Power Distribution - Low current breakers or fuses?

[diviney]

Hello:

The 2009 Power Distribution module utilizes the regular "Maxi" breakers in the 40 A slots. This is fine for high-current users like drive motors, etc. We would wire these with the usual AWG 10 and all is well.

I am baffled by the 30 amp slots however. It was disappointing to find that these slots do NOT accept the standard ATC / ATO fuses. Instead, the documentation refers to VB3 breakers. These slots should be useful for lower current distribution such as the 12V supply to the Digital Sidecars, Analog Breakouts, etc. For these functions, it is desirable to use smaller wire gauges (let's say maybe AWG 20 or similar). This would demand fusing these circuits for ~ 5A in order to safely protect the wiring. We were not given any breakers less than 20A with the controller kit. What's worse, I can't seem to locate any useful references to "VB3" breakers or fuses to purchase them.

Does anyone know if / where we can obtain breakers / fuses compatible with the 30A slots in the 2009 PD module (in particular, low current values)? I can't see running 14 gauge wire around to all the low-power circuits just because we can't get a breaker less than 20A for the PD module...

-Tom

[R.C.]

Originally Posted by diviney

Hello:
Does anyone know if / where we can obtain breakers / fuses compatible with the 30A slots in the 2009 PD module (in particular, low current values)? I can't see running 14 gauge wire around to all the low-power circuits just because we can't get a breaker less than 20A for the PD module...

-Tom

Not a hundred percent sure, but AndyMark sells 20,30 and 40A for the controller. Here is a link to their site: www.andymark.biz

[Vikesrock]

The rules that are released on Saturday will specify the fuses and wire gauges needed for various connections.

Fuses will likely not be legal for use in these slots (this is complete speculation).

[dmcguire3006]

Fuses are not allowed, from 2008 Manual "The Robot Rev G"

"Items specifically PROHIBITED from use on the ROBOT include:
...
Circuit breakers different from those provided in the Kit Of Parts."

I know because I bought some Fuses at the local auto parts store and learned I couldn't use them!

[usbcd36]

This is not necessarily true for the 2009 competition, but it probably will be.

Still don't understand what they have against small fuses for low-powered devices. If it's all about safety, it's worse to use a large breaker for small wire.

What has been suggested in the past on CD is that a smaller fuse panel be used as a subpanel off of a breaker in the main panel. It's not sanctioned by FIRST, though, which is what I fault them for. I wish they would tell teams exactly what they want to see in this situation.

[DonRotolo]

There is (well, was - we'll find out Saturday about 2009) no restriction on using low value fuses after the 20A breaker. See <R82>, <R83> and <R84> from 2008.

[diviney]

What's so odd here is that the released documentation (specifically Chapter 3 Component Data Sheets, page 13) specifically states "A 5A breaker is acceptible here", and it uses AWG 22 wire to feed the auxillary functions.

To me, a 5A protection device is more than "acceptible", it should be required with AWG 22 wiring.

I have yet to find any fuses (allowed or not) or breakers in the required format for this VB3 panel that are less than 20A.

This inconsistancy must be addressed somehow. I am anxiously awaiting the Kickoff tomorrow to see if the encrypted doc's shed any more light on this.

[Russ Beavis]

http://www.snapaction.net/pdf/vb3.pdf

Assuming that the 2009 game and robot rules allow them, wouldn't the above breakers work just fine in the PD?

They've got ratings from 3 to 30A. These are the parts that the PD was designed to utilize in the "30A" position (refer to the following datasheet, chapter 3.2.2).

http://www.usfirst.org/uploadedFiles...s-Rev-0-5a.pdf

Russ

[diviney]

I Just looked at <R82>, <R83>, <R84>, from last year, and I don't see how an interpretation can be made either way regarding down-stream fusing.

What seems clearer is <R45> which specifically prohibits "Fuse panels different from those provided in the Kit Of Parts".

If you search the 2008 rules for "fuse", it turns out that fuses were specifically allowed. This statement appears in <R45>: "In addition to the required branch power circuit breakers, smaller value fuses or breakers
may be incorporated into custom circuits for additional protection".

I agree with "usbcd36" that it should be a requirement to protect the wire. If you run 20 gauge wire, it makes no sense to try and "protect" it with a 20A breaker. The wire will act as a fuse which is exactly what you don't want.

-Tom

[Joe Ross]

Originally Posted by Russ Beavis

http://www.snapaction.net/pdf/vb3.pdf

Assuming that the 2009 game and robot rules allow them, wouldn't the above breakers work just fine in the PD?

