I see this on cars a lot, and sometimes on other hardware - a ground strap, which has two interesting features:
#1: it’s a flat strap, rather than a simple round cable/wire.
#2: it’s braided, rather than being spiral-wound like all/most of the other wire in the device.
Those extra features no doubt incur extra cost. So why are they like that?
a braided strap can flex much more than a stranded cable, and is much less likely to suffer a break due to fatigue.
An electrical engineer once told me that it’s because the skin effect allows braided wire to carry more AC current at high frequencies. At least that’s why it’s used in high frequency applications like in computers. I suspect it’s also used in other situations due to its mechanical flexibility.
This is indeed a consideration. Some high-voltage conductors are actually hollow for this reason. The current tends to flow at the outside of a conductor, so why not get rid of the part that doesn’t help that much? A braided (or stranded) conductor of equivalent cross-sectional area tends to offer less resistance/impedance than a solid conductor in certain applications. There is also slightly better heat dissipation in a braided conductor. Whether it is a significant advantage in any particular application is up to people smarter than I am.
If you are talking about a “ground bond” connection (like from the engine block to the frame, or battery), then the reason is flexibility - a solid or stranded wire would be extremely stiff and would fail under vibration. This is also why they are flat.
When I hear the term “ground strap” I think of the static-discharge straps that trucks drag. Those also need to be flexible, and so might be braided, but don’t need to worry about any high-frequency currents.
in those cases it would be Litz wire, not necessarily just a simple braid.
I’m not an EE, but I think the individual strands need to be insulated from each other in order to mitigate the skin effect. Such conductors do exist, but the ones I’m thinking of (often seen under a car’s hood) typically have no insulation whatsoever.
So why does this type of construction seem to be reserved exclusively for ground straps (and not, say, the hot sides of circuits)?
But why do we sometimes se stranded power cables and braided ground cables in the same place?
My WAG is that ground wires are not insulated, and a stranded wire (twisted bunch of thin wires) would not hold together well if it’s not encased in insulation.
My WAG is so that you’ll know it’s a ground wire that’s not supposed to be connected to something at the other end. Unbraided? Loose wire that needs to be reattached. Braided? Ground strap, leave it alone.
I’m not sure how it works in cars, but in appliances and machinery, the ground wire is an important safety feature. If it breaks or comes loose, the device could become dangerous. OTOH, if the hot wire breaks, the thing just stops working. So extra care (and expense) is taken to use a very flexible wire and bond it at multiple points to anything metal.
they don’t need to be insulated; they’re already at ground potential so they can’t “short” to anything. plus, they’re typically used in applications where one of the connection points moves around e.g. engine to chassis. a car/trucks engine moves around quite a bit on its mounts.
Ground straps are often flat and braided.
All else being equal (i.e. having the same length and cross-sectional area), a flat conductor will have less inductance than a round conductor. Less inductance = less impedance. Low impedance is very important in the ground bonding world.
A braided strap is flexible; a solid strap is rigid. A solid strap requires precisely-placed mounting holes, and the strap will be easily damaged when subjected to mechanical stress & strain. Therefore you often see braided strap used in industrial applications where there’s movement, vibration, and temperature cycles. Solid straps are sometimes used in RF installations.
Flat, braided conductors are perfect for ground bonding. They don’t need to be insulated (though they sometimes are), they’re short, they’re low profile, and they come with lugs which makes installation a breeze.
“Ground wires” on a vehicle are really “return wires.” They carry the load’s return current back to the alternator. Even though they’re electrically connected to ground (vehicle’s chassis), current-carrying return wires need to be insulated to reduce corrosion, to protect the strands from chaffing, and to hold the strands together. They should be round, too, for a variety of reasons: 1) flat wires are not flexible in all directions, 2) insulated flat wires are expensive, 3) insulated flat wires are very difficult to strip, 4) insulated flat wires are difficult to mate to a connector, 5) round, insulated wire is far more common than insulated flat wire.
Braided ground straps are a compromise on grounding straps in many cases where cost and flexibility is important.
While both a solid strand, braid and strap are fairly equal for DC grounds, braided cable is better for RF frequencies.
