Auto-Reset circuit breakers--how do they work?

[DonTom]

Such as the Littlefuse 0813040.xp.

This is a 40-amp auto-reset 12 VDC circuit breaker. I will use it for my question.

Say it opens after a 50-amp load. Then what? When does it try again and what happens if the 50-amp load is still there? How often does it try to reset? How long does it stay closed before it opens again? And how long does it stay open before it closes on the 50-amp load again?

I cannot find any specs on these auto-reset breakers, such as how long they take to reset at different overload currents. I don't need any accuracy here, just mainly the expected reset times open and closed again.

-Don- Cinder Hills, Coconino Forest, AZ

 

[CharlesinGA]

They just go open and closed, and open and closed, over and over. They don't have any limit, at least not until they burn up.

Times, dunno, ones I have seen cool and reset rather quickly, say, within 30 seconds or less, in my experience.

Charles

 

[DonTom]

I just now found this. But what happen if into a dead short? Say 10,000% of rating? Still resets and closes every ten seconds into a dead short?

Seems 10 seconds to reclose is too fast.

 

[Lou Schneider]

A self-resetting breaker acts like a thermostat, the current flowing through it heats a bi-metallic element that curls away from a weak magnet when it's heated. When excess current causes enough heat to overcome the magnetic attraction the contact is pulled open, breaking the circuit.

Once the contact opens the bi-metallic element cools and returns the contact towards the magnet, resetting when the magnet captures the contact.

The current at which it trips is determined by the resistance of the bi-metallic element and the contacts themselves. Repeated cycling will cause pitting on the contacts, increasing their resistance and heat production so the breaker starts tripping at lower currents.

As far as extreme overcurrents, yes the breaker will continue cycling. But if there's a ridiculous amount of current there will be a ridiculous amount of heat generated in the bi-metallic element so each closure will be extremely short. Until the contacts burn up, then they will remain open.

 

[DonTom]

at least not until they burn up.

Yeah, I assume at some point it turns into a fuse and blows!

-Don- Cinder Hills, Coconino Forest, AZ

 

[DonTom]

Until the contacts burn up, then they will remain open.

Are they designed to act as a fuse at some point? IOW, safely open and stay that way?

-Don- Cinder Hills, Coconino Forest, AZ

 

[Lou Schneider]

Are they designed to act as a fuse at some point? IOW, safely open and stay that way?

-Don- Cinder Hills, Coconino Forest, AZ

Only after the contacts burn up. Remember, the objective is to keep the wiring from burning up and it takes some time to heat wire to that extent, especially if it has a chance to cool between cycles. In an application like electric brakes it's better to have numerous short brake applications than no brakes at all.

 

[John From Detroit]

There are two or 3 types of self resetting circuity breakers

One type is basically a thermostat... as current flows the bi-metal "Spring" thing heats. when TOO MUCH flows it gets hot enough to "Turn off".. then when it cools down.. Back on. Old fashion turn signal blinkers worked this way.. Well some did.

The other king of old fashion blinker had a heat wire wrapped around the Bi-metal ( I won't describe how it worked)

"Short Stop" (a line in a brand) Breakers have that as well.. here is how they work (Had a deffective one at one time )( I kind of like thest units.

First. just like the above. current flows through the bi-metal strip and when it gets overly heated CLICK and it opens. but now there is a heat wire wrapped around it and a VERY SMALL amount of current continues to flow throw that wire keepign it hot. Till you remove the load. then it cools off and resets.

 

[Tom]

In addition to the above responses, for the folks who prefer an audio/visual explanation, here's a simple YouTube video explaining it.

 

[DonTom]

In addition to the above responses, for the folks who prefer an audio/visual explanation, here's a simple YouTube video explaining it.

I should have been clearer. I knew how they work regarding the bi-metals responding to the heat.

My only questions were regarding the times involved in opening after an overload and the time to reclose.

Anyway, I have all my questions answered well enough.

