Multimeter - Safety

Safety

All but the most inexpensive multimeters include a fuse, or two fuses, which will sometimes prevent damage to the multimeter from a current overload on the highest current range. A common error when operating a multimeter is to set the meter to measure resistance or current and then connect it directly to a low-impedance voltage source. Unfused meters are often quickly destroyed by such errors; fused meters often survive. Fuses used in meters will carry the maximum measuring current of the instrument, but are intended to clear if operator error exposes the meter to a low-impedance fault. Meters with unsafe fusing are not uncommon, this situation has led to the creation of the IEC61010 categories.

Digital meters are rated into four categories based on their intended application, as set forth by IEC 61010 -1 and echoed by country and regional standards groups such as the CEN EN61010 standard.

  • Category I: used where equipment is not directly connected to the mains.
  • Category II: used on single phase mains final sub-circuits.
  • Category III: used on permanently installed loads such as distribution panels, motors, and 3 phase appliance outlets.
  • Category IV: used on locations where fault current levels can be very high, such as supply service entrances, main panels, supply meters and primary over-voltage protection equipment.

Each category also specifies maximum transient voltages for selected measuring ranges in the meter. Category-rated meters also feature protections from over-current faults.

On meters that allow interfacing with computers, optical isolation may protect attached equipment against high voltage in the measured circuit.

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