Inductive Ratings For Mechanical Limit Switches
Many control applications involve high inductive loads such as starters, contactors, and relay coils, along with solenoids and clutches. You must know the inductive rating of your mechanical limit switch to apply it correctly. Generally, there are three types of inductive ratings:
Resistive or non-inductive
- Inductive (pilot duty)
1. Resistive or Non-Inductive Rating
The resistive rating indicates the resistive load only that the contacts make or break. Generally, resistive ratings are based on a 75% minimum power factor for AC. Heavy-duty applications that use resistive ratings include control or signal of light bulbs and electric heaters.
2. Inductive (Pilot Duty) Rating
The inductive rating indicates the non-motor inductive load, such as the contactors, relays, and other remotely controlled devices that the contacts can make or break. These ratings usually are based on a 35% power factor for AC.
3. Continuous Rating
The continuous rating indicates the load that the contacts can carry continuously without making or breaking the circuit. Applications with continuous ratings involve continuous current flow in some motors.
The inductive rating is always less than the resistive or continuous rating. When contacts break an inductive circuit, the inductance in the load tends to keep the current flowing in the same direction. The result is an arc across the contacts, which causes the heating and burning of the contacts. Because of the extra heat generated, the allowable inductive current must be less than the resistive current for equal contact life.
Snap switching resistance is the total electrical resistance that a snap switch adds to the circuit and consists of:
The conducting path includes all terminals, inserts, stationary contact material resistance, movable blade assembly, and any other parts in the conducting circuit.
Constriction resistance is the resistance caused by limited mating surfaces through which the load must pass. If viewed through a microscope, you would see that that the movable and stationary contact tips touch at very few points, presenting increased resistance to the current. If the current is high enough, the points of constriction are softened and enlarged through thermal effects, and the resistance decreases.
It is common for silver to accumulate surface resistance due to a chemical reaction with its environment. The most common reaction is with sulfur and oxygen, which creates a sulfide and an oxide of silver. Such a surface is known to have some resistance. A low voltage instrument, such as an ohmmeter, is used to read film resistance in this application. These surfaces, however, have the characteristic of being self-cleaning.
When current passes through a surface with film resistance, it creates heat that reduces the compound to pure silver again and restores the contact to a low ohmic value. This characteristic makes silver an excellent selection for contact materials.
Why are gold contacts necessary?
The shelf life of a heavy duty limit switch can be affected by the contact material. Gold plated contacts help prolong the life of limit switches. During storage, silver contacts accumulate sulfate and oxide layers, but gold resists these film particles. Namco high-temperature switches and nuclear qualified limit switches come standard with gold-plated contacts.
Standard contacts can be gold plated to reduce switch resistance but may not provide a sufficient increase in reliability to warrant the added cost. For example, if the contact material underneath is exposed at any point, due to excessive current or normal mechanical wear, a contaminating film may develop. This film will eventually creep over the plated portion, affecting the long- term reliability of the mechanical limit switch.
Contamination in the form of foreign material can also produce resistance. Carefully controlled production processes prevent contamination during assembly of snap switches.
Mechanical limit switches are ideal for industrial control equipment where voltages are relatively high – normally 120 volts or higher – and current levels are high- normally .25 amperes or higher. An increase in switch resistance may appear to be more critical in high energy circuits because it represents a greater percentage of total circuit impedance; however, the arcing produced breaks down or burns away the contaminants, reducing the actual resistance seen by the land.
Switch resistance in dry circuit (low voltage, low current) applications posses a larger threat, but the resistance has to be quite high to affect operation significantly. For example, a solid-state circuit operating at 20 volts and 1 ma has a circuit resistance of 20 divided by 1 (1 x 10-3) or 20,000 ohms. A Namco Snap-Lock® switch with 10 ohms resistance would have little effect. The problem arises with film or particle resistance, which can be quite high or even present an open circuit. Namco Snap-Lock® switches are not ideal for dry circuits. Voltages and currents in dry circuits are not high enough to reduce the silver sulfide or silver oxide to pure silver or to burn away other contaminants present.
Also, an ohmmeter test on Snap-Lock switches is unreliable. The most common voltage source in an ohmmeter is a 1.5 volt battery, and to snap switch contacts, this is just another dry circuit load. Several ohmmeter readings of the same contacts may vary from a few hundredths of an ohm to several ohms, yet the contacts will work perfectly with nominal coil load.
What is Snap-Lock Technology?
Quick make and quick break (snap action) contacts reduce the arcing time and allow higher inductive ratings than with slow make or break contacts. AC inductive coil loads have a momentary inrush current of approximately ten times the sealed current. Contacts must be able to break or interrupt the inrush current in an emergency.
It is important to know which factors affect the current rating and what the minimum recommended contact current is for your mechanical limit switch. The higher the current, the lower the contact resistance. Maximum continuous current ratings are included in product catalogs and datasheets.
Recommendations for Applications and Testing of Limit Switch Contacts
Typically, Namco recommends their mechanical limit switches for use with industrial control devices, not dry circuits. If lower voltages are present, the current drawn through the contacts should be on the order of .25 amperes to maintain proper continuity.
To test continuity in the field, we recommend using a six-volt, .25 ampere pilot light. Do not use an ohmmeter to test continuity. An ohmmeter is a reliable test only if the snap switch is to be used with a dry circuit, and, as stated above, this usually is not recommended.