These questions started because, for some unknown reason, my 40-amp blade fuse blew that goes to my DC2DCC input in this motorhome. I am not sure how or when it blew, but I later noticed I wasn't house battery charging while driving. Last night I replaced it with a 40-amp auto-reset CB and I was wondering if that was wise. Sounds like I will be okay this way.

Thanks for all the info. to all who replied.

-Don- Cinder Hills, Coconino Forest, AZ

 

[Tom]

I should have been clearer. I knew how they work regarding the bi-metals responding to the heat.

The YouTube link was mainly for folks visiting who may benefit having a non-tech answer, especially with a large or short-circuit load. IOW poof! grab the fire extinguisher &/or call the fire department.

 

[PancakeBill]

My question is whether they are directional. usually a steel colored post and a copper colored post, which side to power, which side to load, or does it make a difference?

 

[Lou Schneider]

My question is whether they are directional. usually a steel colored post and a copper colored post, which side to power, which side to load, or does it make a difference?

It does make a difference, DC arcs are hard to extinguish and the contacts are designed to stop an arc better in one direction than the other.

 

[Sunnyfalls]

Yeah...DC auto resetting circuit breakers...

DC auto resetting circuit breakers are typically polarized meaning that the Source (normally the battery) MUST be connected to a specific terminal and the Load (normally the DC Distribution Panel) MUST be connected to a specific terminal. This is because an electromagnet needing a specific polarization is used to increase the electric arc distance when the auto resetting circuit breaker is initially tripped to extinguish this electric arc. If a DC auto resetting circuit breaker is installed such that its polarization requirement is NOT followed, when it is initially tripped this electric arc may never get extinguished and this may cause the circuit breaker to catch fire as shown here:

High quality auto resetting circuit breakers have Ignition Protection to prevent this from occurring. However, this was NOT the case for our Casita trailer OEM 12VDC 40A polarized auto resetting circuit breaker…but at least our Casita trailer OEM auto resetting circuit was installed correctly.

But the astute reader might then wonder what happens when the Charger is charging the battery which then reverses things by making the Charger the Source and the battery the Load which then violates the polarization requirements of the installed auto resetting circuit breaker. If the amperage rating of the auto resetting circuit breaker and the wire were properly sized to be greater than the maximum amperage that the Charger can physically generate, nothing bad happens as the auto resetting circuit breaker never gets tripped and the wire can handle the maximum current generated by the Charger. Unfortunately, Casita Travel Trailer installed a crappy WFCO Charger/Converter that can potentially generate 55A when charging lithium batteries (except it couldn't reliably accomplish this...so we had to replace it) and they only installed a 40A auto resetting circuit breaker...which we considered a potential safety issue...and which we resolved by using a reliable Progressive Dynamics Charger/Converter that can only physically generate 45A and by using a 50A auto resetting breaker having Ignition Protection. We also put a 60A fuse in series with the auto resetting circuit breaker to ensure that it doesn't keep resetting when faced with a continuous excess amperage event.

The only reason that an auto resetting circuit breaker should be used is when the location of the circuit breaker is difficult to access and you want to avoid this inconvenience for situations where you may have frequent nuisance tripping.

 

[DonTom]

My question is whether they are directional. usually a steel colored post and a copper colored post, which side to power, which side to load, or does it make a difference?

The "copper colored post" is the power in. On the CB itself, on the black insulation, it will say "bat".

The "steel colored post" will say "AUX" for the load. Hard to notice either unless you're looking for such.

And yes, there is a difference, but I see Lou & SunnyFalls have already explained that part.

-Don- Cinder Hills, Coconino Forest, AZ

 

[John From Detroit]

My only questions were regarding the times involved in opening after an overload and the time to reclose.

That varies depending on the make and model..Some are desighed with a larger "hang time" for motors. Some with a shorter for other things.

 

[PancakeBill]

I have one between the starting battery and the aux battery, As above it could really be either direction. I might just swap out with a fuse.