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Prompted by an answer on Concerns about replacing a worn out light switch

Also related: When to use holes instead of side terminals to wire an outlet

To my knowledge, electrical components (switches, outlets, breakers, basically all the home-voltage kinda things) will have one of the 3 connector types:

  • screw wrap - the wire end is wrapped around a screw, and the screw tightened to hold the wire.
  • screw down - The wire end is left straight, a screw is tightened to hold the wire, either directly or via a pressure plate.
  • push in / friction - The wire end is left straight, and pushed between an angled plate which holds the wire via pressure in the design.

There are many, many variations on these themes (plastic housings, attempts to make a release device for friction fasteners) but they all seem to boil down to these 3. I'd like to know the pros and cons of each of them; but in particular I'd like to know what can or will happen when they fail.

For example, the push-in are fastest to wire.

Other than having to take time to twist the wire end, the two screw options seem basically identical. Okay, it would take significantly more force to pull a wire out of a wrapped connector than a straight one, but I expect we're into impractical pulling force on the wire either way. The straight wire ones seem to be less common though, probably as it's more complex to manufacture.

I see a lot of dislike for the push-in connectors. Personally, I find them a huge pain to re-wire, and it's usually simplest to just cut the wire off and start again. But if something's wired with push-in and working, how can/will it fail? What symptoms, particularly early signs, indicate impending failure?

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Electricians prefer the "screw wrap" method because it gives them a reason to use those little holes on the sides of wire strippers. :) –  JYelton Oct 10 '13 at 18:26
1  
Electricians prefer the "screw wrap" because it's a lot less prone to failure. If the screw (or plate, in the case of push-in's) comes loose over the next 20-or-so years, the only one of those that will still hold the wire firmly is the screw-wrap. –  BlueRaja - Danny Pflughoeft Oct 10 '13 at 19:27
    
@BlueRaja I was making a joke about the underlying reason. I agree with you, the "screw wrap" is the most proven and reliable. –  JYelton Oct 10 '13 at 19:51
    
Electricians prefer "screw wrap", because that's what they've been doing for years and it's worked great for a long time. –  Tester101 Oct 11 '13 at 1:36
    
people dislike push-in connectors, because the first generation were poorly designed. It's also easy to screw up the connection, and not push the wire in far enough. If you don't bend the wires into the box properly when installing the device, strain can be put on the connection leading to pull out. –  Tester101 Oct 11 '13 at 1:41

2 Answers 2

up vote 1 down vote accepted

Any wire connector can fail. Screws can loosen with vibration, stress, corrosion, temperature fluctuations, and so on. Push-in (or stab-in) connections can fail for the same reasons. Crimp and pin connections, friction lock, and even soldered joints can fail.

The real question is: what's the most effective wire connection given the operating conditions, parts and installation cost, lifetime expectancy, use case, and so on.

For example, where I work (we manufacture large electric signs), we use all manner of connection methods, and they vary based on these factors. One such factor is serviceability. If a connection is expected to be rugged and reliable but disconnected for safety reasons, we may use screw terminal blocks (which only accept straight wire) or friction lock (which requires use of a screwdriver to release, similar to the push-in connectors on outlets and switches).

Outlets shouldn't require frequent service nor replacement, nor should experience extreme vibration. They also won't (usually) have wide temperature swings, which leaves the stress from normal usage (connecting and disconnecting plugs) which can wiggle things loose over time.

A well-made outlet can have quite good push-in connectors; by "good" I mean they hold solidly for years and aren't likely to fail with normal use. However it can be difficult to tell during installation whether the inserted wire has made good contact or not. Also if one happens to be faulty, it may accept the wire but provide little feedback that the locking mechanism is less than optimal. Pulling on the wire after installation is one way to tell, but if you're using the push-in connectors, are you spending the extra time to do the pull test? Probably not. Screw terminals are easy to visually inspect and determine if solid contact is being made and that the wire is firmly held by the screw or plate.

So while push-in or friction lock connectors can be quick and useful in certain applications (with appropriate confirmation of operation), I recommend wrapping wire around the screw terminals for the most durable connection that's easily verifiable. Usage and vibration can still loosen the screw, and in such case the wire hook will be less likely to separate from the outlet than a straight wire. In all cases where the screw comes loose, however, arc fault can occur, causing problems.

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The 'pro' of the push-in connection is that it's fast.

The 'con' is that it's not as secure as the screw terminals. A push-in connection is the wire being pushed into a a piece of metal that 'pinches' the wire. It can hold the wire, but the wire can still move a lot more (twist and turn) than it can when screwed down to a screw terminal.

The other con is that you can't provide any secondary means of securing. I prefer to wire to the screw terminals, and then I wrap the outlet (covering the terminals) with a layer or two of electrical tape. That offers a bit of secondary protection keeping any other wires potentially making a connection where they shouldn't.

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