What would a safe robot look like?

[kamocat]

I'm not talking about mechanical safety here - that's well understood.
I'm referring to the safety of the electrical system, and protection of its components. Namely:

• insulation
• motor protectors
• battery protection

I'm aware that the primary purpose of the auto-resetting breakers is for the protection of the wire.

EDIT: I'd also like to discuss connectors, keying, and color-coding, but that's safety in the testing and maintenance sense. I will reserve that for another thread.

[kamocat]

I'll start with insulation.
To be perfectly honest, I have my own thoughts about what is acceptable. However, I'm curious what the consensus is.

Is electrical tape okay as an insulator?

Is heat-shrink okay as an insulator?

To what degree? (Would it be acceptable to take a bare piece of wire, wrap it in electrical tape, and put it on your robot?)

How much copper can be left exposed?

[Fletch1373]

Originally Posted by kamocat

Is electrical tape okay as an insulator?

I would say yes, provided it's done right. You want to make sure the tape overlaps the insulation. I prefer to have it overlap at least as much as there is copper exposed(or even better, just cut off the exposed copper). You also want to make sure the end is covered(either by pinching the tape together, or my prefered way of folding the end over before I make the final wrap with tape)

Originally Posted by kamocat

Is heat-shrink okay as an insulator?

See comments about electrical tape. Heatshrink tubing should primarily be used for insulating and holding together soldered connections. I wouldn't use it simply to cover the exposed end of a wire.

Originally Posted by kamocat

Would it be acceptable to take a bare piece of wire, wrap it in electrical tape, and put it on your robot?

I have done this in the past when removing a component from the robot. It's not ideal, but if you plan to replace that component in the near future and you make sure the wire is properly covered and tied down, then I would say it's fine.

Originally Posted by kamocat

How much copper can be left exposed?

I wouldn't leave any exposed copper. Crimped connections should have the wire insulation inside the covering, otherwise should be taped appropriately. Soldered connections should be covered in heatshrink tube(see above). And loose wires should be appropriately covered or removed completely

[kamocat]

Are there any other opinions?

Originally Posted by Fletch1373

I wouldn't leave any exposed copper. Crimped connections should have the wire insulation inside the covering, otherwise should be taped appropriately. Soldered connections should be covered in heatshrink tube(see above). And loose wires should be appropriately covered or removed completely

What would you do in the case of the terminals on the battery, or PD board, or the main breaker?
Is it okay to leave the nut exposed so it can be serviced?

Or should it be somehow wrapped in electrical tape, or goobered in hot glue?

Or is there a more appropriate way of insulating this that allows it to be serviced?

[Jon Stratis]

Ideally, there would be no exposed wires/connectors on the machine. From a practical standpoint, that's not really possible - the screw terminals on the Jaguars/victors, the bolts on the main circuit breaker, and the bolts on the power distribution board are generally all exposed. While you can wrap them up (as you suggested, electrical tape or hot glue can cover them up), doing so really reduces the utility and intended use of those items - they're designed like that for easy access, and should remain as such. For the battery... we completely cover the terminals with heat shrink. No exceptions - in fact, it's a rule (<R37> C).

For everything else, all wire should be completely covered. The spade connectors we (and probably just about every other team) use come with insulation that not only covers the connection to the wire, but extends past the connectors a bit. This makes it very easy to overlap the insulation on the wire and the insulation on the connector, without compromising the connection. Most other connectors in common use have similar overlapping insulation available.

For covering splices, heat shrink is always the optimal choice. That's what it's made for. While electrical tape certainly works (provided you overlap it with the insulation on the wire), it's suboptimal. Over time, it will come loose. A corner gets loose, which then lets anything rubbing against it pull it up more and more... a problem you don't have with heat shrink. That doesn't mean you can't use electrical tape. It's just something to be aware of and think through the usage of that wire.


On our robot, all electronic components sit on a ESD-safe material. All of our wires are securely mounted with zip ties. Almost all of our wires are twisted pair. As a result, if any one wire comes loose during competition, it simply can't move enough to short to the frame. It may short to it's companion, but that will simply trip the circuit breaker.

Further, there are no direct connections between any of our electrical board components and the motors/sensors on the robot - we using intermediate connectors (Anderson PowerPole) between the board and the robot. The housing on these connectors wraps completely around the contacts, which will prevent most accidental shorts if one of them is left loose accidentally.

Further, our wires and connectors are all color coded. The PowerPole connectors are coded according to current rating, and different color connectors don't mate properly. This ensure at the points of contact that we know both sides are rated the same. Likewise, our wires are color coded - white/black twisted pair for 12 gauge (40A breakers), red/black for 14 gauge (30A breakers). Sure, we can also use 18 gauge (20A breakers)... but we generally don't keep any of that at our build space. It doesn't hurt anything to use 14 gauge with the 20A breakers, and doing so prevents any problems if the wrong breaker is put in (after all, the 20A and 30A breakers look a lot alike!).

