When shopping for electrical test and measurement tools, you will often come across category designations I – IV. These may confuse you, or make you think that you need to look for a certain category, or that one category is better than another. As far as applications go, this may be true. Collectively, these specify specific uses and ratings for electrical test and measurement tools, so we've decided to put this article together to remove any confusion and make it all plain and simple.
Before delving into the various categories, it's important to understand exactly why a tool is certified in a particular way. First, when dealing with electricity, you have to understand if the tool is designed for a specific application. Many people don't stop to think about what happens if the test probes are not sufficiently insulated, and too much voltage is applied, causing arcing. Or, if too much current is passed through the meter, it will more or less cause it to explode.
These are not just imagined, each of these incidents has happened time and time again to electricians and hobbyists who decide to use incorrect equipment for testing and measuring current and voltage. Having said that, let's take a closer look at the four main measurement categories for power tools.
Measurement categories can be broken down into four basic designations:
CAT I measurement category
This electrical measurement category is used to measure voltages from specially protected secondary circuits. Such voltage measurements include signal levels, special equipment, limited energy parts of equipment, circuits and electronic equipment powered by regulated low voltage sources. The likelihood of any significant level of hazard or overload being present in these usage categories is very low.
CAT II Measurement Category
This is good enough for socket outlet circuits or plug-in loads (also known as "local level distribution"). This also includes measurements made on household appliances, portable tools and similar modules.
CAT III measurement category
Distribution wiring applies to this group, including the "source" bus, feeders, and branch circuits. Also, permanently install or "hardwire" loads and distribution panels. Other examples are higher voltage wiring including power cables, bus bars, junction boxes, switches and stationary motors permanently connected to fixtures.
CAT IV measurement category
This is the "installation source" or utility grade application, such as any external cabling. This category refers to the measurement of primary overcurrent protection devices and ripple control units.
In short, the higher the electrical measurement category, the greater the risk of a so-called "arc blast" – a situation where high voltages can overload circuits and cause electrical (and physical) damage. Arc blast can ruin your day…or life. The higher the available short-circuit fault current, the higher the category. While CAT II voltage ratings may be higher than CAT III ratings (such as CAT II 1000V vs. CAT III 600V) – the higher CAT rating is almost always the safer rating.
What will happen and what are the protections?
- Problem: Transient arcing (lightning, load switching)
Protection: Independently certified to meet CAT III-1000 V or CAT IV 600 V
- Problem: Voltage contact on conduction or resistance
Protection: Overload protection in OHM up to the meter's rated voltage
- Problem: Measuring voltage with the test leads in the current jack (short circuit!)
Protection: High energy fuse rated at meter rated voltage; use meter/tester without current jack
- Problem: Electric shock from accidental contact with live components
Protection: Test leads double insulated, recessed/shrouded; finger guards; CAT III – 1000V; replace if damaged
- Problem: Using Meter or Tester Above Rated Voltage
Protection: God's Will
Electrical Measurement Category Comparison
|Rated voltage||Four categories||Three categories||Secondary||Three categories||Secondary||category one|
|ohm||2 ohms||2 ohms||12 ohms||2 ohms||12 ohms||30 ohms|
Reading this chart of electrical measurement categories is easy. If the specification informs the user that the tool is rated for 300V CAT II and 600V CAT I, the module can withstand impulse voltages up to 2500V. This type of specification will additionally inform the user that the equipment must not be connected to MAINs CAT II circuits when operating above the specified 300V. Of course, tools or equipment rated in this way should not be used with Class III or IV circuits.
Certifications and Standards
The IEC develops standards but does not test or enforce compliance at the industry or product level. Thus, a manufacturer can claim to be "designed to comply" with a standard without any evidence to substantiate their claim.
This is especially concerning for new products sourced from China and sold under generic names (without major manufacturer branding and oversight). To obtain UL Listing, CSA or TUV Certification, a manufacturer must pay and submit the product to the listing agency to actually test (often destructively) the product for compliance with the standard.
Important note: In most cases, just look for the listing body's logo on the meter.
The bottom line for you: match the tool to the application, and pay attention to certifications and specifications. If you don't, the results can be devastating.