Written by: Craig F. Nelson, Electrical Specialist CESCP, CESW Certified
You will notice that gloves are rated in AC and DC voltages. Class 00, for example, is rated 500 Vac/750 Vdc. Why are they rated lower for AC than DC, or why are they rated higher for DC than AC? Anybody ever wonder about that? I hope somebody has!
What is AC in Voltage?
AC, or alternating current, refers to the type of electrical current in which the voltage periodically reverses direction. Unlike DC (direct current), where the voltage flows in one direction, AC voltage oscillates, changing polarity at a set frequency (typically 60 Hz in the U.S. and 50 Hz in many other countries). This type of voltage is used in most household and industrial electrical systems because it is more efficient for transmitting power over long distances and is compatible with transformers for adjusting voltage levels.
What is DC in Voltage?
DC (direct current like your car battery) is a constant voltage. Let’s compare both quickly
It has a positive and negative pole that never change. It doesn’t fluctuate like AC (alternating current) does, and which is constantly changing in magnitude and direction, switching from positive to negative and back again. This, of course, is the electricity that comes off the power grid.
How often that change in polarity takes place per second is known as frequency, measured in Hertz (and often referred to as cycles), and in the US power system, it is 60 times per second or 60 Hertz (aka 60-cycle) electricity. As noted above, it reverses 120 times per second, having a positive charge for half the cycle and negative the other half. This generates what is known as a sinewave. You’ve seen these displayed on the power quality meters, scopemeters, etc.
Where Does RMS Come In?
In order to measure this constantly changing voltage magnitude, they invented something known as rms or root mean square measurement. You are familiar with the many multimeters that tout “true rms” ac measurement capability. What they do is use a microprocessor to capture a bunch of points measured along the entire sinewave, make a calculation and give you the equivalent of all those points as a rms voltage. The more points it can capture, the more accurate the calculation. And you pay for that, too!
Although rms is kind of an average, it is more sophisticated than that. I stay away from the term “average” because averaging is a different measurement method that must have a perfect sinewave to be accurate while rms measurement doesn’t. And it will just confuse everybody. Just a hint, though. Know that they are NOT the same. And some of our less expensive multimeters actually DO use average measurement.
AC vs DC Voltage – Why Their Ratings Differ
So where am I going with this? Well, the distance between the most positive voltage and the most negative voltage is called peak voltage. That’s the maximum voltage level the sinewave produces each second. The rms value of the peak voltage is found by multiplying the peak by .707 or in other words, rms is about 71% of peak. The reverse of that is to find the peak when you know the rms voltage. To do that, you multiply the rms voltage by 1.414 and that gives you the peak voltage. Again, this calculation assumes a perfect sinewave and is only approximate since the actual ac power source can have variations in the sinewave.
The ac voltage rating of the Class 00 glove is 500 volts. If you multiply that by 1.414, you get 707 volts. In round numbers, peak is about 1.5 times the rms value of the ac voltage or 750 volts. How about that? (There’s a reason to be a bit more generous with this calculation having to do with transients and waveform distortion, but let’s keep it simple.)
Notice that the dc voltage rating is 750 volts which is about equal to the peak ac voltage. So… there isn’t really any difference between the two except the way they are measured. We don’t express an ac voltage rating as a peak value, but if we did, then the two voltages would be about the same.
What this boils down to is the voltage ratings are the same; just measured differently. You might be interested to know that I explain this in electrical safety training courses because many electricians never think about why there’s a difference, either, even though they understand the way ac voltage is measured.
If anybody ever asks what the difference is, and they might, now you’ll know!
Now that you have a good idea of what the difference is, shop our glove selections and read up on our PPE Rotation Testing here.