[Tristan Lall]

Are we talking about being safe enough to pass inspection, or safe enough for some other purpose? (In other words, you want to hear about best practices rather than sufficient practices, right?)

[kamocat]

In this thread, I'm concerned with neither the rules nor inspection.

So yes, I'm talking about best practices.
(In the rules, it's legal to take two bare pieces of wire, wrap them each in black electrical tape, put a stripe down one with a marker, and use them together. The one with the stripe is positive. I hope you agree that this is not acceptable in terms of safety or color coding.)

[Tristan Lall]

If you can spare the weight (which is non-trivial), you may want to consider using 10 AWG on all of the 30 A and 40 A circuits. Less resistance, so more efficient.

My favourite wire is the finely-stranded stuff with silicone insulation (or this), but it's expensive and hard to find. Next best is the very soft—maybe natural rubber?—opaque insulation, again on very finely-stranded wire. Tinted-clear-PVC-insulated car audio wire is what we end up with most of the time; it's still flexible, but the thicker insulation is occasionally problematic with some shielded crimp connectors. Ordinary appliance wire isn't that bad either, but it's very aggravating to work with in tight spaces (or when attaching and detaching things). Although solid wire is allowed, I would avoid it like the plague for ordinary wiring (it doesn't take bending well, and needs different connectors to work right).

Speaking of connectors, my preference these days is to use crimp connectors, and a big lever-action crimper and the correct size of crimping dies. Properly attached, you can pull (hard) on them, and they won't come off—in fact, that's the test you need to use. Solderable connectors are fine too, but they're a lot more difficult to do right (in that choice of solder, iron temperature, amount of wicking, surface preparation, etc. are all important). I wouldn't recommend both soldering and crimping the same connector; I suspect that the thermal stresses of soldering are not going to interact favourably with the cold-working of the crimping process.

Also, those trapezoid-shaped crimp connectors from Terminal Supply that FIRST gives us? They're a trap. Nobody else seems to sell compatible ones, so you're best buying what's available locally in bulk, so that you always have matching sets of connectors. I do like spade terminals with integral insulation for connections that don't need to be made and broken too often (e.g. less than 5 times per season). (Added advantage: no heatshrink or tape to worry about.) For stuff that's always coming in and out, just buy Anderson PowerPoles or some other similar connector that's designed for repeated cycles. (The Dean's Ultra Plug connectors are decent too; a little tricky to solder, and unshielded, but designed for high current and keyed. But don't use the off-brand ones in this case; they usually use a very weak brass leaf spring on the contact which wears out easily.)

For any wire or connector that doesn't have something similar built in, I like using heatshrink tubing (clear preferred) as both insulation and strain relief.

I always liked ring terminals for speed controller connections because they don't fall out until you lose the screw. Unfortunately, someone put captive screws on the grey Jaguars and didn't tell us—on those models, the ring terminals are a liability because the screw has to come out, and in so doing it releases filings into the Jaguar (which lacks conformal coating). Now I'm willing to consider the fork terminals with upturned tines.

And let me just say, conformal coating is fantastic stuff. For FRC applications, if you've got a bare circuit board, this is just the stuff for it. Too bad that both for rules reasons and for heat dissipation reasons, it's probably a bad idea to add it to the Jaguars.

For the WAGO connectors, just follow the datasheets. There are specific ways to prepare the wire involving stripping back a particular amount of insulation—this varies by connector and wire type. (Crimp-on square-section ferrules are an option for these connectors; that might be something to try too.)

And don't use the FRC terminal strips (or any others, for that matter). They're likely points of failure, and can be replaced with a pair of connectors anyway.

[DonRotolo]

This is difficult because the topic is very broad. Assuming we mean 'best practices' here.

Insulation on the wire needs to be undamaged and applied by the manufacturer, in the proper color for the application. Ordinary PVC is acceptable in most cases, heat is the greatest issue that may require better insulation materials.

The wire insulation must be protected from damage, particularly chafing and rubbing - best practice is to cover both the wire with a formed insulator (not tape, but tubing, mesh sleeve, conduit, etc) and the chafe point with a wear-resistant cushioning material (e.g. felt tape, velcro loop, softer plastics, leather, cardboard, etc.).

Additional protection against damage is provided by bundling wires together neatly (with a clean lay) and fastening the wire or wire bundle (using zip ties, tape, etc) to rigid supports. Bundles large enough to be rigid require less frequent but stronger fastening.

Electrical tape provides both mechanical and electrical insulation. Tape must be at least three layers thick, with edges overlapped at least 1/3, to offer reasonable insulation properties. Good practice is to wrap tape with a 50% overlap (total is 2 thicknesses) and then a second layer in the opposite direction with 1/3 overlap.

Wire, even stranded wire, should be protected from bending to the extent possible. When wires must flex in their function, maintain as large a bend radius as possible. Flexing wires must not be bundled, or if they are then very loosely. Best practice is to end all binding just past the fixed end supports, enclosing the unbundled wires in a flexible covering large enough to allow the wires to move freely within. Highly-flexing wires should use finer stranding. Solid wire cannot be permitted to flex at all.

