How are charger polarity fuses wired?

[tinglett]

This is really a "hmm....how do they really work" question. I don't have an actual problem to solve today at least! As I started typing this post I realized I could investigate (and did).

On my 12v fuse panel I have a pair of "polarity protection" fuses. My original WFCO 8955 used a pair of 40A fuses, and my new PD4655LIV has a pair of 30A fuses. I was thinking power to/from the battery flowed through these fuses, and thus was limited by these fuses. But this can't be true as I just watched over 40A flow through those 30A fuses. And it occurred to me I really don't understand this as fuses can't "detect" polarity anyway. I was lamenting the fact that I didn't take better pictures of the new 12v fuse panel, but then had a "doh!" moment when I realized my WFCO 12v panel is sitting on a shelf. See attached photos.

What I observe is that the fuses are tied off in parallel. They connect the battery + to the charger +. So all the current produced by the charger does go through these fuses. And if you hook the battery + cable to the - (i.e., wrong polarity) you are creating a direct short from the charger + to ground. So the fuses blow.

Now I'm wondering why they use two fuses. I suppose redundancy is good for protection like this, but do you guys suppose the main reason is that 60A or 80A automotive fuses are hard to find?

[Morich]

I'm with you on that question if I understand which way the board is turned, etc. The idea of "polarity protections fuse" is new to me but if the fuses are set as these "hornshoes I've drawn with power coming in and out as the arrows, it does seem the only thing they can do is prevent the total "nuclear meltdown" situation. My view would be that two 40 put in parallel will carry 80 until one of them gives up, so you are going to require something like you mention, a full blown short to ground to get one blown and then the other pretty quickly after!
It does seem like an odd way as putting two items on the board would seem to make it slower to assemble and the cost of one 80 amp versus two 40 and an extra fuse mount seems a bit odd. I often wonder about the way boards are made but more often because I can't see the trace. This time I can see it but not make sense of why.
Just to confuse us?

I may have to do more thought on this as I found a forum post that mentions these being a common item in some trailers. They seem to also use two 40 amp! I guess I never really went that deep in looking for trouble on the panels. Maybe I have one in my Winnebago and never wondered at all?
https://www.jaycoowners.com/forums/f...xud-38144.html
Post number three shows do different panels with them. Looks like a logical thing on trailers where a person is far more likely to moving the batteries in and out, possibly?

[tinglett]

Yep, you got the orientation right. This surprised me as I was assuming one fuse was "protecting" the +, and the other the -. But I've seen more than 30A go to the battery, and today I was seeing over 40A! My current fuses are 30A.

This is a mystery solved, but still puzzling why they make them that way. This also reminds me that these fuses aren't there to protect my battery wiring from being over current. There is a breaker under the rig that does that. Now I'm wondering what size that breaker is.

[Old Navy]

I had assumed like you guys that it was one fuse on the VCC and one on ground. 40A is the largest size ATC fuse, so maybe they were trying to keep with one fuse type?