Explanation of IEC Category Ratings
Hardware: Switches
Problem: I am using a switch in my application, and the input specifications list a maximum switching voltage that includes a designation of CAT I. What exactly is CAT I voltage, and how is it different from CAT II, CAT III, or CAT IV?
Solution: The International Electrotechnical Commission (IEC) has defined four voltage category ratings, and an instrument's ability to withstand voltage spikes applied through a specified level of resistance defines its category rating. The table below lists the voltage category ratings.
| Rated Voltage |
CAT IV |
CAT III |
CAT II |
CAT I |
| 150 V |
4,000 V |
2,500 V |
1,500 V |
800 V |
| 300 V |
6,000 V |
4,000 V |
2,500 V |
1,500 V |
| 600 V |
8,000 V |
6,000 V |
4,000 V |
2,500 V |
| 1000 V |
12,000 V |
8,000 V |
6,000 V |
4,000 V |
| Resistance |
2 ohms |
2 ohms |
12 ohms |
30 ohms |
CAT I - Electronic
- Protected electronic equipment
- Equipment connected to (source) circuits in which measures are taken to limit transient overvoltage to an appropriately low level
- Any high-voltage, low-energy source derived from a high winding-resistance transformer, such as the high-votage section of a copier.
- Low-voltage (< 1000 V) test equipment.
CAT II - Single-phase, receptacle-connected loads
- Appliances, portable tools, and other similar light industrial/household loads
- Outlet and long-branch circuits
- Outlets at more than 30 ft from CAT III source
- Outlets at more than 60 ft from CAT IV source.
CAT III - 3-phase distribution, including single-phase commercial lighting
- Equipment in fixed installations, such as switchgear and polyphase motors
- Bus and feeder in industrial plants
- Feeders and short branch circuits, distribution panel devices
- Appliance/equipment outlets with short connections to service entrance.
CAT IV - 3-phase at utility connection, outdoor conductors
- Origin of installation, or where low-voltage connection is made to utility power
- Electricity meters, primary overcurrent protection equipment
- Outside and service entrance, service drop from pole to building, runs between meter and panel
- Overhead line to detached building, underground line to well pump.
Related Links: Developer Zone Tutorial: What are Measurement Categories (CAT I, CAT II, etc.)?Developer Zone Tutorial: Isolation and Safetry Standards for Electronic Instruments
Attachments:
Report Date: 05/20/2004
Last Updated: 02/13/2008
Document ID: 39J8JMFP