And that really only scratches the surface.

[kamocat]

What about voltage and temperature ratings?
I know we don't use anything greater than 24v, and nothing gets especially hot under normal use. However, I might have a wire run that occasionally comes in contact with a motor body, which may be hot.
What can I expect from an unmarked piece of wire? Could it be anything? Or are there certain standards all wire must be produced to?

How about electrical tape? What is the voltage and temperature rating on 3 wraps of that?

[DonRotolo]

If we keep it up like this, we'll eventually get there. This is good.

Temperature: Cold usually is not an issue, and any wire we'd encounter is plenty good at 0C (32F). Hot is the one we need to watch. "normal" wire is good from 0C to maybe 60C, maybe 75C, and that's generally fine for FIRST stuff. Hot is often less than 60C here. But when wiring near things that get hot, try to stay away, or be sure you understand how hot it can get and use appropriate wire. Automotive wire is generally -40C to 120C. It isn't the Wire that's rated, it's the insulation! Some wires can exceed 400C happily, but these don't come cheap.

Voltage: We do not usually see anything in excess of 24 volts, and doubling that for a safety factor still leaves us with just a few mils of insulation thickness necessary. The conclusion is that the insulation is more for mechanical protection than for voltage-arc resistance. We do not use magnet (enamel-coated) wire because the coating is not mechanically robust, not because it can't handle the voltage punch-through.

Tape: 3 layers of Scotch 33 tape is good for at least 250 Volts. One layer is easily OK for 48 volts, but you want best practices, not minimums, right? Tape must be UL approved, or it can only be used for identifying wires. For example, masking or duct tape has no recognized insulating power. Duct tape makes a dandy chafe guard though.

There are tapes available in greater thicknesses to accommodate higher voltages.

Tape temperature rating is assumed 0C - 40C unless marked (or check the manufacturer specs). It might be better, but if unknown, assume the worst. Cheap tape is awful stuff, the adhesive bleeds and ages. Scotch "33" is the standard, or perhaps "88" or their colored tapes "35".

Markings: I *Believe* UL-approval requires the wire to be marked with gauge, insulation type & voltage rating, and manufacturer. Good wire is always marked. Cheap wire may not meet UL standards, you get what you pay for. It is often good-enough, and I have seen very high-quality wire without markings, particularly if part of a cable assembly (e.g., inside an extension cord) having markings on an outer insulating layer. Also very small wires are impractical to mark. 24 ga is not very small.

Contrary to something i wrote before, I just remembered that safety ground wires are OK to be uninsulated, since they are intended to carry fault currents. If insulated, it must be green or marked green at each end.

I didn't mention heat shrink before. It is almost a replacement for manufactured insulation, but better used for marking, additional mechanical protection, and bundling. On very small wires it does replace tape. In my book it is just as good as low-end wire insulation, others may differ in that opinion.

[kamocat]

Cool.

I'm still curious about that tape. 3M says Scotch 33 has a dielectic strength of 1,150 Milivolts.
Doesn't that mean one wrap can insulate a 1.15 volt line from ground-potential, and three wraps can insulate a 3.45 volt line? (I am, of course, referring to a DC system)

[Fletch1373]

Originally Posted by kamocat

What would you do in the case of the terminals on the battery, or PD board, or the main breaker? Is it okay to leave the nut exposed so it can be serviced? Or should it be somehow wrapped in electrical tape, or goobered in hot glue? Or is there a more appropriate way of insulating this that allows it to be serviced?

The terminals on the battery are always wrapped in electrical tape(I haven't seen many teams doing it any other way, except to leave them uncovered, which I of course ask them to cover them). The Main Breaker and PD board are a little different. As an inspector, I've asked many teams to cover them at least loosely with electrical tape, even though it's not specifically listed in the rules. I still feel it's a good practice to follow and I've rarely had any issues from a team I've requested it of.

I've never required a team to wrap it tightly, or threatened to fail them in inspection unless they do it. Simply explaining my reasoning is more than adequate for every team I've inspected.

[Mr.G]

You will have to ask Al......

[Greg Young]

Originally Posted by kamocat

Cool.

I'm still curious about that tape. 3M says Scotch 33 has a dielectic strength of 1,150 Milivolts.
Doesn't that mean one wrap can insulate a 1.15 volt line from ground-potential, and three wraps can insulate a 3.45 volt line? (I am, of course, referring to a DC system)

Interesting. Vinyl generally has a dielectric strength around 1000 volts/mil, varying with details of the chemistry of the vinyl. I suspect that spec should be 1,150 volts/mil, so 7 mil Scotch 33 has a dielectric strength of 8050 volts under ideal conditions. Conditions in use are far from ideal so they rate it for 600V.

Scotch needs to use a proofreader instead of a spell checker.