My team is 4213
At worlds, we were told during inspection that our robot had current running through its frame. It was within the allowed limit but Is still a problem. Has anyone else had this problem? How can it be fixed?
Check if any of your wires are not correctly attached and touching the frame.
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One thing we experienced which caused us troubles was having 12 AWG motor leads with thin silicone insulation touching our aluminum frame. There was no metal contact but the resistance was low enough that we had to wrap the part in contact with electrical tape. I still don’t know how that is possible but it was very clearly our problem so you might want to check that. Either way, just go around probing with your multimeter until you can isolate where your electrical contacts your frame.
My team had just such a problem that we resolved this year. There is a writeup in our 2022 build thread that goes into depth on how we tracked it down and what the culprit (in our case) was.
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Be systematic:
- Remove the battery from the robot
- Using a DVM or multimeter, measure resistance from the +/red lead to the robot frame.
- Now remove the breakers, one at a time, until the measured resistance goes up.
This will tell you which branch or branches of your wiring harness is giving you trouble. Inspect these wires for any broken insulation or sloppy crimps.
I just had to troubleshoot a shorted wire in my car this morning using the same technique.
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Continue with the fuses as well if you don’t find the issue via the breakers. Items connected to the roboRIO or other electrical components protected by fuses can also short to the frame. The craziest one I’ve experienced is a Sharp IR distance sensor that has a conductive plastic housing.
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Do all of what everyone else said. I would also check the wires first, as it can be a quick spot and would save a lot of time. A lot of the times an incorrect crimp, especially around where the power to the Rio, radio, vrm if you have one, pcm, etc can easily touch the frame depending on where its placed. If the wire is exposed then that would be the issue. Near motors as well. If you had wires chewed up or beat up by something but kept those it may be time to fix them. Just make sure to stay safe!
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Can you give me a number on the resistance from power leads to frame?
Silicon covered wire is very easily penetrated by sharp corners as the insulation is so soft. It is possible to cause a short to frame and then to move the wire and not see a penetration. It takes a trained eye.
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At Wisconsin this year my first inspection (ever, of course) had a frame short, and if I understood correctly (because someone who could figure it out much faster relieved me), it was a defective compressor that was shorted to its own frame. So just make sure you don’t give up if it seems like you went by process of elimination and had no luck when you’re down to a few components left!
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I was there for that compressor, found by the LRI. That is the first time I have ever seen that type of failure. We chose not to open the defective device but I suspect it was damaged in assembly at the manufacturer.
We had a situation where a falcon ground was shorted to chassis (via the motor controller part of the falcon). Removing breakers only separates the chassis from the “high” side of voltage, so if you remove all the breakers it could be a case where a ground wire is connected to chassis somehow (potentially through a faulty cots component).
Assuming the problem was found by using a dmm between the negative terminal of the pdp/pdh and chassis, I’d recommend disconnecting all motors (not the breakers) if it’s not too inconvenient first and verifying the problem goes away and then plugging them in one at a time to find the problem
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As Al said, knowing the resistance to frame is important in these cases. We’ll often see numbers in the M-ohm range, those aren’t a problem - but it’s easy not to realize it’s that high with the auto-ranging multimeters every team has. I know I have several inspectors every year pull me over to look at a team with a frame short, and it turns out they just missed seeing the range. For those wondering, you can see these sorts of extremely high numbers through a number of different areas - for example, used CIM motors accumulate some dust inside that can cause a high resistance short to the case, and from there to the frame.
more direct shorts can happen for a number of reasons. Some of the ones i’ve seen in the past few years:
- A USB cable that got worn through on a sharp edge
- A sensor wire trapped between the edge of a piece of polycarb and the frame, eventually getting worn through
- An encoder that shorted to the frame through the mounting screw
- A compressor that had an internal short
- A Falcon that had an internal short (I believe it was a large, internal solder joint that touched the frame)
It can be tough to get everything electrically isolated these days, but that’s really the only way to go about finding it - unplug stuff until it goes away, and then start plugging stuff back in until it returns!
