Keeping the main breaker from tripping

[Ether]

Originally Posted by brycen66

This is the system we have implemented into our practice robot and the results so far look good.

Have you compared the readings to a precision ammeter to determine the accuracy over the current range?

[brycen66]

Originally Posted by Ether

Have you compared the readings to a precision ammeter to determine the accuracy over the current range?

No, we compared it to the voltage drop data

[Al Skierkiewicz]

Jared,
In some ways, replacing a tripped breaker is a good practice. When that breaker trips under high loads, the load is actually fairly inductive. As such when the contacts open under load, there is some pitting of the contact that takes place. This pitting can raise the series resistance, ultimately raising the heat generated for a fixed current.
These breakers are unlike the protection breakers used in your home. Those generally will have a design that both protects long term high currents and sudden overloads. The trip curve for these breakers has a wide range because they are simple (cheap) and provide the coverage needed for our designs.
No one has mentioned this yet so let me add this to the mix. (Yes, I am the one who said the king had no clothes.) If I observe a team tripping the main breaker for any reason other than a catastrophic electrical failure, I would not consider them a good alliance partner. If we need you to play an entire match to win, why would we take the chance at a good robot who trips the main breaker in the hopes it won't happen in a critical match. Personally, I would rather have an OK robot who doesn't break over a great robot that sometimes sits idle in the middle of the playing field. When I see a team spraying their breaker, I have to wonder...are they covering up a design flaw or are they just being cautious?

For those who think spray is illegal can you tell me why you think that?
For the peltier, I would consider the rule about being accessible and visible would not be accomplished by putting the breaker in an insulated box.

For general knowledge... when you see one or more of us run out on the field when a robot stops during a match, the first thing we are looking at is the main breaker status, followed by indicators on the DSC, cRio, PD and radio. This sweep will give us an indication of what might have occurred. It is the standard list of checks that was formed following the Einstein weekend a few years ago. We want to be sure every team is given every chance to compete.

[FrankJ]

"Canned air" is generally not actually air. Some versions are flammable. If you are using one of those versions, I would avoid spraying it on energized components.

[adciv]

Originally Posted by Al Skierkiewicz

For those who think spray is illegal can you tell me why you think that?

I'm basing this off my experience with other types of circuit breakers and what we would permit with them. This is regardless of if it works or not.

Yes, ambient temperatures affect thermal circuit breakers. Using any sort of device to intentionally cool the breaker violates the intent of the safety mechanism. If a team is popping breakers after X time, they are then exceeding the ratings. The only reason for attempting to cool it down is to "cold soak" the breaker to attempt to uprate the breaker and increase the current required to trip it. I see little difference between intent in using compressed air and packing dry ice around a breaker during a match.

Now, if a team wishes to cherry pick which breaker they want to use, that is a different matter. But intentionally manipulating the environment around the breaker to prevent the breaker from tripping is a direct attempt to bypass the safety mechanism the breaker provides.

[Cory]

Originally Posted by adciv

I'm basing this off my experience with other types of circuit breakers and what we would permit with them. This is regardless of if it works or not.

Yes, ambient temperatures affect thermal circuit breakers. Using any sort of device to intentionally cool the breaker violates the intent of the safety mechanism. If a team is popping breakers after X time, they are then exceeding the ratings. The only reason for attempting to cool it down is to "cold soak" the breaker to attempt to uprate the breaker and increase the current required to trip it. I see little difference between intent in using compressed air and packing dry ice around a breaker during a match.

Now, if a team wishes to cherry pick which breaker they want to use, that is a different matter. But intentionally manipulating the environment around the breaker to prevent the breaker from tripping is a direct attempt to bypass the safety mechanism the breaker provides.

Your fallacy is in assuming the intent of the breaker is to limit temperature of the bar inside and not to limit current through the breaker.

If the breaker under normal operating conditions (ie: starting at roomp temp) can reach current A before tripping, due to temperature A, then if a match is played shortly thereafter, and the temperature of the bar has not returned to room temp, it will actually trip at some current less than A.

How is restoring the breaker to the properties it has at room temperature a safety risk? If you accept that the breaker is meant to operate at room temperature in a safe manner, there is no justification for claiming that cooling it to room temperature rapidly is any different than operating it as if it were always at room temperature.

[Jared]

As I read this thread, I'm more and more on the fence about this issue. I'm starting to thinks it's fine, mainly because of the point that Cory brought up: the breaker spray doesn't really keep the breaker any colder than ambient temperature, it just resets the temperature to what is was before.

My original opinion that the breaker spray was against the rules was from somebody who made the argument that it was similar to tampering with a thermostat by tricking it into thinking the room is colder than it is. I now realize that this isn't really an effective analogy.

[Al Skierkiewicz]

I would agree that a permanent solution to cooling (keeping the breaker below ambient) the internal temperature of the breaker would be a violation of safety protection for which the breaker is ultimately designed.
The breakers on the robot are intended to prevent fires due to high current in any circuit on the robot. The wiring from the battery to the main breaker is the only circuit not protected. All breakers are meant to protect the wiring for this reason, not the load. The current rating vs. wire size is a derivative of the NEC specifications for open frame wiring. We slightly derate the tables based on the short time in use. These are not continuous (24 hour) currents. Even with the protection, we sometimes do see fires or at least significant smoke events all the time.
While we are on the subject, if you remove a motor(s) but leave the wiring, you must remove the breaker feeding that branch. (Consider this a 'best practice" method.)

