The Safest Connectors

Safest in what way? Pull-out? Isolation / finger safety? High vibration aplications? Ability to remove wires safely and repeatedly?

It depends on your application. A screw can be modeled as a stiff, cam powered compression spring.

Engilis,

Your post has two very different ways the question can be interpreted: 1) For terminating wire, or 2) for bolting busbars or terminals. I hope you could add more details about your question to clarify this; or perhaps an example of what you are questioning.

Different metals (and to an extent, even different alloys) have different rates of thermal expansion/contraction. Also, different metals and alloys have different values for their ductility (ability to deform under stress). When you bolt two electrical conductors together and pass current through them, the internal resistance causes them to heat up until they reach an equilibrium temperature determined by the heat gain from the resistance and the heat loss through (mostly) radiation and convection. Calculating this equilibrium temperature is not easy for the average electrician or electrical engineer. But many decades of experience have developed tables that give worst case temperature rise for a given: environment, current flow, and conductor or busbar size. In the USA, the National Electrical Code places the most commonly used tables in article 310.

As you go through multiple cycles of heating and cooling (expansion and contraction), you want the resistance of the connection to remain unchanged. If all the components have a similar coefficient of thermal expansion, then bolting to a specified torque will work quite well. If the ductility of the metals is small and the elastic limit of the bolt is not exceeded (this elastic limit is less than the limiting tensile strength), again the same bolting will work quite well. Otherwise, some form of spring is needed to absorb the change in dimension while maintaining the required tightness of the connection.

Enter the cone washer, commonly called the bellville (spelling?) washer or spring. It is applied under the bolt or screw head with the flat side against the material being bolted. Tightening it to its specified torque will give the required compression of the bolted pieces and still retain a portion of its cone shape so thermal expansion is accommodated.

Back to your original question: At times a screw connection is very safe. A spring pressurized connector (if properly done) is more forgiving and can last longer. Busbars of aluminum alloys must be bolted with bellville washers; a good torque wrench is required.

--JMM

Very good balanced response. I would add that the fundamental reason for bolts, springs, etc., is to insure a gas tight joint over the life of the connection. Corrosion is responsible for most electrical connection degradation. Therefore, anything that keeps gas and fluids out of connection interfaces is a great help in producing reliable electrical connections. So, as a connection engineer, you must first provide enough cross sectional area to manage temperature rise due to joule heating. Then you must assure that the joint remains gas and fluid proof.

thanks for your good answer...