Well, here’s an interesting idea: the service loop. Ever heard of it? We haven’t!
In the video, the presenter explains the service loop serves two purposes: on the one hand it may provide strain relief, but chiefly these loops are installed so there will be extra available slack in the cable if you need to rewire it some day to change the configuration of your pinout.
One major problem with the service loop may be that the single turn is enough to create an inductor which will then induce noise and cross-talk all over the place. Our rule of thumb is always to completely unroll wires and cables before using them. Do you have a theory about the benefits or problems with service loops? If you do, we’d love to hear what you think in the comments!
If you’re interested in strain relief, we’ve covered that before, and you don’t need a service loop to do it! Check out Cheap Strain Relief By Casting Hot Glue In A 3D Print and Arduino Uno Strain Relief.
Thanks to [Oliver] for writing in to let us know about this intriguing and somewhat controversial idea.
This is mainly a thing (well the only time I’ve ever seen them) for D38999 (or their derivatives, anything that’s similar to mil-spec circular) connectors, where the connector and contacts can be Absurdly Expensive so rewiring them is helpful.
I was never taught to create small coils as a “service loop” in circular connectors. The extra exposed wire in a cable was there for servicing, but there is a much simpler, and quicker method to bring the sheathed area of the cable closer to the pins, and make sure it is well within the clamp area of the connector hood. Since the hood is screwed onto the connector, a counter-clockwise twist of the cable, while compressing the wires towards the back of the connector allows for movement of the wires if the cable manages to break free of the clamp. It also adds a bit of tightening torque to the hood to the connector while clamped. Of course, the wire size is important here, as shown in the video, the wires are relatively large. If the individual wire size is very small, the torque would not be as much, and the strength also much weaker. Besides that, cutting back the sheath a bit to replace pins is always an option, requiring a “service loop” to be incorporated into the cable itself.
The majority of my experience with service loops wasn’t within connectors. It was within equipment racks, where the equipment was on slides, and service loops were required to allow the equipment to be pulled out for in rack service (i.e. video cassette tape recorders need the tape path and heads to be cleaned).
Yeah this had been my experience too minimize the length of exposed wire. Most half decent connector types now have a metal strain relief you attach to the outer sheath for strain relief purposes that sits inside the cable connector that you attach AFTER you have soldered or crimped your pins. If a cable needed to change configuration half way into its service life i would simply deconstruct the whole thing, cut the connector off and desolder the wires without risk of burning the insulation which can easily happen if you are changing some inner conductors.
The service loops i was always more familiar with was stuff like ethernet cable runs that might have to be regerminated in the future or coaxial antenna cables on high masts that it would be very costly to rerun and a loss of signal though a extension cable was unacceptable.
Yes. This^^^ exactly.
Could you solve the inductor problem by having it go around itself and loop the other direction an equal amount?
Good question. I’m not sure! But even if it was possible it seems like it would be difficult to do right.
what inductor problem? the inductance of such a small loop is minute and anything fast enough that it could possible affect it wouldn’t be run in wires like that
Inductance would be negligible for most uses except at very high frequencies. I guess this technique is for DC and low enough frequencies, for example carrying signals for machine control and analog audio and video up to a few MHz shouldn’t be problematic.
This is probably only a problem with thicker gauges of multi stranded kabel, but I am worried that the connector is crimped before the wire is looped. That could lead to strands breaking on the outer side of the loop.
If you want it extra safe it should be crimped after the loop is made.
I’ve never seen this done on connectors, but it’s fairly ubiquitous on cabling in industrial automation. Its main drawbacks are:
– Sending (and receiving) magnetic fields in new and exciting directions
– Bundling limits for high-current cables
– Storage: do you put this in the already-stuffed cabinet or at every endpoint? Both?
– Cost: not a big deal in low-qty installations, but can add up at a scale
– Appearance: I had a PM that had a strict rule against service loops simply because they make the install look less tidy
Why would you put a service loop INSIDE the connector? Just loop some of the wire outside the connector. If and when the time comes that it needs to be rewired, chop off the connector and redo it. The larger diameter of the external service loop probably creates far less inductive issues then small loops of wire inside the plug.
“Chop off the connector and redo it”
Sure, unless the cable is already the exact length it needs to be as part of a complex wiring loom.
Also if the application is sensitive enough that the inductance of a single turn of wire will cause problems, you have bigger problems with parasitics than just this.
So;
That connector you are about to “chop off” just has one bad wire, and the attachment method is with a pin extractor replacement pin and a crimp tool. Even if the connector has only four wires, in most cases you are saving time money and maintaining reliability by repairing the one or two connections.
These days….
The temptation is to trash the cable assembly and buy another one from China.
yeah I’m not feeling it. The service loop could be part of the shielded cable. But I mostly work with RJ connectors, you can’t tweak a single pin.
How about a frequency chart to indicate at what frequency this starts being a bad idea.
Do a loop clockwise then back on itself widdershins. Just like that the spell was broken.
As Tesla would say: Go wireless maaan
Ironically when creating such loops in stuff feeding wireless it is especially an issue I would think.
I work for an automotive wiring company (but don’t do wiring myself) and we commonly put service loops in, usually it is at the client’s request.
There are other kinds of service loops; like the extra wire in the harness going to a large switch or a circuit board. The service loop here is designed to get the circuit board or large switch out of the way for other work on the unit being built or repaired.
Another use case is for avoiding moisture ingress. At the cable company, we used these as a “drip loop” to mitigate moisture accumulated on an aerial service drop from reaching the connectors, which are more vulnerable to ingress – extending the useful life of the installation.
For twisted pairs, inductance in a service loop is cancelled by the current in each wire flowing in the opposite direction of the other wire in the pair.
For wired telephone or wired internet, the service loop provides the extra needed for repairs when the end of the cable gets broken.
I work in motorsport. There seems to be a lot of misinformation here. I get that there are different types of service loops, but this has nothing to do with water ingress. You don’t see these unless you remove the shrink boot. Here’s a really good article on motorsport wiring: https://www.rbracing-rsr.com/wiring_ecu.html