Overview
This lecture explains CAT (category) safety ratings on electrical multimeters, their significance in HVAC work, and provides guidance on choosing the right meter for various applications.
Understanding CAT Ratings
- CAT ratings indicate the safety level of a multimeter for specific electrical environments based on transient voltage risks.
- The International Electrotechnical Commission (IEC) sets CAT ratings: I, II, III, and IV.
- Higher CAT categories allow use in riskier (closer to power source) scenarios, e.g., CAT I for electronics, CAT IV for utility service.
- You can use a higher-category meter for lower-category tasks, but not vice versa.
CAT Ratings in HVAC Applications
- Most HVAC/R-specific meters are rated CAT III or CAT IV; some may also specify different voltages for each category.
- CAT II 1000V / CAT III 600V meters are suitable for most residential/light commercial HVAC work.
- For heavier commercial or industrial work (voltages above 600V), check that your meter's CAT rating and voltage are adequate.
CAT Ratings and Electrical Characteristics
- CAT ratings are determined using the concept of transient voltage, not just constant voltage.
- Multimeter impedance decreases as CAT rating increases: CAT I (30 ohms), CAT II (12 ohms), CAT III/IV (2 ohms).
- Lower impedance allows the meter to safely handle higher current without arcing.
Differences Between CAT III and CAT IV
- CAT III and CAT IV both have 2 ohms impedance, but CAT IV meters have higher transient voltage protection.
- CAT IV offers better safety for sudden voltage spikes, especially in utility or main service applications.
Meters with Multiple CAT Ratings
- Some meters list dual ratings, e.g., CAT III 1000V / CAT IV 600V, meaning they are safe for up to 1000V in CAT III environments, 600V in CAT IV.
- These meters are generally ideal for typical HVAC/R use.
Key Terms & Definitions
- CAT Rating — A safety category for electrical testers indicating maximum voltage and environment suitability.
- Transient Voltage — Short, high-voltage spikes that can occur in electrical systems.
- Impedance — The combination of resistance and reactance in a circuit; affects current flow.
- Ohm’s Law — Formula: V = I × R (Voltage = Current × Resistance).
Action Items / Next Steps
- Confirm your multimeter's CAT rating matches or exceeds the environments and voltages you work with.
- Review Ohm’s Law and understand how resistance and current interact in electrical circuits.
- For further learning, watch the “Multimeter 101 for HVAC” video linked in the lecture.