ABSTRACT
Mineral particles are present almost everywhere on our planet and finding some mineral particulate on electrical contacts should be no surprise. The effects of dust particulate have been discussed in Chapter 2 are are not the focus of this chapter. Williamson et al. described the effect of mineral particles on contact surfaces in terms of surface finish, particulate size, particulate concentration, and contact force (46). They show that as particle size decreases and contact surface roughness increases to the point where the ratio of the particulate size to mean plane height of contact surface irregularities is unity, the probability of high resistance resulting from these particles is low. Also as the contact load is increased beyond what is required to produce a contact area equal to the cross-sectional area of particulate on the contacts, the probability of the particulate creating high resistance decreases rapidly. These same principles apply to arcing contacts in the initial unarced condition but become more difficult to interpret after arcing of the contact surface, due to immense changes that occur in the particulate distribution and surface topography as a result of the arcing (47).
