Contact Performance in Relays


5.5 Types of Loads

The types of contact loads to be considered in relay design may be divided into four broad categories: 1. Dry circuits, 2. low level loads, 3. intermediate loads, and 4. heavy loads in the so-called rated-load range. Each of these categories presents its peculiar problems:

1. Dry circuits are usually considered to be loads that are not opened or closed by the contacts, that is, currents may flow through the contacts after closure and before opening, but the contact does not directly control the load. It is advisable, however, before concluding that the contact is dry to consider the possibility of the cable capacitance's being discharged through the contacts at the time of closing.

2. Low level switching ordinarily is considered to be in range of microamperes or a few milliamperes, with the open-circuit voltage below the melting voltage of the contact material. This category covers a wide range of the currents, however, are well below those at which arcing occurs.

3. Intermediate contact loads are those for which the current is below the minimum necessary for a momentary arcing condition. Fifty to 400 milliamperes at 26 Vdc is representative for this range.

4. Heavy contact loads are those that cause some degree of contact arcing under normal operation.

Dry circuits: The performance of contact under dry circuit conditions is affected by many parameters. Contact material, contact force, contact wipe, cleanliness of contact surfaces, type of environment, and magnitude of current of voltage all influence their behavior. By definition, a contact is considered to be dry if it does not make or break current. There are, however many applications falling within this category in which contact may be required to carry appreciable current. If the initial interface resistance of some other part of the circuit. This resistance may decrease sharply or be affected very little, depending upon the surface condition. If organic films are the cause of initially high resistance, appreciable voltage may produce low and stable resistance. On the other hand, resistance may remain high if it is caused by particulate contamination. Over the wide load range encompassed by the dry contact concept, it is not possible to generalize on probable performance. It is possible, however, to evaluate contact performance for a particular set, or a limited range, of conditions within the broad dry contact category. Usually gold alloy contacts, or gold alloys working against palladium, offer the best performance under these conditions. The suitability of a contact material or combination of materials, however, depends on the interactions of the parameters mentioned earlier.

Low level loads: As in the case of dry circuits, contact performance at low levels is determined by materials, design parameters, and performance criteria. Within this range organic films may affect performance significantly. Although it is virtually impossible to eliminate the absorption of organic films, cleaning techniques, controlled environment, and design may reduce their effects and assure satisfactory performance. It is within this range of low level contact loads that polymerization of adsorbed organic material or condensates on contact surfaces of materials in the platinum family can be particularly troublesome. When organic materials are present and microscopic sliding occurs between contacts of the platinum group, the heat developed by friction causes substance, cause high and unstable resistance. These polymers, in the form of powdery substance, cause high and unstable resistance. Increased contact force and greater wipe between contacts upon closure may alleviate but will not eliminate the trouble. The difficulty usually, but not necessarily, is associated with relatively large numbers of operations. Vibration of relay contact springs also has been responsible, on occasion, for considerable contact trouble of this type. The use of gold, gold alloys or gold alloys in combination with palladium practically eliminates the polymer problem. However, gold or gold alloy contacts of 22K or higher gold content, when working against each other, can susceptible to sticking or cold welding. This tendency must be considered in design and application.

Intermediate loads: In the intermediate load range, slight arcing may occur on closure or opening of contact. Usually the erosion of contact in the intermediate range progresses at a much slower rate than at voltages and currents close to the maximum contact rating. Loss of contact force, therefore, is not a problem. However, arcing in the presence of adsorbed or condensed organic material and the contact surfaces causes carbonization of these materials. The voltages and currents in the intermediate range are not high enough to break down completely the relatively high resistance of the carbonaceous deposits. As a result, appreciable contact resistance sometimes develops within several thousands of operations. In general, hermetically sealed relays are more likely to be affected than open type designs. High current contactors and circuit breakers are not affected significantly because of the high currents and voltages that are switched and the relatively high contact forces. The difficulty created by carbonaceous deposits can be minimized in smaller general purpose relay designs by choice of insulating materials and design of the contact actuating systems, and in hermetically sealed relays by baking and evacuating techniques or by isolation of the contact from organic materials. Usually the resistance developed under intermediate load conditions, while significant, is tolerable from a circuit functioning standpoint. However, in certain critical cases the contact resistance may be troublesome. One set of conditions that should be avoided, if at all possible, is imposing arcing and low energy loads on the same contact.

Heavy loads: Ordinarily, contact must operate at or close to, the rated load function satisfactorily for their required life. The amount of carbonaceous material formed under these load conditions may be considerably greater than the quantity observed at lower loads. However, the actual area of contact of the mating surfaces us usually relatively free from this material and the voltage and currents and high enough to break down the resistance of deposits in the immediate contacting areas.

Occasionally in hermetically sealed relays, if insufficient clearance is provided between contacts and a grounded case, flashover to the case due to gaseous ionization may occur upon repeated opening of the contact load. This condition, while undesirable, usually does not damage the relay when it is ungrounded. The effect is about the same as that of a repeated opening of the contacts on the contacts themselves. If the relay case is isolated from ground, however, consideration should be given to the personal hazard of possible having a relay case at a potential well above ground because of a circuit trouble.

Much has been written concerning the use of relays to switch three-phase loads. Difficulties that have occurred were caused by misapplication. The transient and steady state voltages between contacts in a three-phase or direct-current application. Irrespective of the type of load, the voltages that may appear between adjacent contacts should be well below the dielectric withstanding voltage.