How reliable are connector strips with plug-in terminals and how do I select and use them properly?

Question

Here's a close-up photo of a connector strip with plug-in terminals

It's used for connecting 220 volt mains wires – you can see a bare metal wire coming in – that's 4mm² aluminum wire. Each of the four holes has some clever spring-loaded contacts inside so that when you insert a wire there it is grabbed and held in place and presumably good contact is maintained. The terminals of the two holes where the wires come in are connected to each other inside and the other two holes terminals are connected to each other but not to the terminals of the first two holes. So the connector can be used to join four wires pairwise.

How reliable are such connectors? Do I use them or plain old connectors where each wire is held by a screw? How do I select such connectors and use them properly? What are possible caveats?

Answer 1

Call me old school, but I am not a fan of some of the new friction grabbing splice devices out there. Similar spring connectors have been used on spec grade outlets for many years for 14awg copper. I have seen many failures and intermittent connection problems with these push springs. My theory, and just a theory mind you, is that there is not enough actual contact surface between the wire and the connection device and that the contact tension cannot be tightened. Over time expansion and contraction of the wire, surface tarnishing and dust can compromise the integrity of the connection. This happens with copper and much more with aluminum. As the surface area of the connection shrinks from any of those factors, heat and arcing can be created in the connection. Eventually failure can result. If you look at the comparison with a good wire nut or screw fastening terminal strip, you will see that there is much more metal to metal contact, the conductors can be twisted together and the tension can be tightened at any time with a twist or a turn of a screwdriver.

An other concern I have is the use of these type of connectors with aluminium wire. I have had nothing but bad experience with alu branch feeder wiring over the years. You must be absolutely sure the device is Al/Cu rated at minimum and that connections are as tight as possible. On larger wires such as service entries where alu wire is still used regularly, a substance known as NOLOX is always used on these connection to fight corrosion and assure good conductivity between the wire and landing connector. I also use NOLOX on any dryer or range connections that have alu wire. Alu wire tarnishes very quickly and expands and contracts faster and more than copper, therefore needs to be secured firmly and protected from corrosion wherever possible.

The last reason I am not a fan of friction fit connectors is that they can be miserable to work with after initial installation. Yes, they are fast easy to install the first time, but if you need to modify a connection or trouble shoot a problem where these connectors are installed, it is pain if not impossible to remove wires temporally. Reinserting a wire often doesn't have the same grip as before.

I may be all wet on this one, but I'm not ready to give up my trusty high quality wire nuts and screwdriver yet.