Bench Talk

Figure 1: NKK’s Illuminated rocker switch with voltage, current ratings and certification markings. In most cases these certifications –logos or makings need to appear on the switch themselves or a designator is added to the end or next to the switch part number in order to be valid.

Have you ever tried to use a switch in an application and wondered whether the difference between the AC and DC rating on switches really matters? ­­There is a fallacy that any switch can be used so long as its current rating exceeds the maximum load requirements in the circuit. Who cares if it’s AC or DC, it’s only a 12V_DC system right? Wrong; when it comes to switches, the differences in current-carrying capacity are dramatically different between AC and DC circuits and this is typically reflected in the switch’s AC and DC ratings (see Figure 1.)

In its most common form, a switch is a manual electromechanical device, used to turn on or off power to an electrical circuit. Typically represented by the letter “S” in schematics, switches are available in a number of different types: push-button, rotary, toggle, rocker, slide, breakers, DIP, reed, and snap action (micro switches), to name a few. Although there are relatively few switch mechanisms per se, the number of switch varieties extends into the thousands. Nonetheless, all are switches and for the most part accomplish the same thing —make and break the electrical connection in a circuit.

Slow or Fast, AC or DC?

However, it is the speed with which the circuit is broken that makes a difference. Slow or quick-break; should switch contacts be broken slowly or quickly? It depends on whether the electricity is AC or DC. This may seem odd since electricity is electricity. But AC varies in magnitude and direction while DC maintains a steady unidirectional flow, and an interesting phenomenon exhibits itself when AC and DC circuits are broken. Consider an AC and a DC circuit, each carrying the same amperage. When an AC circuit is slowly broken, the arc or spark is extinguished quickly — a desirable condition (AC naturally has "current zeros" twice a cycle.) Conversely, when a DC circuit is slowly broken, the arc can be drawn much longer before it is extinguished. This is an undesirable condition which leads to pitting of switch contacts, which leads to overheating and premature failure of the switch, which can also lead to fire!

Apart from a switch’s mechanical function, an application’s electrical demands are also what will determine whether or not a particular switch is up to the task. Are you using it to switch a resistive or inductive load?

Loads: Inductive or Resistive?

Knowing what kind of load the switch will be switching makes a difference in the currents and voltages the switch gets exposed to. If the switch is being used to energize a resistive load (R), e.g., an incandescent light or heater, then current rises instantly when the load is turned on and reaches its steady-state value without first rising to a higher value.

However, if the load to be energized is an inductive load (L), for example, an electric motor or transformer, then the load will pull a large amount of current (an inrush current) when first energized. After a few cycles or seconds the current "settles down" to the full-load running current, but inductive loads cause excessive voltages to appear whenever they are switched. In this case, you need to ensure that the switch ratings meet or exceed what the circuit will require or the life of the switch will be greatly diminished or the switch can fail.

A switch’s DC Voltage (V_DC) rating is typically always lower than the AC Voltage (V_AC) rating at the same current (Amps) rating. For example: A switch rated for 20 amps at 125V_AC or (10 amps at 250V_AC), would typically have a current rating of less than 1 Amp at 125V_DC, and yet the only difference was going from 125 volts AC to 125 volts DC.

Figure 2: The "quick-break" technology built into today's switches ensures no arches or sparks are generated when the switch is thrown, unlike what happens in the movie “Young Frankenstein."

Ultimately, the deciding factors of whether the switch you’ve picked out will work or not come down to the following:

1. Is the switch being used in an AC or DC circuit?

2. Are you switching a resistive or an inductive load?

Certifications for Switches

A word of caution: There are many different switches in the market that would probably work for your specific application, but having a switch that has been certified to a standard ensures that the switch will function both mechanically and electrically as intended by the supplier. Hence, it is very important to choose a switch that meets anyone of the following certifications.

UL (Underwriters Laboratories): UL provides safety-related certification, validation, testing, inspection, auditing, advising and training services to a wide range of clients, including manufacturers, retailers, policymakers, regulators, service companies, and consumers.

CSA (Canadian Standards Association): CSA is accredited by the Standards Council of Canada, a crown corporation which promotes efficient and effective standardization in Canada.

VDE (Verband der Elektrotechnik): The Association for Electrical, Electronic & Information Technologies is one of the largest technical and scientific associations in Europe.

In the future, the next time you wonder whether the difference between the AC and DC rating on a switch really matters, you can positively say yes and you’ll know why!

Recommended Resources:

SWITCH FUNDAMENTALS

https://www.aeroelectric.com/Reference_Docs/Switches/CH_Switch_Training_Manual.pdf