Sep 4, 2019

Molded case circuit breaker with new characteristic curve


elcome dear friends of electrical engineering. In this article you will learn about a successful selectivity optimization within molded case circuit-breakers from Siemens. Have fun while reading.

Excellent selectivity values through cost-effective combination

Especially in an industrial environment, it has enormous advantages if, in the event of a fault, only the protection equipment that is directly upstream is tripped:  as only the affected feeder is turned off, all other system components remain in operation and more significant downtime is avoided. In addition to this, the cause of the fault can be located quickly. Until now, however, the required selectivity values could only be achieved with a combination of two molded case circuit breakers. With the Siemens 3VA molded case circuit breakers, a significantly more cost-effective combination of molded case circuit breaker and fuse can be selectively graded throughout – thanks to the new electronic trip unit ETU340 ELISA.

ETU340ELISA, Kompaktleistungsschalter 3VA, Kennlinienvergleich, Siemens, schutztechnik, niederspannung, selektivität
Quelle: Siemens

Molded case circuit breakers are amongst the most important protective components in power distribution. They reliably protect both downstream system components such as cables and lines, as well as electrical devices, from overload and short circuits. Fuses, on the other hand, as a lower-cost alternative to molded case circuit breakers, are generally used in the last or penultimate protection level. In modern low-voltage systems, in many cases, fuses with gG characteristics are used. This means: all-range fuses for general use. These individually protect smaller feeders or loads.

If fuses are used, these inevitably meet molded case circuit breakers in the sub-distribution or main distribution boards. Until now, this combination couldn’t be optimally selectively and cost-effectively graded. In standard molded case circuit breakers, the tripping characteristics are significantly different to that of a fuse (current-time characteristics). This means that the tripping characteristic curves of upstream molded case circuit breakers and downstream fuses can overlap. Selectivity is therefore usually lost as a result: the upstream circuit breaker trips and turns off all downstream feeders and loads. Until now this problem could only be solved with larger molded case circuit breakers, which often created unnecessarily high costs.

A development by Siemens has now created an alternative option that is more efficient in all respects: The implementation of a fuse characteristic curve (as opposed to a conventional LSI characteristic curve) in a molded case circuit breaker allows, for the first time, selective grading to a downstream fuse. Circuit breakers upstream of the fuses can therefore be smaller than was possible for conventional solutions. As well as being a technical optimization, it also leads to substantial cost savings.

New current-time characteristic curve simplifies configuration

The ETU340 ELISA has similar tripping characteristics to a fuse in the entire overcurrent range. This means that the distance between the rated value of the fuse and the rated current In for the molded case circuit breaker is reduced to the lowest possible value within which complete selectivity can be achieved. The result: the rated current intervals between the circuit breaker and the fuse can be selected as needed. This means that the circuit breakers have lower rated currents, so that smaller, more economical sizes can be installed.

A sample characteristic curve shows what this means in practice (see Fig. 1): with a downstream 100 A fuse, previous selectivity considerations would require an upstream molded case circuit breaker with a rated current In of at least 250 A. With the ETU340 ELISA, on the other hand, there is already full selectivity between a downstream 100 A NH fuse system and an upstream 3VA2 molded case circuit breaker with a rated current In of 160 A. The implementation of selectivity is therefore much more economical.

ETU340ELISA, Kompaktleistungsschalter 3VA, Kennlinienvergleich, Siemens, schutztechnik, niederspannung, selektivität
Kennlinienvergleich zwischen Kompaktleistungsschalter3VA2 mit konventioneller LSI-Kennlinie (links) und mit der neuen ETU340ELISA-Kennlinie (rechts)

As well as efficient implementation of full selectivity, the ETU340 ELISA offers further advantages with regard to planning, commissioning and operation. Configuration is as simple as that of a fuse. Implementation of characteristic curves in the ETU340 ELISA 3VA2 molded case circuit breaker allows a rated current sliding scale for the circuit breaker   at a rate of 1.6: 1 (similar to a fuse). This creates additional graded levels in low-voltage distribution. Adjustability of the ELISA tripping characteristics – adjustable rated current – means that smaller, streamlined cable diameters can be selected. This is because the response value of the overload release can be adjusted for individual system currents. Additional steps are not necessary. Errors when setting the short-circuit protection, and therefore also the selectivity settings, are automatically eliminated, as the short-circuit protection is not adjustable. An additional external expansion module, EFB300, allows output signals from the ETU, for example with the reason for tripping (overload or short circuit), or an overload warning. After tripping and error elimination, the molded case circuit breaker can be immediately switched on again; no additional spare fuses are necessary.

Modular molded case circuit breaker system

The electronic characteristic curves with ETU340 ELISA are available in the 3- and 4-pole molded case circuit breaker of the 3VA2 series, with a rated current of 100 A to 1.000 A. The 3VA molded case circuit breaker series from Siemens is part of the Sentron portfolio for low-voltage power distribution. It covers a broad spectrum of use, from standard applications in buildings to sophisticated industrial systems. The 3VA2 series, which also encompasses the molded case circuit breaker with ELISA characteristic curve, is designed for use in applications with higher technical requirements, and a rated current of up to 1.000 A. The molded case circuit breakers can be custom configured with over 500 accessories, with the option for a modular expansion to include numerous functions.

Bottom line

The electronic trip unit ETU340 ELISA for the 3VA molded case circuit breaker series from Siemens has a fundamentally new current-time characteristic curve. Overlap with the characteristic curve of downstream fuses is prevented. With full selectivity, the upstream molded case circuit breaker can therefore be designed to be smaller and more cost-effective. In addition to this, the new function optimizes effort and expense in planning, commissioning and operation.

Source of all images: Siemens