Fuse protects the motor; compared to circuit breaker protection, it has a higher breaking current (currently testable up to 300KA); installation and maintenance costs are low. This article will discuss practical applications based on the IEC and UL standard systems, covering both low-voltage and high-voltage fuses.
1. Low-voltage fuses:
1) Characteristics: IEC60269 standard (including Chinese standard GB/T13539, as the Chinese standard fully follows the IEC standard, implementing mandatory CCC certification or voluntary CQC certification): According to characteristics, fuses are divided into gM (fuse-links protecting the full range of motor circuit breaking capacity*) and aM (fuse-links protecting partial range of motor circuit breaking capacity). That is, gM fuses protect against both overload and short-circuit currents in the circuit; aM fuses only protect against short-circuit currents. In practice, gG and gM types can be used interchangeably, and gG and aM types are commonly paired.
*Definition of fuse (FUSE) and fuse-link (FUSE-LINK) according to IEV441-18.
2) Size classification of IEC standard fuses: Because IEC needs to accommodate many compatible sizes, they can be divided into tubular, NH (NT)**, British BS type, German D type, and American CC/J/T/L types***.
NH (NT)**: The new IEC standard uses the name NH. NH type fuses have a straight-edged, right-angled appearance, can include the body, and have dual indicators for blade direction; NT type fuses have a curved appearance, generally with a blade direction indicator; both are interchangeable in performance during use.
American CC/J/T/L types***: The new IEC standard is compatible with UL standard sizes. These fuses have unique advantages in safety design concepts, breaking current, size, etc., which will be explained later.
3) Selection considerations:
(1) Motor rated voltage;
The fuse's rated voltage must be higher than the motor's rated voltage and voltage fluctuations to prevent damage to equipment caused by arc voltage after fuse melting (which can reach up to 2.5 times the rated voltage);
(2) Motor rated current;
The motor's rated current is not the same as the fuse's rated current. As shown in the figure below (aM motor short-circuit protection schematic): current-limiting fuses must cut off fault current before it damages the motor; the fuse's rated current must consider several factors.
(3) Ambient temperature, cooling system, and altitude where the motor is used;
The operating temperature of fuses is generally defined between 20-30°C. Above 30°C, the rated current must be derated according to the temperature correction curve provided by the fuse manufacturer; if cooling methods such as air cooling are used, their effect on rated current must also be considered; IEC standard specifies fuse use at altitudes not exceeding 2000 meters, above which the rated current must be derated by 0.5% for every 100 meters increase in altitude;
(4) Effects of starting capacitors, grid harmonics, and recovery voltage after fuse melting on the motor;
Starting capacitors, grid harmonics, etc., impact the normal use of fuses and require increasing the fuse's rated value;
(5) Protection coordination between fuses, contactors, and thermal protection relays.
Since short-circuit type fuses only protect against short-circuit faults (the figure below shows the aM protection range as specified by IEC standards), the main reason is that under overload conditions, heat generated in the circuit causes fuse short-circuiting in a high-temperature environment, damaging the fuse structure and affecting other components in the circuit. Therefore, thermal relays are needed for protection within 60 seconds.