To Ferrule or Not to Ferrule?

We recently posted about a spectacular 3D-printer fire that was thankfully caught and extinguished before spreading to the hacker’s house or injuring his family. Analyzing the remains of the printer, the hacker determined that the fire was caused when a loose grub screw let the extruder’s heater cartridge fall out and touch the ABS fan shroud. It ran full-on and set things on fire.

A number of us have similar 3D printers, so the comments for this article were understandably lively, but one comment stood out by listing a number of best practices for wiring, including the use of ferrules. In particular, many 3D printers connect the heated bed, which draws a lot of current, with screw terminals to the motherboard. While not the cause of the fire in the original post, melted terminal blocks are a common complaint with many DIY 3D printer kits, and one reason is that simply jamming thick stranded wire into a screw terminal and hoping for the best can result in increased resistance, and heat, at the joint. In such situations, the absolutely right thing to do is to crimp on a ferrule. So let’s talk about that.


A Series of Tubes

So what is a ferrule anyway? In general, any kind of band or clamp that’s used to attach, reinforce, or secure objects to each other. That’s a broad definition that covers everything from the aglets applied to shoelace ends to prevent unraveling to the stout metal clamps used to connect wire ropes together. But in the world of electrical wiring, ferrules have a more specific definition, and very different purposes than ferrules used for purely mechanical applications.

An electrical wiring ferrule is a soft metal tube that is crimped onto the end of a stranded wire to improve the wire’s connection characteristics. Most ferrules are made of copper, which is usually plated with tin. The ferrule is sized for a specific gauge of wire, both in terms of its diameter and its length. A ferrule is not just a simple cylinder, though — it has a lip or flare formed into one end that serves to collect and consolidate the individual strands of the wire as they are inserted into the ferruule.

Cross-section of an insulated ferrule installed on stranded wire. Source: Weidmüller Interface GmbH & Co. KG

The flare in most ferrules is not immediately apparent because it’s usually encased in a conical plastic cable entry sleeve. This sleeve acts as a transition between the insulation of the wire and the ferrule itself, and also serves to corral all those loose strands into the lumen of the ferrule. Unlike in more traditional crimp connections, the plastic sleeve of a ferrule is not compressed during installation. It stays intact around the insulation, and provides some measure of strain relief after installation by moving the bend radius of the wire away from the end of the insulation. Most ferrule sleeves are color coded for wire size in the DIN 46228 standard, which confusingly has two different codes, French and German, for the same cross-sectional area in square millimeters.

Making Stranded Wire Solid

If it sounds like ferrules are more a European thing than an American one, that’s with good reason. In order to get CE certification, electrical equipment must terminate stranded wire entering a screw or spring terminal with ferrules. There’s no such regulation in the US, and so it’s not common to see ferrules used in American equipment. But ferrules have specific advantages that are hard to deny, and their adoption appears to be spreading because they make good engineering sense.

To understand the principle, clip a small piece of insulated stranded wire of any gauge. Stranded wire is flexible, which is one of the reasons it’s used rather than solid wire in mobile applications and where vibration can occur. But it’s still somewhat stiff thanks in part to the insulation, which wraps the strands of the conductor, keeps them all in intimate contact, and maintains the twist, or lay, of the separate strands. Now strip off a bit of insulation from one end. You’ll notice that in most cases, the lay of the strands is at least partially disturbed — they untwist a little. Strip off more insulation and the strands get more and more separated. Take off all the insulation and the conductor will lose all structural integrity, falling into individual strands.

This is the essential problem that ferrules solve: they maintain the close association of strands in the conductor after the wire has been stripped and allow the connection to conduct its full rated current. Without ferrules, stripped stranded wires compressed in screw terminals tend to splay apart, reducing the number of individual strands that are in firm contact with the terminal. The resistance of such terminations is much higher than properly ferruled connections.Stranded wires with ferrules perform much better than without them. Source: Weidmüller Interface GmbH & Co. KG

Squeezing Your Problems Away

There’s more to a ferruled connection than reduced resistance, though. Like other crimped connections, the strands inside a properly applied ferrule undergo tremendous pressure, in the process stretching axially and deforming radially. The stretching action tends to disrupt and displace surface oxidation on the strands, while the radial compression tends to remove the air spaces between the strands. These tend to make the crimped connection better at resisting oxidation than uncrimped wire, increasing the longevity of the connection.

So are ferrules the way to go for the home gamer? On the whole, I’d say yes. Ferrules have obvious advantages over plain stranded wire, and in high-current applications, I would insist on using them with screw terminals or anywhere that the strain relief into the shielding is helpful. Plus, they lend a clean, professional look to a project, so even if the application is non-critical, I tend to include them on my stranded wire connections. It’s not without its cost to tool up for ferrules, of course, but at $30 for a kit with an assortment of ferrules and a proper ratcheting crimp tool, it’s not too bad.

Thanks to [NobodyInParticular] for suggesting this story.

[Featued images: KNIPEX, Ferrules Direct]

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