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Measurement Categories CAT I, CAT II, CAT III, CAT IV

What are electrical measurement categories CAT I, CAT II, CAT III, CAT IV ?

 

Measurement Categories CAT I, CAT II, CAT III, CAT IV are used to rate test instruments on their ability to resist a voltage spike, which is applied through a specific resistance. The higher the category, the more risk there that a high voltage can overload a circuit and cause electrical and physical damage. Usually, the higher the CAT (category) rating, the safer the rating.
 
 
Before looking into the various categories it’s important to understand exactly why tools are certified in particular ways. First off, when dealing with electricity, you MUST understand that the tool is either designed for a particular application or not. Many people don’t stop and think what would happen if the test probes, for example, don’t have enough insulation and too much voltage is applied to them, causing an arc. Or if too much current passes through the meter, causing it to more or less explode. These aren’t just imaginations, each one of these events has happened time and time again to electricians and amateurs alike who decided to use improper equipment to test and measure current and voltage. With that said, let’s take a closer look at the four primary measurement categories for electrical tools.
 

CAT I

This category is for measurements of voltages from specially protected secondary circuits. Such voltage measurements include signal levels, special equipment, limited-energy parts of equipment, circuits powered by regulated low-voltage sources, and electronics. These categories of use present very little chance for danger or overloading on any significant level.
 

CAT II

This is sufficient for a receptacle outlet circuit or plug-in loads, also referred to as “local-level electrical distribution”. This would also include measurements performed on household appliances, portable tools, and similar modules.
 

CAT III

Distribution wiring are qualified for this group, including “mains” bus, feeders and branch circuits. Also, permanently installed or “hard-wired” loads and distribution boards. Other examples are higher voltage wiring, including power cables, bus bars, junction boxes, switches, and stationary motors with permanent connections to fixed installations
 

CAT IV

This is “Origin of installation” or utility level applications such as any outside cable run. This category refers to measurements on primary over-current protection devices and on ripple control units.

 

Using the chart below we deduce the following: This specification informs the user this module is rated for 300V CAT II and 600V CAT I. In other words, this module can withstand up to 2,500V impulse voltage. This specification additionally informs the user this module must not be connected to MAINs CAT II circuits when operated above 300V. Lastly, the user should not use this module with Category III or IV circuits.

 

Rated Voltage IEC 61010-1
2nd Edition
UL 61010B-1
(UL 31111-1)
 
  CAT IV  CAT III CAT II CAT III CAT II CAT I  
150 V 4000 V   2500 V 1500 V 2500 V  1500 V  800 V  
300 V 6000 V  4000 V  2500 V 4000 V  2500 V  1500 V  
 600 V  8000 V  6000 V  4000 V  6000 V  4000 V  2500 V  
 1000 V  12 kV  8000 V  6000 V  8000 V  6000 V  4000 V  
 Resistance  2 ohms  2 ohms 12 ohms 2 ohms 12 ohms 30 ohms  

 

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