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3 Phase Power Meter – Buy Phase Sequence Meters
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.
- 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.
On meters that allow interfacing with computers, optical isolation may protect attached equipment against high voltage in the measured circuit.
Good quality multimeters designed to meet CAT II and above ratings will include High Rupture Capacity ceramic fuses typically rated at more the 20kA breaking capacity. They will also include high energy overvoltage MOV (Metal Oxide Varistor) protection, and circuit over-current protection in the form of a Polyswitch.
A general-purpose electronics DMM is generally considered adequate for measurements at signal levels greater than one millivolt or one microampere, or below about 100 megohms—levels far from the theoretical limits of sensitivity. Other instruments—essentially similar, but with higher sensitivity—are used for accurate measurements of very small or very large quantities. These include nanovoltmeters, electrometers (for very low currents, and voltages with very high source resistance, such as one teraohm) andpicoammeters. These measurements are limited by available technology, and ultimately by inherent thermal noise.