[Gdeaver]

Tim Allen would be proud of First this year. "more power" This year a large portion of First teams will be pushing the the robot power system to the limit. A large number of robots are going to push power draw beyond rating and eat up safety factor of many components of the power system. There are going to be problems. Saw many examples of this last year. Smart teams will develop power management strategies and be able to push the power system with out failures and reliability problems. Those who don't will learn the hard way of what happens when you push systems beyond their safe operating zone. Scouting needs to track teams that have "power problems". Bad alliance choice. Get smart or suffer. The new control system has the resource to help teams with power management. This is a comprehensive systems problem and is complex. Teams will need to put much effort into this. Positive is that this is an excellent opportunity for students to learn the process of data collection, analysis and the development of strategies based on this. The bad part what we are teaching future engineers. "More Power" is not an attitude future engineers should embrace. First could address this problem by limiting motor choice for drive trains. That would get the forum hopping. Or more like the real world, First could attach cost to the robot power. Like each cim above 4 costs a team xx points every match. With the new PDB they could asses a xx point penalty for every total robot power draw above xxxx watts. We should teach the cost of power.

[FrankJ]

You are already penalized for using too much power. You trip the main breaker. As Al says, that should move you down the pick list for alliance selection. Kinda like mecanum wheels. Maybe that should be a scouting question.

[adciv]

Originally Posted by Cory

Your fallacy is in assuming the intent of the breaker is to limit temperature of the bar inside and not to limit current through the breaker.

If the breaker under normal operating conditions (ie: starting at roomp temp) can reach current A before tripping, due to temperature A, then if a match is played shortly thereafter, and the temperature of the bar has not returned to room temp, it will actually trip at some current less than A.

How is restoring the breaker to the properties it has at room temperature a safety risk? If you accept that the breaker is meant to operate at room temperature in a safe manner, there is no justification for claiming that cooling it to room temperature rapidly is any different than operating it as if it were always at room temperature.

No, I assume the intent of the breaker is to limit the temperature of the wiring. Your fallacy is in assuming cooling down the breaker resets the system. A breaker, including this one, is not intended to only protect against high current short duration short circuits but also longer duration overloads. This is why the breaker will trip after an overload which occurs for 30 seconds. Depending on the overload of some breakers, it can require one hour to trip. It depends on the amount of overload.

By cooling the breaker, you are trying to bypass the safety mechanism it represents which includes long duration overloads such as you are apparently inducing in your design. How long does it for the breaker to trip when you practice?

[Tom Line]

Originally Posted by Al Skierkiewicz

I would agree that a permanent solution to cooling (keeping the breaker below ambient) the internal temperature of the breaker would be a violation of safety protection for which the breaker is ultimately designed.
The breakers on the robot are intended to prevent fires due to high current in any circuit on the robot. The wiring from the battery to the main breaker is the only circuit not protected. All breakers are meant to protect the wiring for this reason, not the load. The current rating vs. wire size is a derivative of the NEC specifications for open frame wiring. We slightly derate the tables based on the short time in use. These are not continuous (24 hour) currents. Even with the protection, we sometimes do see fires or at least significant smoke events all the time.
While we are on the subject, if you remove a motor(s) but leave the wiring, you must remove the breaker feeding that branch. (Consider this a 'best practice" method.)

That's right up there with always making sure you have female pwms on both ends of the wires connected to the digital sidecar, roborio, etc. It prevents someone from unplugging a wire from a motor or sensor and accidentally shorting the power pin to ground. I've always assumed that's the same reason the prongs always go into your wall at home, but never verified it.

[maths222]

Originally Posted by Tom Line

I've always assumed that's the same reason the prongs always go into your wall at home, but never verified it.

This one is really more about safety than short circuts. Touching live prong would be quite dangerous and undesireable, so the outlet design offers a level of safety. European plugs are actually better, because your finger can never touch energized metal, but the US design is better than having prongs stick out of the wall.

[Mr V]

Originally Posted by adciv

No, I assume the intent of the breaker is to limit the temperature of the wiring. Your fallacy is in assuming cooling down the breaker resets the system. A breaker, including this one, is not intended to only protect against high current short duration short circuits but also longer duration overloads. This is why the breaker will trip after an overload which occurs for 30 seconds. Depending on the overload of some breakers, it can require one hour to trip. It depends on the amount of overload.

By cooling the breaker, you are trying to bypass the safety mechanism it represents which includes long duration overloads such as you are apparently inducing in your design. How long does it for the breaker to trip when you practice?

A circuit breaker is a circuit protection device so its purpose it to protect the circuit, ie the wiring. So yes it is to prevent the wiring from getting hot enough to cause the insulation to start to flow, melt, or catch on fire.

So I feel that cooling the breaker is a bad idea because you are "resetting it" to ambient temps but not doing the same for the wiring which it is supposed to protect.

[GeeTwo]

Originally Posted by brycen66

Place a steel washer or ring(any ferrous metal works) around one of your main wires. Then cut a slot in a steel washer and place a linear hall sensor in the slot. We epoxied ours in to keep it intact. The hall sensor outputs voltage which is directly related to the total robot current, and it can be plugged directly into the analog breakout. This is the system we have implemented into our practice robot and the results so far look good.
http://www.jameco.com/webapp/wcs/sto...001_1915940_-1

For anyone trying to implement this --the voltage produced by the ring (which merely needs to be conductive, not ferrous) is directly related to the RATE OF CHANGE of the current. You will need to keep a running total of the voltage to get a good value for current draw. I don't see how you're going to keep from having significant drift in a practice session unless you pause and recalibrate regularly, though perhaps over the duration of an FRC match it wouldn't be too bad. This trick works well for A/C meters because the current is constantly changing (and adds up to zero), so that the average voltage induced over a 1/60 sec cycle is proportional to the RMS current flow. For DC, no such luck - if you draw a steady curent, whether 1A or 110A, the voltage will be zero after it stabilizes.