Fuse Question

Steven Lake

I've got a tiny question about fuses. I've noticed a number of people recommending recently that, instead of the standard brick fuses for smaller setups, using the self resetting automatic breakers like what's used in cars. I admit, for an unattended remote logging station or animal monitor like we use, that'd be good as you don't go offline for hours if a fuse pops. But I also see almost like a "water hammer" effect if you've got a consistent overload situation over an extended amount of time with the fuse kicking in and out, in and out constantly until the overload corrects itself. Would there be a practical use for one of these self resetting switches anywhere in the design, or are there certain areas that a dead throw breaker (ie, it goes off and has to be manually reset) or even a brick fuse would be more preferable?

My thinking would be that between the panel and the charge controller that a brick fuse or dead throw beaker would be preferable, but between the battery and charge controller, or even the battery and the device it's powering, a self resetting would be more preferable, as it's not likely that you'll have more than a brief overload condition from time to time that will quickly reset itself and not persist, especially if the circuit is broken and reset by the breaker. Anyhow, what's your thoughts on this? If self resetting can be used on the entire setup, I'd like to use those instead as it saves me time having to haul my butt out to the back 40 in the winter in 6ft drifts and replace fuses when stuff goes boom. It doesn't happen very often, but the monitors have been known from time to time to pop a fuse when sending updates.

BB.

Hmm... If the fuse/breaker is sized for the wiring, then the fuse tripping is protecting the wiring.

If the wiring can manage more current, then you have to decide if the occasional high loads and resetting means you have a "bad load" or too small of fuse.

I guess you could go with two devices. One single trip fuse sized for the wiring, and a second smaller self resetting fuse/breaker that protects/reacts to the improperly running load. That would give you the safety protection for the wiring and the self resetting if the load goes side ways.

The down side--By design fuses and breakers are the most "unreliable" part of the electrical system (by design)... Having two in series does not help that part.

-Bill

Steven Lake

Well, the problem here is we're not protecting the wiring so much as we're protecting the monitor itself. The units are designed to phone home every so often to report in, and do so using a baseline frequency of like 14 watts or something for the first attempt. If it can't connect with the base it keeps doubling the signal strength until it does. The problem is, after a certain point the unit overheats and turns itself a very expensive pile of smoldering goo. I know that because we've already had two of them do that to us. Supposedly they were designed to prevent that via a whole bunch of fancy internal overload stuff. But clearly not. Anyhow, to prevent that I added a 10a brick fuse in line and it succeeded in stopping them from melting down. The problem now is that when they blow the fuse, especially the ones WAY out back in the rear areas, I end up having to hop on the 4 wheeler, tracker, snowmobile, etc and drive ALL the way out there to replace the fuse to bring the monitor back up. Hence why I was considering the self resetting breaker. That way it'd protect the unit and keep it from burning up while at the same time keeping me from having to haul my butt way out across the farm at all hours of the day and night to replace blown fuses. As far as the panel to battery side of things I haven't had a single blown fuse yet. It's been the battery to monitor side that's been the issue. Again, hence why I was wondering if the self resetting fuses would be a good idea, or if they had some unwelcome downside to them, such as the electrical equivalent of the "water hammer" effect. I can still keep using brick fuses if that's best, but I figured to try and find something that made things easier.

BB.

OK... Understand your needs better.

In general, the effect you are looking for is sometimes called "inductive kickback".

http://www.coilgun.info/theoryinductors/inductivekickback.htm

Basically, when you have current flowing through an "inductive" circuit (coils, DC motor, etc.), when the switch opens, the coil/inductive device wants to continue the current flow--And can generate enough voltage to actually cause an arc across the switch as it is opening.

Most DC Electronic circuits really do not have much in the way of inductance, and there is nothing really different between a fuse/breaker opening or a switch being turned off.

If you do have and inductive kickback problem, you can install a flyback diode (or snubbing diode) that allows current to flow as the inductor "discharges".

On other issue--If you have wide temperature ranges, many breakers/fuses are thermally operated (heating element causes fuse to blow/switch to open). So, make sure that the unit used will open at the current you need over the operating temperature range.

-Bill

Steven Lake

Oh, well, that does simplify things a bit. The unit doesn't have any DC motors, but it does have a transmitter coil, so I'll have to toss a monitor on that and see what it does as it's reporting in and see if I see any of that kickback as it's switched off and on. If not, then it sounds like the self resetting circuit breaker will meet my needs. I just didn't want to hammer the poor monitor while it was kicking the breaker in and out repeatedly during times of heavy load. But it sounds like it really won't. Or shouldn't anyhow. I still need to check it though to be sure. Anyhow, thanks for the tip.

solarpat

I think that the self resetting ocpd's would be a good idea. The one thing I would not do is putting fuses in line with breakers, , if you have to do that you make sure that the devices are series rated. If they are not then a lot of times they will blow the larger ocpd without tripping the smaller and that would defeat the purpose.also there are protective devices in motor controllers called "heaters" that are thermal overload devices but some of them do reset automatically.

Steven Lake

Yeah, this isn't a motor. It's an analog transmitter.