Grounding - Thread-forming vs Thread-cutting screws

System
System Posts: 2,511 admin
in PV Installers Forum - NEC, Wiring, Installation #1
What is the difference between thread-cutting screws and thread-forming screws? And can both be used to for PV module grounding/bonding?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,658 admin
    #2
    Re: Grounding - Thread-forming vs Thread-cutting screws

    Usually, the NEC is very specific... If the parts/system is listed with A, B, and C--but there is no mention of D--then D is not legal.

    Thread cutting vs forming... From a manufacturing/engineering point of view, I much prefer thread forming screws (and taps, etc.)...

    Thread cutting is just what is sounds like, the sharp edge cuts shavings that then have to be cleaned up (or the piece can pierce insulation and get in the mechanicals) whereas thread forming leaves no chips behind. A thread forming tap or screw instead of having one or more sharp edges to cut--has three "lobes" that stick out and shape the material into a thread.

    Also, formed threads tend to be stronger (material is "rolled" or "pressed" into the thread shape--generally leaving a stronger surface/material behind).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #3
    Re: Grounding - Thread-forming vs Thread-cutting screws

    Thread Cutting, has a sharp self-drilling point.

    Thread Forming needs a pilot hole, and then makes it's own threads, usually very secure & tight. Much better long term connection than thread cutting. Also very metal sepcific, SS screw for aluminum frame.
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  • newenergy
    newenergy Solar Expert Posts: 291 ✭✭✭✭✭✭
    #4
    Re: Grounding - Thread-forming vs Thread-cutting screws

    I didn't really know the difference, but from what has been described and what I've seen:

    We call the screws that do their own drilling w/o a pilot hole self-tappers. All the ones I've seen have a very course thread and I don't believe they make an acceptable ground because of that. I think you are supposed to have two complete threads in contact with the material you are grounding.

    The thread forming screws that come with some grounding lug kits generally fit right into the grounding whole of the modules - so no extra pilot hole is needed.

    These are still kind of easy to mess up and the grounding lugs I get lately (mostly from sunpower) come with a nut and a star washer for the back. You have to get a little wrench in there to tighten it up, but it makes for a very good connection.

    I'd still rather use WEEBs though, but Sunpower doesn't like them and most of my jobs end up being Sunpower.
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  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    #5
    Re: Grounding - Thread-forming vs Thread-cutting screws

    The screws with a drill bit tip (sometimes known as TEK screws) are "sheet metal" screws. I.e., they are only really suitable for thin material (normally, material which is thin enough to fit *between* the threads of the screw). The threads are widely spaced and also they stick out a good distance from the body of the screw.

    Then there are "self-tappers" which are like a normal machine screw, but have a notch or notches cut into the end, and the end is usually a bit tapered. That way they can be easily started in a pilot hole, and will cut their own threads. These have more threads per inch than a sheet metal screw. These are suitable for material that is a bit thicker (like the metal used for electrical boxes). Most of the "green ground screws" will be of this type.

    For material that is even thicker, it's proper to drill a pilot hole and cut the threads with a tap.


    All three will create some metal chips. The TEKs from the drill point, the self-tappers and the tap from the thread cutting, though normally self-tappers make the least amount of chips.

    The frames that I have seen on PV panels are normally aluminum, and also too thick for the TEK screws. For these I would drill and tap, or drill and use a self-tapping machine screw.

    Even better would be to use a lockwasher and nut on the backside. I would prefer that, since I don't much trust threads in aluminum unless the aluminum is thick enough to be considered "plate" rather than "sheet metal".
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  • System2
    System2 Posts: 6,290 admin
    #6
    Re: Grounding - Thread-forming vs Thread-cutting screws

    I understand that the 2008 NEC allows for thread forming screws, but there appears to be a few different types of thread forming screws. From a grounding perspective, is there a difference between using Type A, AB or C?

    Brief descriptions of each from an internet search

    Type A point: A thread forming screw for use in thin metal .015 to .050 thick. Used with drilled, punched or nested holes in sheet metal, resin impregnated plywood, asbestos combinations, among others. Not recommended for new design.

    Type AB point: A thread forming screw combining locating point of Type A with thread size and pitch of Type B. Normal limitations of type B apply.

    Type B point: A thread forming screw for use in heavier metal .050 to .200 thick. Larger root diameter with finer thread pitch for light and heavy sheet metal non-ferrous castings, plastics, impregnated plywoods, asbestos combinations, and other materials.
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  • Robin Gudgel
    Robin Gudgel Registered Users, Solar Expert Posts: 58 ✭✭
    #7
    Re: Grounding - Thread-forming vs Thread-cutting screws

    Type A screws have a sharp point. UL does not allow these to be used where they may drill into a wire. Type B has a blunt tip. That is what most companies use.
    UL does not allow Thread cutting screws in electronic enclosures due to filings that can short things out. We use Thread forming type B that are also called taptite.
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  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    #8
    Re: Grounding - Thread-forming vs Thread-cutting screws

    Another point to make when talking electrical connections - especially when involving aluminum is that the goal is a gas tight point of contact in order to ensure good conductivity. Besides the desire to eliminate loose chips, a thread forming screw will displace the material removing the surface oxidation and leave a gas-tight connection that should last.
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