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Posted on 01 May 2019

Introduction to IEC 60269

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gS Type Fuses

In circuits where there are no overloads requiring the gG fuse curve it is possible to use a gS fuse and optimise the protection by reducing the peak current and then improving the coordination with all associated switches and components.

By Charles Müllert, Ferraz Shawmut

 

The gS type fuses are new fuses defined by the IEC 60269-4 standard. They are fuses designed for the protection of semi conductors and cables.

The IEC 60269-4 4 main sections:
• IEC 60269-1: general section
• IEC 60269-2: protection systems with fuses for authorized persons (industrial applications)
• IEC 60269-3: protection systems with fuses for unskilled persons (household applications)
• IEC 60269-4: protection systems with fuses for the protection of semi conductors (special applications)

IEC 60269 fuse types (or classes) are defined with two letters.

The IEC 60269 defines two large families of fuses with the first letter:
• The « g » fuse: general purpose fuse; it will interrupt all faults between the lowest fusing current and the breaking capacity. Such a fuse is able to protect against overloads and short circuits.
• The « a » fuse: this fuse can not interrupt faults below a specified level. Such a fuse is generally used to provide short circuit protection only. The « a » fuse must be associated to another protective device designed to interrupt overloads.

The second letter indicates the utilization category (or application type):
G = cable and conductor protection
M = motor circuit protection
R = semi conductor protection
S = semi conductor protection
Tr = transformer protection
N = North American conductor protection
D = North American “Time Delay” for Motor circuit protection

Most well-known fuses are : gG, aM, gM, aR, gR, etc.

Note: FERRAZ SHAWMUT fuses type URB,URC, URD, URE etc are all IEC aR type fuse. The UR- designation is necessary because FERRAZ SHAWMUT offers a lot of fuses having the same rated voltage and rated current but with different I²t . They can not be in a same group with the same designation. However the marking of the fuse shows the URD designation as well as the aR designation as per IEC 60269.

The gS fuse is then a fuse for semi conductor protection able to interrupt low overloads and short circuits. The gR fuse is able as well to interrupt low overloads and short circuits. The difference between the two fuse types is only in the non melting current value.

The gS fuse will not melt when the overload is below 1.25 In (In is the rated current of the fuse). The gR fuse will not melt when the overload is below 1.10 In (In is the rated current of the fuse).

The top of the prearc curve (Time / current curve) of the gS fuse and the gG fuse are defined in the same manner. For both fuses the conventionnal non melting current is 1.25 In and the conventionnal melting current is 1.60 In.

The curves are different for times between 1 hour and 1 miliisecond:
• the gG fuse must comply with non melting gates and melting gates between 10 s and 100 ms.
• the gS fuse does not need to comply with any gates and then can melt for currents lower than the non melting gates of the gG fuse.

This is why the gS fuse can be fast enough to protect semi conductors.

As an example figure 1 shows the difference between the prearc curves of the gS 250 A fuse and the gG 250 A fuse as well as the large difference of the i².

Difference between the prearc curves of the gS 250 A fuse and the gG 250 A fuse as well as the large difference of the i²

Introduction to the 690 V gS fuses in the FERRAZ SHAWMUT NH technology

This fuse range is using the NH technology (figure 2) so that all corresponding fuse holders and fusegears can be used. For such fuses it is not necessary to derate their operating current as their power consumption comply with the maximum wat losses of the standard accessories. In an existing circuit protected by a gG fuse it is not recommended to propose its replacement by a gS fuse. As a matter of fact the gS fuse, due to its speed, can be subjected to undesired interruption of normal transient currents (inrush currents or currents resulting from acceptable overvoltages). It is then possible to replace a gG fuse by a gS fuse with the same current rating in circuits where there are not overloads requiring the non melting gates of the gG fuses. Therefore circuits feeding a current converter controlling all overloads can be protected by a gS fuse. This fuse will be able to protect the conductors as well as the semiconductors of the converter.

This fuse range is using the NH technology

These fuses have temperature rises much lower than very fast acting URD type (table 1).

Temperature rises

Consequently the calculation of the rated current of a gS fuse is using easier corrective coefficients (table 2).

Using easier corrective coefficients

It can be seen the ratio between the rated current of an URD fuse and a gS fuse is often 1.5 and sometimes more. The rated current of the gS fuse will always be smaller than the rated current of an URD type fuse.

For this reason the i²t of the URD fuse must not be compared to the I²t of a gS fuse having the same rated current. For example (table 3) the gS 250 A fuse must be compared to a PSC URD 400 A or the gS 400 A must A gS fuse may replace 2 fuses.

The rated current of the gS fuse will always be smaller than the rated current of an URD type fuse

In an installation there are often circuits where two different types of fuses are fitted in two different cubicles at each end of a cable (figure 3). They are in series. A new calculation of the fuse protecting the speed variator shows the NH 690 V URD fuse rated 450 A can be replaced by a 690 V gS 315 A fuse. But the same 690 V gS 315 A fuse can replace as well the gG 300 A fuse upstream. As a matter of fact the gS fuse is able to protect the 185 mm² cable connecting the two cubicles to each other.

There are often circuits where two different types of fuses are fitted in series

Finally it is not necessary to use a fuse inside the cubicle containing the speed variator since it is enough to replace the gG 300 A fuse by the gS 315 A fuse in the upstream cubicle (figure 4). This brings also the advantage to reduce the temperature rise inside the cubicle containing the speed variator. be compared to a PSC URD 630 A or even to a 700 A according to the type of equipment to be protected.

It is enough to replace the gG 300 A fuse by the gS 315 A fuse in the upstream cubicle

Conclusion

The gS 690 V fuses are well adapted to the protection of cables and static converters. Moreover in circuits where there are no overloads requiring the gG fuse curve it is possible to use a gS fuse and optimise the protection by reducing the peak current and then improving the coordination with all associated switches and components.

 

 

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