The Formation of the Electric Arc during the Opening of Contacts

An arc will always form between opening contacts if the circuit current and the voltage that appears across the contacts is greater than a minimum value. The arc formation depends entirely upon the properties of the contact material and the arc always initiates in the metal vapour from the contacts themselves (Slade, 2013).

From equation (5) the contact resistance 𝑅_𝐶 is given by

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where again, a is the radius of the real area of contact, H is the material or Vickers hardness, ρ is the resistivity and F is the force holding the contacts together.

As the contacts separate, the holding force F → 0 and therefore the area of contact a → 0, producing an increase in the contact resistance 𝑅𝑐. This increase in the contact

resistance 𝑅𝑐 leads to the voltage drop 𝑈𝑐across the contact also increasing.

𝑈_𝑐 = 𝐼𝑅 (22)

From 2.4.3 above the relationship between voltage and temperature has been discussed, an approximation to this can be represented by the temperature of the contact 𝑇_𝑐 being given by

𝑇_𝑐2 _{= 𝑇}

02+ 𝑈𝑐2× 107 K (23)

where 𝑇0 is the ambient temperature. Work carried out by (Wakatsuki, 2008) shows a

stage will be reached when the temperature of the contact spot will equal to the melting point 𝑇_𝑚 of the contact material.

The values from table 2.4 below and calculated values from equation (23) are depicted in figure 2.6 below for a wide range of contact materials and shows the correlation between results.

Figure 2.6. Showing the relationship between the calculated and measured melting temperature and the measure voltage drop across the contact (Wakatsuki, 2008).

When a contact reaches this melting stage and the contact pairs continue to draw apart, a molten bridge is formed between them, this happens at low currents (Utsumi, 1969), (Miyajima, 1998) and (Ishida, 2004), slow (Mcbride, 2012) and high speed opening contacts (Slade, 1971), (Koren, 1975) and (Slade, 1972) and even in a vacuum (Slade, 2008). This molten bridge continues to form and is drawn until it ruptures due to instability. After this rupture of the molten metal bridge, an arc forms

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in its vicinity. A typical change in the initial voltage drop across the contacts is shown in Figure 2.7 (Slade, 2013).

Figure 2.7. Showing the change in the initial voltage drop across the contacts as the arc develops (Slade. 2013).

These voltage characteristics can be described using the four stages shown in figure 2.8 (Haug, 1990) & (Slade, 2010):

Figure 2.8. The four stages of molten metal bridge rupture and metal phase arc formation.

Stage (a): Once the molten metal bridge has formed its rate of change of voltage is

about 2 × 103 _Vs–1_{. As the contacts continue to open and the bridge is drawn further it}

becomes unstable. There are a number of physical reasons for this instability, including surface tension effects, boiling of the highest temperature region, convective flows of molten metal resulting from the temperature variation between the bridge roots and the high-temperature region.

The bridge will eventually rupture, releasing metal vapour into the contact gap when the voltage across it, Ub, is close to the calculated boiling voltage Ub1 of the contact

materials, i.e.:

𝑈_𝑏 = √𝐿(𝑇_𝑏12 _{+ 𝑇}

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where L is the Lorenz constant and 𝑇𝑏1 K is the boiling temperature as in the table 2.4

below.

Metal Breaking Voltage(Ub) Calculated Boiling Voltage(Ub1) Boiling

Temperature(Tb1) K Ag 0.75 0.77 2485 Cu 0.8 0.89 2870 W 1.7 1.82 5800 Au 0.9 0.97 3090 Ni 1.2 0.97 3140 Sn 0.7 0.87 2780

Table 2.4. Showing the Boiling Voltage (Ub1) with the Break Voltage (Ub) for Various Metals (Slade,

2013)

Stage (b): Once the bridge ruptures the voltage across the contacts rises very rapidly

without a discontinuity from about 103 _Vs–1 _{to about 10}9 _Vs–1

This rate of rise of the voltage will depend upon the dimensions of the molten metal bridge just before its rupture. After the bridge rupture a very high pressure, perhaps as high as 100 atmospheres (Haug, 1990) & (Slade, 2010), very low electrical conductivity, metal vapour exists between the contacts. This region can then be considered to be a capacitor with a very small capacitance. Because the circuit’s inductance prevents a rapid change in current charge flows from the circuit inductance into this small capacitor causing the very high dV/dt. The metal vapour volume expands rapidly into the surrounding lower pressure ambient and as it does its pressure also decreases rapidly. When the pressure of the metal vapour decreases to 3–6 atmospheres conduction is initiated with a voltage across the contacts of a few 10’s of volts.

At these pressures the discharge that forms is the “pseudo arc” (Puchkarev, 1997), (Ebling, 1991) where the current is conducted by ions. During this stage the electrons required for charge neutrality will be introduced into the discharge from secondary emission resulting from ion impact at the cathode. As the original molten metal bridge will have material from both the cathode and the anode, net transfer of material from the anode to the cathode is expected and is indeed observed (Haug, 1990) & (Slade, 2010).

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Figure 2.9. The transition from molten metal bridge to the metal phase arc (Haug, 1990): (a) Voltage, neutral copper radiation (Cu I), Cu contacts, 50 A, 100 V and (b) voltage, ionized copper radiation (Cu II), Cu contacts, 50 A, 100 V.

Stage (c): As the pressure of the metal vapour continues to decrease to about 1–2

atmospheres, the pseudo arc transitions into the usual arc discharge with an arc voltage impressed across the contacts whose value is about that of the minimum arc voltage expected for an arc operating in the contacts’ metal vapour (i.e., 𝑈𝑚𝑖𝑛 ≈ 10 −

20 V). Here again net material transfer will be from anode to cathode. It is only in a vacuum ambient that the arc continues to operate in metal vapour evaporated from the contacts themselves (Slade 1972), (Slade, 2008) & (Slade, 2008b). In order to sustain this arc a minimum arc current is also required.

Stage (d): At this stage as the contacts continue to open and the arc between them

gradually transitions from the metallic phase arc to the ambient, gaseous phase arc with most of the current now carried by electrons. The whole sequence is illustrated in Figure 2.10 (Slade, 2013). As the contacts continue to open this metallic phase arc transitions into an arc operating in the ambient atmosphere.

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Figure 2.10. Showing the opening sequence of an electrical contact; the formation of the molten bridge; its rupture and arc formation (Slade 2010).