Contact Performance in Relays
5.3 Effects of Load Currents
Physical conditions at the contact interface: The chemical and catalytic
actions of these various contact materials are accelerated because, for an
instant of time at each switching operation, the metal at the actual point of
contact may be exceedingly hot, molten, and often vaporized. Arcing
temperatures at the instant of circuit closure or interruption will be on the
order of 3,000 to 6,000° K. In this temperature range, all metals melt and
probably boil, and all common chemical compounds tend to decompose.
Softening and melting voltage: For each metal in the form of a short
electrical conductor-such as a point of contact at a contact interface-there
is a current density above which heat will be evolved faster than it can be
conducted away until temperature equilibrium is reached at the melting point
of the contact metal. The voltage drop across the contact interface is the
electrical characteristic necessary for such a current density to maintain
this metal softening temperature. This is commonly referred to as the
"softening voltage". As the contact interface softens, the area in contact
increases and the interface resistance decreases. This contact resistance
decrease is referred to as the "softening drop". Increasing the contact
current well beyond that necessary to produce softening will raise the contact
interface temperature to such an extent that melting occurs. The melting
temperature is the upper limit for temperature in solid contacts and the
corresponding maximum contact voltage drop is called the "melting voltage".
Any further increase in current results in additional contact melting causing
the contacts to sink together and resolidify as they cool, increasing the
contact area and decreasing the contact resistance.
Typical softening and melting voltages at 23°C are as follows:
| Material |
Softening Voltage |
Melting Voltage |
| Silver |
0.09 |
0.37 |
| Gold |
0.08 |
0.43 |
| Palladium |
- |
0.57 |
Contact arcing: Conditions for voltage breakdown of a contact gap depend
on various geometric and environmental factors. For a static gap, initiation of
an arc discharge can be related to the type of gas (air, nitrogen, hydrogen,
argon, helium), gas pressure, and gap length. Secondary variables that
influence this relationship are contact shape, material and surface texture,
gaseous contaminants such as water content, and the degree to which the gas in
the gap is being subjected to ionization. Discharge is initiated when ions
that may normally exist in the voltage-charged gap are accelerated
sufficiently to generate more ions and have a number of "mean free path
lengths" in which to accomplish this. The minimum voltage for this kind of
breakdown is approximately 320 volts at any pressure for air, and at normal
atmospheric pressure the gap length is approximately 0.003 in. For both
shorter and longer gaps, the breakdown voltage is greater unless the gap
becomes exceedingly small (in the Angstrom range). For materially shorter
gaps, the voltage breakdown follows the rules for vacuum discharge or direct
molecular ionization.
Arcs will form in a voltage stress of approximately one-half million volts per
centimeter when the gap is small compared to the mean free path length of the
has molecules (or ions) in the gap. Therefore, when the gap is sufficiently
small, much lower voltages are capable of initiating breakdown. Typical
values are 100 volts for a gap length of 10,000 angstroms (0.001 mm), 10 volts
for 1,000 angstroms, or 1 volt for 100 angstroms. At these extremely small
gaps, the contact damage caused by the breakdown is accentuated by the
discharge of the air capacitor formed by the very closely spaced contacts (and
the nearby parts of the circuit). This discharge takes place as the closing
contacts are about to meet. This arc is normally invisible and not to be
confused with visible arcing (frequently seen on the contact closure), which
is actually caused by contact reopening during contact bounce.
Metal transfer: Since the high current density in contact a-spots at the
first instant of closure can cause metal melting and probably boiling, even at
fractional ampere loads, there tends to be some metal transfer at contact
closure. These phenomena become matters of practical concern when contact
life must extend over many millions of operations, especially when significant
circuit or line capacitance must be discharged in addition to the contact gap
capacitance.
When contact separation takes place, the last instant of metallic conduction
is subject to the conditions of small contact area and very light force. This
results in molten or boiling metal (sometimes called a bridge) that becomes a
copious source of electron emission and of the ionized, positively charged
vapor of the contact metal immediately thereafter. These effects meet the
requirements for generating the plasma (electrified particles) for an arc
discharge when the circuit power can supply the energy needed at a rate
sufficient to maintaining the plasma. The amount of energy dissipated in the
arc is the principle factor in governing the amount of metal transfer during
arcing.
Inrush transients at contact closure: Contact damage is frequently caused
by current surges at the instant of closure when the contact forces are light.
Contact sliding and bouncing probably take place, and the load current is
often many times the steady state value. A microscopic weld or "bridge" will
frequently form at the point of contact closure. In dc circuits this bridge
usually ruptures asymmetrically at the next contact opening, resulting in
metal transfer. In ac circuits, a net loss of contact material usually
occurs. The metal vapor that condenses in the vicinity of the actual contact
area is normally black and is frequently mistaken for carbon.
Loads that produce transients at contact closure are as follows:
1. Tungsten lamps whose cold resistance is 7 to 10% of their hot
resistance.
2. Transformers and ballasts that may cause transients 5 to 20 times their
normal currents when switched in their inputs.
3. AC solenoids and some kinds of motors.
4. Capacitors placed across contacts or loads with inadequate
series-connected, current limiting resistance.
To meet these circuit conditions, the relay designer may elect to employ:
1. Heavy duty contacts and high contact force to minimize contact bounce on
closure
2. Contact materials with the highest possible electrical and thermal
conduction, usually silver or silver alloys.
3. Contact material additives to inhibit welding, such as cadmium or cadmium
oxide.
4. Relay actuator mechanisms to enhance weld-breaking ability of contacts.
The circuit designer can add small values of series resistance to the circuit
to reduce current surges.