have an "amp meter" that will go up to 20 amps or a little more?

If so, instert the ammeter in the circuit that blows the fuse, in place of the fuse.

Then horse around and tap this bump that etc, to see if you can locate a short.

When the Univolt is charging, the battery voltage is a little higher, which shouldn't contribute to the issue, but it could be, again, depending on which exact fuse is popping.

Andy

[HowieE]

It is vary strange that 2 fuses different would blow. They have nothing in common but the battery and the ground circuit.

The first condition that comes to mind that would effect 2 circuits would be reduced voltage. As voltage drops current goes up for a given load.

I would put a volt meter on the source side of the panel and have someone watch it while you do your testing. If in fact the voltage is low with a new battery I would check ever connection between the battery and the panel for tightness and condition and the battery and the chassis ground connection.

A bad or loose connection can cause a voltage drop.

[Inland RV Center, In]

Originally Posted by HowieE

It As voltage drops current goes up for a given load.

Howie.

In any pure DC circuit when the voltage drops, the currect also drops. The Ohms law formula is E=IR or I= E or R= E.
R I

In AC work, where there is an inductive load, then your absolutely correct.
E=IZ Z being impedance which is a combination of resistance and reactance.

Andy

[HowieE]

Touche

I stand corrected.

[Inland RV Center, In]

Originally Posted by HowieE

Touche

I stand corrected.

Howie.

Glad to help, but things being things, the formulas didn't post correctly.

So I will do it the long way.

E=IR and I is equal to E over R, and R is equal to E over I.

Sorry.

Andy

[fotochop]

Thanks for your replies, guys. Unfortunately I don't have an ammeter and all the battery connections seem solid. I thought I'd try bypassing the thermostat and touch the two thermo wires together at the heater to see if the heater fired up like it usually does. This only blew another fuse. I guess its time to call in an expert...someone with an ammeter and a whole lot more know-how than me ;-) I do have a couple of hot shots I can call.

I checked my home circuit breaker and verified that the circuit I'm plugged into is actually a 20-amp (garage) circuit. Now, I thought that was sufficient to keep my battery charged at home, run an electric drill inside the trailer, etc, as long as I didn't run the a/c. Am I mistaken in that assumption? If so, I can call a home electrician and get it amped up to 30 amps, but that doesn't seem like the problem here...

I won't bring the trailer to my local white-box rv retailer, though, as their constant put-downs in search of a sale/trade-in are a bit much for me... they just can't handle Airstreamers in any form...

[TomW]

Or is it "lightbulb" ?

Anyway, if you are comfortable doing so, before calling someone with an ammeter, wire a 12 volt light bulb in series with a fuse, and connect the creation to the fuse block in place of a fuse.

If nothing is turned on, the lightbulb will be off (no current flow). If the bulb glows, look at how bright it is glowing. A bright light means dead short; dim light means something is on.

Turn everything off, and play with the mirror or whatever. You might be fortunate to see the light flash on & off as you get closer to the real source of trouble.

Tom
p.s. The fuse in series is not required for this short-term test - it was included to keep everyone from focusing on its absence.

[Inland RV Center, In]

Originally Posted by fotochop

Thanks for your replies, guys. Unfortunately I don't have an ammeter and all the battery connections seem solid. I thought I'd try bypassing the thermostat and touch the two thermo wires together at the heater to see if the heater fired up like it usually does. This only blew another fuse. I guess its time to call in an expert...someone with an ammeter and a whole lot more know-how than me ;-) I do have a couple of hot shots I can call.

I checked my home circuit breaker and verified that the circuit I'm plugged into is actually a 20-amp (garage) circuit. Now, I thought that was sufficient to keep my battery charged at home, run an electric drill inside the trailer, etc, as long as I didn't run the a/c. Am I mistaken in that assumption? If so, I can call a home electrician and get it amped up to 30 amps, but that doesn't seem like the problem here...

I won't bring the trailer to my local white-box rv retailer, though, as their constant put-downs in search of a sale/trade-in are a bit much for me... they just can't handle Airstreamers in any form...

If the furnace is original, quite well the motor went south.

If that's the case, the fuse will blow.

Your 20 amo city power service is ok, for now.

But you still have not said what is the amp rating of the 12 volt fuses.

Andy

[fotochop]

The two fuses that are blowing are both 15 amp (the new ones are 15 amp "AGC"). That's what was in there originally and I was reluctant to try anything higher.

The furnace is a new Suburban NT30, which replaced the old NT22. The sticker on front the new Suburban says 5 amps.

