Ratings and speciications
The published ratings for a switch will invariably start with the voltage and current rating of the contacts, showing both DC and AC ratings. These ratings should be taken as absolute maxima if the highest standard of reliability is required. For example, if a switch is rated at 30 V, 4 A DC and 250 V 2 A AC, then it is reasonable to use the switch with 30 V, 4 A either AC or DC, and up to 250 V 2 A AC, but not to exceed the absolute limits of 250 V AC or 30 V DC, and the absolute limits of 4 A at low voltage and 2 A at high voltage. You should not, in other words, work on the basis that you can trade voltage for current and assume that if the voltage is less than 30 V, then the current can be more than 4 A. As has been indicated earlier, the voltage rating and the current rating for a switch are imposed through different constraints, and a change in one quantity does not necessarily affect the other, unlike power dissipation in a load.
The other important quantity, which is not always quoted, is contact resistance. Vhen this is quoted, it will be in units of mD, typically in the region of 5-20 mD. The figure that is quoted is usually the initial contact resistance, meaning the contact of the switch as supplied, since contact resistance tends to increase with the age of a switch when the switch is put into service. The rise in contact resistance will be greater if the switch does not use wiping contacts, if the environment is corrosive, and if arcing is frequent. Contact resistance is seldom of the greatest importance in non- signal switches, and for that reason is seldom quoted for these switches.
The reliability of a switch will be quoted in terms of the average number of operations to failure under accelerated test conditions. These test conditions may be unusually severe, so that for ‘average’ applications it is normal to find that the actual life of a switch is greater than the quoted amount. In many cases this may be quite irrelevant, because the equipment in which the switch is installed will have failed or been scrapped long before the switch fails. The expected life is usually quoted as two figures, the mechanical life and the electrical life. The mechanical life expresses the average number of operations before a mechanical failure, such as a broken spring or contact leaf. This is usually longer than the elec- trical life, which is the average number of operations up to the point of elec- trical failure through unacceptable contact resistance, contact welding, or failure to make contact. A typical figure for this quantity is 50 000 operations. To put this into perspective, if a piece of equipment is switched on and off once a day in a 5-day week cycle, the switch failure can be expected at about 96 years. In other words, a switch in a reasonable environment is not likely to be the component that determines the effective life of a piece of equipment.
The problem is that a switch, of all the components in an electronic circuit, is least likely to be located in a favourable environment. This applies particularly to front panel switches that will be exposed to shop- floor atmosphere and a good deal of heavy mechanical use. Vhereas the other components are encased, possibly encapsulated, and do not, in the main, have moving contacts, a panel switch is located where the surround- ing atmosphere can reach the contacts.
It is also subject to large forces on its actuating levers. Manual operation of a switch can vary from gentle pressure to violent prodding of a push button or fast flicking of a toggle lever. The actuating mechanism must be strong enough to withstand the extremes of treatment that are likely, but must not, as far as possible, allow such treatment to affect the contacts. The provision of strong and freely moving actuating parts makes it difficult to seal the interior of a switch in any way, and because of the effects of arcing, sealing is seldom desirable in any case. Expected life is therefore a quantity that can be very misleading unless it has been measured under conditions that approximate to the working conditions of the switch.