Ok fellow scientists :) What is so special about the reverse polarity fuse circuit in a converter compared with just having a fuse on the positive output wire?I yanked the 7355 from the trailer to put in the truck camper, and that means it is now like a portable charger with 120v input (no neg wire though, just hot and neutral with a 7355) and pos and neg DC wires output.Its two 30 amp reverse polarity fuses are still on the DC fuse panel back in the trailer, doing nothing now. Somebody said here a while back that each 30 did half so it was like 60 (on the 55 amp converter)So how can I wire up a set of reverse polarity fuses for the loose 7355, or can I just depend on a 60 amp fuse on the positive wire?(AFAIK, I am also supposed to run a chassis ground wire for it now that it is no longer in its original power centre, same as you would for a deck mount.)Thanks.

The purpose is to prevent destroying the Power converter when you hook up the positive and Negative cables at the battery in reverse. It rarely works. But, sometimes it does. Doug

Use T rated ultra fast reaction fuses. Refer to good inverter reading like Outback or Magnum for an in-depth explanation of their purpose and function. I like inverter and charger products that scream polarity orientation via color. I.e. red wire to red terminal.

Hi doug,Reverse polarity fuses worked for a shop that managed to put one of the twelve volt jars in series with the others--presenting 24 volts to the converter.They also worked when I was attempting to do double conversion with my Magnum inverter. Double conversion with my old Cobra MSW was no problem. I don't understand why there was a difference.BFL13, yes fuse on the positive but use what mex has wisely recommended.

Not getting the info requested so far.I want to know if there is some special way you have to wire up a couple of 30s however they do it in a converter or over at the DC fuse panel, instead of using a 60 farther out on the pos wire. Why do they use two instead of just one big one? My PowerMax 100amper uses four 40s while the PowerMax 55 amper uses two 40s. The 7355 uses two 30s but they are over on the fuse panel.

Yes the reverse polarity fuses are in series with the output of the converterAre you actually using a single voltage Parallax converter???????? Two lower cost fuses are less expensive and easier to install Nothing more than a matter of cost for the manufacturer IMHO A parallel connection of a fuse is STUPID at best nothing more than a cost saving that the comsumer pays for sooner or later

Reverse Polarity Fuses Circuit Question | Good Sam Community

38 Replies

MEXICOWANDERER
Explorer
Oct 05, 2015
Ever check the response curve between different amperage / same type fuses?
Smaller rated fuses may react faster ?
Chris_Bryant
Explorer II
Oct 05, 2015
Dutch_12078 wrote:

Even though it makes no sense from a cost standpoint to parallel fuses when a larger single fuse costs about the same, I bow to your superior knowledge and experience...

The common ATC fuses they use are not available any larger than 40 amps, which is too small for this application. The maxi fuse is quite a bit larger, and would present layout problems.
Dutch_12078
Explorer II
Oct 05, 2015
KJINTF wrote:
Posted By: Dutch_12078 on 10/04/15 07:44pm

It would make no sense at all for a converter manufacturer to parallel two 30 amp fuses instead of using a single 60 amp since they both sell for the same price. If anyone looked at the schematic or opened up a converter with reverse polarity fusing, they would see that the two fuses are NOT paralleled. Each one is in series with an output, one for the positive side and one for the negative side. The purpose of having both sides fused is intended to prevent a reverse back feed from the battery positive to the converter negative when it's connected incorrectly. The fuse on the positive output also provides normal overload protection when wired correctly of course. As Doug said, the fuses don't always work as intended though.

This is NOT what I have seen in the over 200 converters I have opened up in the past few years. Paralleling a set of fuses has many negative drawbacks electrically but can reduce costs in many ways including wire size, wire routing, part size, parts availability, etc.....

Even though it makes no sense from a cost standpoint to parallel fuses when a larger single fuse costs about the same, I bow to your superior knowledge and experience...
MEXICOWANDERER
Explorer
Oct 05, 2015
In haste I forgot to add that the both legs fuses are what should be used and not necessarily what are used. Most high end inverters use a permanent FUSIBLE LINK as a last resort protector. Sorry for my lapse. I have seen a Trace 5548 burned to the ground with the chassis ground wire fried from using a positive only fuse not protecting reverse input. Started a fire that burned down a shed.
wa8yxm
Explorer III
Oct 05, 2015
To answer your question: What is so special about the reverse polarity fuse in/on the converter v/s an in-line fuse in the wore:

Electronically: NOTHING

Preactically: Some installers, in the name of saving 5¢ may omit the inline fuse and thus toast a very expensive converter instead of blowing a very inexpensive fuse or two. Thus the converter manufacturer puts the fuses in so he won't forget.