One reason is the skin effect, where the cross section gets smaller as frequency rises and resistance goes up but there is another reason, Faraday’s Law.
Faraday’s Law is typically taught as impacting neighboring wires, but a changing magnetic field can also induce a reverse current in a wire itself, or inductance.
Wire being round simply has less surface area than strap or braid and will have higher inductance and resistance than braid or strap.
As noted above, ground straps are not constructed from braided insulated wire. So skin effect is not a consideration from the point of view of the braiding. The flat versus round geometry will have a very minor effect. But the vast majority of a vehicle runs on DC, and has for all useful intents very little high frequency energy. (Hash coming off the points and ignition coil being about it.) Certainly not enough to make skin effect or inductance in the conductors matter (or even be measurable.) Braided ground straps have been in evidence when cars were for all intents devoid of any electrical function past ignition, starter, horn, and lights. Whatever reason a braided strap is chosen for, it isn’t to do with alternating currents. Many current cars don’t use a flat braided strap anyway. Mine doesn’t. Whereas if the design had anything to do with high frequency current you might expect more modern cars to be even more sensitive to the question.
The ground strap simply connects the chassis to the battery over a very short distance. It was probably just difficult to make such a short heavy duty connection reliable using typical automotive stranded wire. (Copper work hardens, so you need to be careful.) The live side typically connects to a range of elements, such as the fuse box and starter solenoid, and does so with a much longer length of wire.
The return current from the starter is usually carried on another braided strap connecting the engine block to the chassis. That one really needs to be braided, as it is short and bridges from the chassis to the vibrating engine sitting on rubber isolation pads.
I have seen on trucks the power steering lines turn into ground straps and burn through while starting the truck, spraying flaming power steering fluid all over the place.
Agree. The resistance due to the skin effect is roughly the same for solid and stranded wire, all else being equal.
While a flat wire certainly has less impedance vs. a round wire, I can see your point that it’s probably not a factor in a vehicle application.
I am guessing this is what happens: a vehicle needs a ground strap. The vehicle designers go to a company that makes ground straps. All the ground straps are flat and braided. Why? Because the ground straps must be designed to work in lots of applications. A low impedance ground strap will work in an application where low impedance is needed and where low impedance is not needed. A braided ground strap will work in applications where flexibility is needed and where flexibility is not need. Therefore it makes sense for a company to make all of their ground straps flexible and have low impedance.
ground wire around homes can be either braided, stranded or solid, and its just that braided looks good. It can be pulled through without kinking and looking messy.
You mentioned not only cars but other hardware.
Choosing the physical format of a grounding conductor can involve whether a DC ground or RF ground is the priority. For a DC ground a simple round cable/wire is OK, although a flat strap may be more physically flexible and offer an easier attach point without a separate fitting.
For RF ground (ie radio equipment) skin effect becomes an important consideration so a flat strap is preferred over a round conductor, and a braided strap is even better due to the increased total surface area. This lowers RF impedance somewhat and provides a better ground path at those frequencies.
However the lower RF impedance of a braided strap may not be that much lower than a flat strap, but in some applications the better physical flexibility of the braided strap may be worthwhile. Even in a DC ground situation where skin effect isn’t an issue, the braided strap will be more flexible. If equipment at each end point is under vibration or is periodically moved, a braided strap could reduce stress on the attach points.
It doesn’t matter if it is insulated or if it oxidizes, the main current flow is not required to switch strands to reduce inductance or to take advantage of the skin effect.
Remember that Electrical fields and Magnetic fields arise to the exact same fundamental physical property and only the relative movement makes them appear different.
The fact that the braid is not insulated or may be oxidized may not be ideal but the impedance will be lower because the magnetic field is wider, and the inverse square law.
Note that while the cars systems may be DC, you have Alternators that are AC, Starters that are high current load, and spark plugs that are effectively spark-gap transmitters.
It may be useful to remember the ever present shifting noise in old AM radios from even pure mechanical ignition systems.
The flexibility is also an option but braided grounds become far more critical in RF applications but their value is also demonstrable in systems like an automobile.
Stranded wire is far less optimal in both high current and RF conditions, and it would require more material which results in higher cost.
