Tripping Characteristics of NH Fuse-links – What Do gG, aM mean..

A fuse protects low-voltage electrical conductors and cables from short circuits and overloads. There are different types of fuses, such as type D, type DO, NV, cylindrical, etc. Various markings on a fuse or fuse link indicate important parameters, including the rated current In (e.g., 20A in Picture 1), rated voltage U (e.g., 500V ), and short-circuit capacity (e.g., 120kA). This article focuses on the meaning of the gG marking, specifically the significance of the letter g (as opposed to a), which indicates the tripping or current-time (I-t) characteristic of the fuse.

Picture 1: low-voltage fuse-link NV 20 A type gG

Overload and Short Circuit

To illustrate the difference between overload and short circuit, consider this simple example:

An overload occurs when an electrical current exceeds the permissible limit of a conductor or cable. The electrical designer selects the appropriate cable cross-section based on the expected load of each circuit, and a matching fuse-link is chosen accordingly (where the rated current in amperes is crucial).

If the current exceeds the permissible limit, the fuse must respond and disconnect the circuit. For example, suppose multiple electrical devices are connected to a 20A fuse due to an incorrectly installed electrical system, leading to a current of 40A (twice the rated current). This is an overload, and the fuse must trip, as a 4mm² cable cannot withstand such a high current for long (leading to insulation degradation or even fire risk due to excessive heating). In this case, the fuse would disconnect in approximately 152 seconds, with some tolerance (see Picture 2).

A short circuit, on the other hand, occurs when the phase conductor (with black or brown insulation) comes into direct contact with the neutral conductor (with blue insulation). Due to the very low resistance of this connection, an extremely high short-circuit current flows—often several hundred or even thousands of amperes. The fuse reacts almost instantly, disconnecting the circuit within a few milliseconds.

Tripping (I-t) Characteristic of a Fuse-link

The tripping characteristic (or I-t characteristic) defines how quickly a fuse-link will interrupt a circuit in the event of an overload or short circuit.

Simply put, this characteristic indicates how fast the fuse will cut off power when an issue occurs in the electrical installation.

There are two main types of fuse tripping characteristics:

1. g-type: Fuses with a g characteristic provide protection across the entire current range and are commonly used in standard electrical installations.
2. a-type: Fuses with an a characteristic reliably trip only at a certain multiple of the rated current (e.g., they trip when the current exceeds approximately 5 times the rated value). These fuses are used for specific applications.

Tripping Characteristic of gG Fuse-links

Picture 2 shows the I-t characteristic of a gG 20A fuse-link.

• The capital letter "G" means that the fuse is intended for general-purpose applications and is the most widely used type.
• The lowercase "g" indicates that the fuse provides protection across the entire overload range (as shown in Picture 2).

If the current is 20A or less, the fuse does not trip. However, when the current exceeds 20A, the fuse trips faster as the current increases. The characteristic curve illustrates four overload scenarios:

• I = 1.3 × In
• I = 1.6 × In
• I = 2 × In
• I = 3 × In

For a short circuit, where the fault current is hundreds or thousands of amperes, the fuse trips immediately (see Picture 2).

Picture 2: printout using the ETISON program, fuse-link 20 A, gG, where it can be seen that, for example, at twice the rated current, the tripping time will be around 152 s

Table 1 shows the tripping times at different overload currents (x In) of a 20A gG fuse, based on Picture 2, which shows the I-t characteristic of a 20A gG fuse-link.

 Table 1: Tripping times at different overload currents (x In ) of a 20 A gG fuse-link.

When is an "a" Type Fuse-link Used?

Picture 3 presents the I-t characteristic of an aM 20A fuse-link.

It clearly shows that in the overload range (2×, 3× In), the aM fuse does not trip at all. This means that for the previously mentioned overload cases, the fuse-link would not interrupt the circuit. It only trips when the current reaches approximately 100A (around 5× the rated current, or slightly earlier).

Thus, aM fuses are suitable only for short-circuit protection.

The capital letter "M" signifies that this fuse is designed for motor protection. In such applications, the fuse effectively protects against short circuits, while the motor and cables are protected against overloads by a bimetallic relay or a motor protection switch.

From Picture 3, it is evident that at 5× the rated current, the tripping time is about 18 seconds, which is acceptable. If a motor had a startup current of 5× its rated current, the fuse would not unnecessarily trip during the motor's startup phase.

Picture 3: printout using the ETISON program, fuse-link 20 A, aM, where it can be seen that, for example, at five times the rated current, the tripping time will be around 18 s

Conclusion

This article demonstrates that using an aM fuse-link for general applications would be very dangerous, as it does not provide overload protection.

Electricians sometimes refer to aM fuses as "slow fuses", but this is misleading. While they are slower than g-type fuses for higher multiples of the rated current (e.g., at 5× In), they do not react at all to overloads below this level.

This means aM fuses should only be used in specific applications where overload protection is provided by a separate device, such as a bimetallic relay or a motor protection switch.