source. Unfused meters are often quickly destroyed by such errors; fused meters often survive. Fuses used in meters must carry the maximum measuring current of the instrument, but are intended to disconnect if operator error exposes the meter to a low-impedance fault. Meters with inadequate or unsafe fusing were not uncommon; this situation has led to the creation of the IEC61010 categories to rate the safety and robustness of meters. 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 subcircuits Category III: used on permanently installed loads such as distribution panels, motors, and three-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 rating also specifies maximum safe 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 be used to protect attached equipment against high voltage in the measured circuit. Good quality multimeters designed to meet Category II and above standards include high rupture capacity (HRC) ceramic fuses typically rated at more than 20 A capacity; these are much less likely to fail explosively than more common glass fuses. They will also include high energy overvoltage MOV (Metal Oxide Varistor) protection, and circuit over-current protection in the form of a Polyswitch. Meters intended for testing in hazardous locations or for use on blasting circuits may require use of a manufacturer-specified battery to maintain their safety rating. DMM alternatives A quality general-purpose electronics DMM is generally considered adequate for measurements at signal levels greater than 1 mV or 1 μA, or below about 100 MΩ; these values are far from the theoretical limits of sensitivity, and are of considerable interest in some circuit design situations. 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 1 TΩ) and picoammeters. Accessories for more typical multimeters permit some of these measurements, as well. Such measurements