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Some FYI, silicone insulation is nice and rubbery, rated for high temperature, and high voltage, but it’s kinda poor against cutting and tearing. Pvc is actually quite a bit more robust in contact with sharp edges!
That said, sharp edges are wires just don’t mix 
On the mega Ohm front, touching both probes when you are hunting will lead to 100k to 10 MOhm readings due to the conductivity of human skin…
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Just to be a bit pedantic, having one frame short (or low resistance) isn’t the same thing as having current flow through the frame. It would bring the electrical potential of the frame to the same as that of whatever is shorted, but you need two such connections to have steady-state current flow.
As others have said, it’s very helpful to know the measured resistance to both of the main power rails. The things to check / how to check could be narrowed by knowing the resistances, but the advice in the thread is sound.
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Over the years, I have read about a variety of faulty components causing a short to the chassis. Unfortunately, sometimes pulling the breakers and fuses is not sufficient to isolate the fault and one has to do what @ahartnet described. I’ve had to do the same thing a few times at work, over the years.
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To add onto this list, I’ve inspected teams that replaced a NEO550 and solved their frame short. According to some other inspectors I worked with, this isn’t the first time they had seen it and allegedly it’s a sign that the motor is dying.
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We had a neo 550 that showed signs of a frame short. Turns out metal shavings got inside the motor causing internal connections to short to the casing. Blew air through the motor and got the shaving out and it was fine. Also using too long of a screw on a neo 550 can hit wires internal to the motor and cause a frame short.
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NEO 550 learning for us this year. The mounting bolts that come with the motor are longer than needed if you are mounting onto 1/16” aluminum. We could not find our short until we figured this out. Went and bought shorter bolts, and the entire problem went away. It was an issue for us for three separate motors. We believe (and did discuss with Rev) that the bolts were lightly touching the internals, probably the windings.
I’ve helped track down a number of these throughout the years. We should probably do a write-up within the community of tips and tricks.
If it is Positive Side only, then you can start tracing from battery to various elements and do the breaker or fuse pulls to help narrow it down.
If it is the negative side… good luck. because so much of an FRC robot shares a “common” negative side, it is much trickier to diagnose. This will often involve pulling negative connections one by one.
Types of things I have found:
LED lights, especially the strip lights were offenders for several years though less so the last couple.
Cameras with grounded chassis (old time offender way less common now)
Sensors: Encoders, potentiometers…
pinched PWM cables (these are frequently what you see gets smoked when something actually gets shorted).
pinched CAN.
Case shorted motors. This is often due to manufacturing defects that sometimes occur more frequently in batches. There were BAne bot 775 that were frequent ofenders years ago. Then 775 pros. Recently I have seen a couple Neos or a couple Falcons. No motors are fully immune as Jon talked about dust accumulation and other factors, but often in a given season, there may be a batch of motors that are especially problematic.
If you see a resistance start low and then get higher, that is often the signs of some sort of logic device or circuit. IE, its starts at 1K (doesn’t pass), but as you keep your meter it climbs to say 10K or more… This is a good time to check anything that has a built in circuit.
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Last time this happened for us, it was a wire that was pinched between the chassis and a cross-aluminum angle brace. It is a pain to discover this during inspection, just because it could take a while to find. Someone with good eyes found our short. I’ve seen other shorts before with devices/sensors being the problem.
Makes me recall a tricky to find short during an oceanography research cruise I was on, when the other group was testing a benthic lander, instruments to go to the sea floor. To sink it, steel weights were attached, and a timer circuit burns a wire to drop these letting it buoy to the surface. Well, it kept burning this wire immediately after they deployed it, instead of at the right time, but things seemed okay at the surface. They eventually worked out that they had a short originating at the watertight electronics cannister, and that seawater was conducting this through the trigger wire.
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