My rig they put in a self resetting circuit breaker that is smaller than the fuses (I have 3 of those 30 amp jobs)
pianotuna
Nomad III
Oct 05, 2015
The OEM wiring for my converter has auto reset circuit breakers--one near the battery bank and the other near the power panel. Since it is a PD unit it also has reverse polarity fuses.
BFL13
Explorer II
Oct 05, 2015
DrewE wrote:
Parallel means side-to-side, with the end leads connected together. So you'd have the wire from the converter, connected to one end of both 30A fuses, and the other ends of the fuses also connected together to one wire going to everything else. The goal is to have half the current flowing through each fuse, basically forming a 60A fuse. If the legs are somewhat unequal for whatever reason, the current won't be evenly split, and one of the fuses would be likely to blow even under non-overload conditions or when a reverse polarity battery connection is made, followed momentarily thereafter by the other one (which would then be carrying the full current). A single 60A fuse would be simpler and better.

Series connection means end to end, the same current passing through everything. Having multiple fuses in series is pointless. Having the fuse(s) in series with the output of the converter, however, is necessary—that's just saying that the wire from the inverter goes to the fuse(s), and then the wire from the other end of the fuse(s) goes to the rest of the world.

I see now why some people have reported that only one of the two fuses has blown. The usual advice being to replace both fuses anyway, since the one that didn't blow might be "stressed"

Also this all shows how come sometimes people report their battery fuse or DC circuit breake at the battery has failed, and also the RP fuses way up at the converter have also blown.

My Xantrex inverters do not seem to have any RP protection, and will be immediately fried if you screw that up. No second chances! OTOH the portable battery chargers, converters, and solar controllers do seem to have internal RP protection depending on the model etc. Got to read the info on them to know for sure in each case.

So every RV pretty much has a fuse at each end of the battery to converter positive wire: the battery fuse and the RP fuses in the converter.
Bobbo
Explorer II
Oct 05, 2015
Each power producing unit needs a fuse near it. The battery needs a fuse near it because it can provide power to the unit. The converter needs a fuse near it because it provides power to the unit, as well as charging the battery.

So, yes, you really need 2 fuses, one at each end of the wire.

That is why people who run a charge wire from their MH battery/alternator to the towed vehicle's battery fuse both ends.
BFL13
Explorer II
Oct 05, 2015
Thanks again. I wondered if those sense wires at either end of the RP fuses group had something to do with the RP protection.

Not clear why the single fuse outside the unit should be close to the converter instead of near the battery where you would get more of the wire protected. I do want to protect both the converter from reverse polarity and the wire as usual with a "battery fuse" Or do I need two fuses, one at each end of the wire? The wire will be about three feet long in this case, but what if it were twenty feet long?

RJ, yes, I now understand your post with the single fuse being in series. I got off track when you said "they" and was still thinking about the two 30s (which are in parallel). All clear now! :)
KJINTF
Explorer
Oct 05, 2015
Posted By: Dutch_12078 on 10/04/15 07:44pm

It would make no sense at all for a converter manufacturer to parallel two 30 amp fuses instead of using a single 60 amp since they both sell for the same price. If anyone looked at the schematic or opened up a converter with reverse polarity fusing, they would see that the two fuses are NOT paralleled. Each one is in series with an output, one for the positive side and one for the negative side. The purpose of having both sides fused is intended to prevent a reverse back feed from the battery positive to the converter negative when it's connected incorrectly. The fuse on the positive output also provides normal overload protection when wired correctly of course. As Doug said, the fuses don't always work as intended though.

This is NOT what I have seen in the over 200 converters I have opened up in the past few years. Paralleling a set of fuses has many negative drawbacks electrically but can reduce costs in many ways including wire size, wire routing, part size, parts availability, etc.....

BLF - It depends on the converter (Yes the PD's do exactly the same "reverse" polarity fuse configuration some even have as many as 4 fuses in parallel, none on the "-" side). You can see in the picture there are sense lines that are used by the brains of the converter.

Bottom line in your case I would put a single 50amp fuse close to the output of the converter and use the proper size wire to the load and not worry about "reverse" protection

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Reverse Polarity Fuses Circuit Question

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