They've got ratings from 3 to 30A. These are the parts that the PD was designed to utilize in the "30A" position (refer to the following datasheet, chapter 3.2.2).

http://www.usfirst.org/uploadedFiles/3-FRC%20Control%20System-Component-Datasheets-Rev-0-5a.pdf

Russ

In a little bit of googling, I haven't found a distributor for Snap Action circuit breakers, other then AndyMark, who just started carrying the same sizes that came in the kit. I haven't called Snap Action, though.

As for the original question, the Snap Action breakers have much better trip characteristics then most fuses/circuit breakers for the application we are using them for. As for why the rules were written that way in the past, I don't know. Like Russ said, with a new control system, I think some of the electrical rules will be re-thought out.

[diviney]

Russ:

Yes - Breaker or Fuse, I don't care. My main point was that I have been unable to find any of these VB3 format protection devices below 20A.

Now that you have shown me the data sheet with values down to 3A, I know they exist [I suppose]... Where can we buy them though?

One final thought on breaker vs. fuse... I agree that breakers may be better for our needs in general because of the heavy inductive loads, stall currents, etc. But for the mundane low power control circuits with fixed well-behaved current draws, why spend $6.00 on a breaker when a $0.10 fuse would be perfectly fine.


-Tom

[dtengineering]

Originally Posted by usbcd36

Still don't understand what they have against small fuses for low-powered devices. If it's all about safety, it's worse to use a large breaker for small wire.

While I agree that it makes sense, from a stricktly engineering perspective, to use a small fuse rather than a large breaker, and I doubt FIRST has anything "against small fuses", there are a couple of reasons why they might choose to limit what can be used:

1) Inspection: Standarizing the power distribution means that there are fewer chances that an inspector might miss an inappropriate value or type of breaker/fuse. While many inspectors are capable of determining whether a particular breaker or fuse would be acceptable based on the robot design, may inspectors are volunteers inspecting a robot for the first time. Limiting the options speeds inspection and prevents disagreements over what size or type of circuit protection is appropriate.

2) Experience: FIRST has been doing this robot design stuff for a while. They may have found that teams that used low value fuses would have them pop too often and that would negatively affect their experience of the game. FIRST may have also observed that using high value breakers does not present a significant safety risk compared to using low value fuses. In this case, why not specifiy the higher current, self-resetting breaker to ensure teams have fewer equipment failures and get a better experience out of the game.

No, I don't know the WHY behind the rules... this is just my speculation... but I do believe that most of the rules have some kind of reasoning behind them and while we may not always agree with the rule or see the reasoning behind it, there is usually (although not always!) something there. After all, the rules were made up by a fairly rational group of people.


Jason

[Russ Beavis]

I've sent an email to Snap-Action regarding availability and distribution. Hopefully I'll hear back soon.

The clips in the PD for the "30A" positions are Keystone 3557-2. In other words, ATO and ATC fuses should fit just fine. Unfortunately, I think that the case may interfere with such short-legged fuses.

Depending on the rules, you might be allowed to "relieve" the openings and install any "small" bladed fuse/breaker that you wish.

As Chief Inspector, I'll have to abide by FIRST rules. But as an EE, I'd certainly appreciate being allowed to tweak the PD's case and use "appropriately-sized" fuses and breakers. I understand the pros/cons of fuses vs. breakers and don't mind teaching students about these devices and "when to use which". As long as the device is securely held within the PD and you use properly-sized wiring downstream, I don't care whether it's a slow-blow fuse, fast-blow fues, resettable breaker or auto-reset breaker. Breakers (especially auto-reset) should always, of course, be used where there is a decent chance of often tripping (ie motor drives).

I agree - why use a $6 breaker with limited availability when a $0.10 fuse that can be purchased at any auto parts store would be just fine.

Russ

[tdlrali]

According to http://www.snapaction.net/pdf/vb3.pdf, the breakers in the same style as the 20A and 30A breakers we get in the KoP are available rated for:
3A, 4A, 5A, 6A, 7.5A, 10A, 12.5A, 15A, 20A, 25A and 30A

[diviney]

At first blush, an ATC / ATO fuse should "fit", but it doesn't. I have tried it. The problem is indeed that the "standard" VB3 breaker has legs which are 0.57" long. The PD was clearly designed around this because the ATC fuses (with 0.25" legs) do not even begin to engage. One would need to create a major relief to accomodate the entire body of the fuse in order to be able to use it.

I have also sent a query to Snap Action. They unfortunately seem to be a single supplier of a one-of-a-kind product. I can't fathom why something that's not an industry standard (and hence very limited availability) was chosen for a function like this.

Please post anything you hear from Snap Action...

Thanks for all the thoughts.

-Tom