Electrical connections including grounding connections, at a microscopic levels will show that the surface contact can vary in different types of connections.
The asperity contact points are very small contacts points which are in microns. These points are distributed across the apparent contact area of connections. Electrical currents that flow through the connection will flow through these asperity or A-Spot contact points. There a resistance that needs to be considered called the constriction resistance. The magnitude on the constriction resistance depends on the number and sizes of the asperity spots.
These asperity spots and constriction resistance will exist even when similar metal like copper to copper are used for the connection. The level of asperity spots are expected to vary for different connection types like mechanical connections, compression connections and exothermic connections.
If any of the asperity interfaces are compromised by corrosion films or contaminants, the constriction resistance will increase. This is the reason why corrosion is a degradation mechanism for connectors. Loss of asperity contact area, or of asperity contacts, due to corrosion or contamination can result in contact interface resistance increases that are sufficient to lead to connector failures.
The constriction resistance can also increase over time in grounding connection upon the application of repeated fault currents.
While standards exist that provide a benchmark for testing of various electrical connections for use in grounding systems comparative inspection of various grounding connection has not been carried out to inspect asperity spots and degradation of constriction resistance.
The power industry accepts and refers to the test methods developed for testing grounding connections in standards like IEEE Std 837-2014, UL 467 and IEC 62561-1. The purpose of this paper to provide complimentary information on the existence of this asperity model and showing some examples of how different connection behave under this model.