Circuit Protection in Typical Camper Power Systems

Jon R

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Oct 7, 2020
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Puget Sound Region, Washington State
We have a lot of posts and discussions about circuit protective device (fuse or circuit breaker) location and sizing for the truck charging wires, the camper battery wire, and branch circuits, but I don’t think i have seen in the few years since I joined the following issue discussed: specific protection of the wire from the camper main positive bus bar or terminal to the camper fuse panel positive bus connection due to multiple sources of fault current.

I sized my camper battery external fuse with the size of that particular wire in mind (6 awg, 80 amp fuse), but recently I was thinking about the fact that, if there’s a short to ground of that wire (really any wire connected to the positive bus, but all the other wires are short, secured well, and entirely located and visible in the battery compartment) while I’m driving on a sunny day, the fault current will initially be the couple of hundred amps the battery delivers plus the 30 amps each outputs of my DC to DC charger and the solar charger. The battery fuse would blow quickly, but then I would potentially have 50 to 60 amps flowing steady-state into the short from the chargers. That could be a fire hazard.

I decided to check the Victron manuals to see if my Orion or Smart MPPT charger have any kind of output short protection. The MPPT manual says it has output short protection, but doesn’t describe the nature of that protection. The Orion 12/12-30 does not say it has output short protection, and instead says its output short current is 60 amps. That would require its input side to receive 70 amps from the truck. My 6 awg wires and 80 amp fuse at the truck battery would allow that.

So, while it sounds as though the solar charger won’t just sit there putting 30 amps into the short, the Orion apparently will make up for it and put 60 amps for a prolonged period into the short until something internally acts or fails and reduces the current. Does anybody know that not to be the case?

I can’t reduce my camper battery fuse size because I want it to be able to charge at up to 60 amps from the two chargers simultaneously. The wires are sized for that. However, I’m thinking of doing two changes:

1) Adding a 40 amp fuse in the wire from the positive bus bar to the camper fuse panel located near the bus bar. That will allow the normal load currents and allow the Iota DLS-30 to charge the battery if it ever decides to grace my lithium battery with 30 amps. It would blow if that wire shorted to ground either from the huge initial battery fault current or from the 60 amp output short current of the Orion.

2) Reducing the size of my fuse at the truck battery to 40 amps so it would hopefully blow if the output of the Orion shorts and the input current rises significantly.

Arguably I don’t need to do both, but I probably should have fused the truck end of the Orion input wire at 40 amps in the first place. I used an 80 amp fuse because it was there and available on the OEM battery fuse block and it was adequate protection for that wire.

Thoughts on this, anyone? Thanks.

Oh, and “spend the money on beer instead” is still being considered as a possible alternative.
 
Good line of inquiry Jon, and one I have not investigated. A fuse/breaker inline to the DC fuse panel would also give one a way to disconnect all power vs I using the silver push/pull kill switch. I wonder if wiring the iota and Orion, each with its own fuse might be a way of handling this?
 
Good line of inquiry Jon, and one I have not investigated. A fuse/breaker inline to the DC fuse panel would also give one a way to disconnect all power vs I using the silver push/pull kill switch. I wonder if wiring the iota and Orion, each with its own fuse might be a way of handling this?
Hi Vic. My understanding is the Iota already has an internal fuse on its output. I assume it’s a 35 or 40 amp fuse on the DLS-30.

If the Orion output delivers 60 amps into a short, then a 40 amp fuse on its output would allow it to deliver 30 amps in bulk charging but blow in a short.

I think a 40 amp fuse where the wire to the fuse panel comes off the bus bar would also be sufficient to protect against a short to ground on that wire because the battery fault current would blow it before or as it blows the 80 amp battery fuse, and that blown 40 amp fuse would isolate the Orion from the short, possibly slightly faster than a fuse just on the Orion output.

I guess either approach would would work. Spacewise in my compartment I have more room to install a fuse holder on the Orion output as you suggested.
 
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Post is about fuses but worth pointing out a little-known aspect of circuit breakers.
That is the rating of the circuit breaker does not mean the circuit breaker will instantaneously trip at that rating. The circuit breaker rating is the highest current it will pass indefinitely without tripping and is NOT the current at which it will trip. In many cases, it can pass a much higher current for several seconds before it trips. If you need to protect against short time power bursts, then you may want to purposely undersize the circuit breaker knowing a circuit breaker can pass several times its rated current for many seconds before it trips (but no lower than the expected normal current load). As shown in the curve below the time to trip is a function of X times the rated current. The higher the current the faster it trips but at a current just above the "rated current" may take several minutes for the circuit breaker to trip and at 2X the rated current can still take 10 to 40 seconds to trip. A circuit breaker does protect against short circuits as a short will pull as much current as the battery can provide thus for smaller rated circuit breakers results in a fast trip. Here is one example for a typical no-name 30 amp circuit breaker you may find on Amazon or Ebay. Most sellers do not post this information so unless you see a curve like this you really don't know what protection you are buying.

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Thanks for this graph! Do I read it correctly, that this “typical” 30A breaker would pass 60A for 10 seconds?
 
This how I read it. 60 amps on a 30 amp breaker would need a minimum 10 seconds to trip and possibly as long as 40 seconds.

It is a useful feature for circuits with motors, i.e. refrigerator, where there may be a high in-rush current at startup. This way the fuse does not trip every time the motor starts but still gives some protection if the circuit goes above its nominal rating for a longer period.
 
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I'm beginning to like fuses more...
I like that they are simple, passive, are not significantly affected by long term extreme environmental exposure (other than corrosion potentially causing them to open prematurely), and they blow quickly with overcurrent if you size them for that.

Plus Blue Sea fuse holders are pretty!
 

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