Man, it really bums me out to be so clueless about electrical relationships...theres a level of complexity there that my brain can't seem to get around (I think back and wish I had FORCED myself to study harder in math class... seems I can't actually learn something until I've SEEN it (a problem with electricity) and worked with it myself) and it bums me out further to think of the interior/overhead panels I'll have to pull out even if someone else does find a short...

[VIKING]

Most shorts are in connections or fixtures, so it probably won't be necessary to pull panels. I opened every switch, outlet, light fixture, etc when I first got my Safari and tightened and inspected every connection. I found three loose ones, and one of those was intermitent. Time well spent.

Rich the Viking

[Inland RV Center, In]

Originally Posted by fotochop

The two fuses that are blowing are both 15 amp (the new ones are 15 amp "AGC"). That's what was in there originally and I was reluctant to try anything higher.

The furnace is a new Suburban NT30, which replaced the old NT22. The sticker on front the new Suburban says 5 amps.

Man, it really bums me out to be so clueless about electrical relationships...theres a level of complexity there that my brain can't seem to get around (I think back and wish I had FORCED myself to study harder in math class... seems I can't actually learn something until I've SEEN it (a problem with electricity) and worked with it myself) and it bums me out further to think of the interior/overhead panels I'll have to pull out even if someone else does find a short...

Now I know what the issue is.

The fuses are supposed to be 20 amp, not 15 amps.

Try that, and then make another post.

AGC 20 amp fuses will, I believe fix the problem. You can get those fuses at any auto parts store.

Andy

[VIKING]

From the original post it sounds like there is something more going on here. I read that the fuse blew when he raised the mirror, which could be coincidence, or it could have a bad connection in the bathroom light fixture. Mine would blink when I moved the mirror, before I tightened up the loose connection in the switch. Maybe worth a look.

Rich the Viking

[fotochop]

Originally Posted by Inland RV Center, In

Now I know what the issue is.

The fuses are supposed to be 20 amp, not 15 amps.

Try that, and then make another post.

AGC 20 amp fuses will, I believe fix the problem. You can get those fuses at any auto parts store.

Andy

Wow, that sounds way too easy! Can I do this safely? Meaning, without a wire heating up and setting the trailer afire?? The wires to that fuse are red and about 1/8 of an inch (sorry, I need to learn my gauges...)

I'll gladly give it a try but may end up calling someone just to feel better about it. I've had other fuses blow at strange times, once while hooking up the light inside the curb storage, for example, where one wire had come loose. I'd really like to know that the trailer is wired correctly, but I've camped in it for a couple years now with no serious electrical problems.

I can only think of one other thing that may not be right. After installing the new heater I installed an LP gas detector by using the hot & ground wires leading to the heater. I just added two jump wires to those and hooked them to the detector. Should I have run dedicated wires back to the fuse panel or something? Thanks for all your help!

[Inland RV Center, In]

Originally Posted by fotochop

Wow, that sounds way too easy! Can I do this safely? Meaning, without a wire heating up and setting the trailer afire?? The wires to that fuse are red and about 1/8 of an inch (sorry, I need to learn my gauges...)

I'll gladly give it a try but may end up calling someone just to feel better about it. I've had other fuses blow at strange times, once while hooking up the light inside the curb storage, for example, where one wire had come loose. I'd really like to know that the trailer is wired correctly, but I've camped in it for a couple years now with no serious electrical problems.

I can only think of one other thing that may not be right. After installing the new heater I installed an LP gas detector by using the hot & ground wires leading to the heater. I just added two jump wires to those and hooked them to the detector. Should I have run dedicated wires back to the fuse panel or something? Thanks for all your help!

Electrical problems, what ever they may be, must first be diagnosed.

Remove the wires for your LP detector, then proceed.

You cannot have any mysteries in the equation, when your trying to find a fault.

Then try the furnace again.

Post again, with those results.

Andy

[Jammer]

When you are on shore power, the DC system runs on, typically, 14.4 volts, instead of 12.7 volts when you're on battery. If the fuses are marginal - undersized - that can cause them to fail.

You want to use the right size fuse. Smaller wires are used in automotive applications for the same fuse size than in household applications, in part because the 14 gauge=15 amps standard in household wiring is thought to be unrealistically conservative in many quarters, and in part because 12 volt systems are more highly engineered and don't rely on plug-connected loads of unpredictable size.

One strategy to troubleshoot is to go up to 2x the fuse size and connect an ammeter and observe the circuit closely while in operation. The ammeter readings can be used to isolate the problem while the large fuse still provides short circuit protection.

[richinny]

post a photo of the bad fuses. a short or a slight overload will blow the fuse differently. try to show the wide side of the metal strip inside. a dry bearing in the motor can cause quite a load when it tries to start to turn.