Why Choose Electronic Protection For DC 24 V Switch Mode Power Supplies?
- Published: Monday, 21 May 2018 13:28
In DC 24 V systems, usually primary pulsed switch mode power supplies are used for voltage supply. They have meanwhile become state-of-the-art and indispensable. They are characterised by a compact design and high reliability in continuous operation.
However, in the overload range they can offer only very limited power reserves. These are often only 1.5 times rated current. In the event of a failure a 20 A switch mode power supply will provide only 20 A x 1.5 = 30 A. Even if an overload or a short circuit would theoretically be higher, the power supply would protect itself. It would turn down the voltage at the output and thus limit the power. This means, however, that all other loads connected to the same power supply would no longer be supplied either. If a thermal or thermal-magnetic protection, e.g. a blade fuse or an MCB, is operated as a protection at such an output, it will not be able - for technical reasons alone - to disconnect. MCBs with a C characteristic require up to 15 times rated current for a quick magnetic trip. For a disconnection in the event of a short circuit, only the fast magnetic trip within 10 ms will be considered. According to the relevant standard EN 61131, sensitive loads such as storage-programmable control units must not be affected within 10 ms. For a circuit breaker with C characteristic and 6 A rated current, the required short circuit current ensuring a quick trip can easily be calculated in a worst case scenario: 6 A x 15 = 90 A.
In purely arithmetical terms, the power supply cannot provide this current. Cable lengths and their impedance additionally benefit this behaviour. Resistance of a 50 m long copper cable with a cross section of 1 mm² can be calculated as follows: 0.0175 Ohm x mm² / m x 50m / 1 mm² = 0.875 Ohm. Limited by the cable cross section, the maximum short circuit current is: I = U/R = 24 V / 0.875 Ohm = 27.42 A.
This is exactly the point where electronic circuit protectors have become standard and definitely indispensable in machinery. The advantages are obvious: The overcurrent protection, designed particularly for these applications, responds exactly to the requirements of the switch mode power supplies: On the one hand it is very fast in the event of a short circuit, and on the other hand it responds a little slower when switching on high current-consuming loads.
Quick and correct connection or disconnection is possible because modern overcurrent protection quickly detects and evaluates not only the current, but also other important measuring values such as voltage or temperature and compares them with specified values. The general trip values are much lower than with conventional thermal-magnetic systems. Electronic overcurrent protection is exactly tailored to the needs of the switch mode power supplies and can easily be adjusted.
Using electronic circuit protectors always has the goal to ensure stable operation of switch mode power supplies, ease of trouble-shooting and maximum machine uptime. Machine and panel builders also often request a modular design, ease of handling and a clear and transparent system design. Electronic circuit protectors meeting these requirements can be mounted side by side and can easily be added later. Replacement of devices can also be accomplished quickly and easily.
E-T-A’s REX system (fig. 1) allows the customer to build up a very economic DC 24 V supply with a modular and cost-effective protection and distribution solution. Without any connection accessories and with minimum wiring time. The modular devices can easily be snapped onto a symmetrical rail and the connector arms automatically provide electrical connection.
If you wish to replace or exchange one of the circuit protectors, it is sufficient to open the left and right connector arm and remove the unit in question. Afterwards the new device is snapped in and the connector arms are closed. The new system saves 50 percent of working time compared to conventional products.
A detailed description of the new system and its components
The EM12 supply modules for the power input of the REX system are available in different versions, providing genuine flexibility with regard to costs and functionalities. Apart from the conventional EM12-T-… supply modules with integral group fault signalling by means of a relay contact, there are also the intelligent and communication-capable EM12D-TIO-… supply modules for IO link as well as EM12D-TMB-… for Modbus-RTU. They provide transmission of a great amount of diagnostic information to the superordinate IO link or Modbus-RTU master. These data serve as a basis for remote maintenance.
They are the basic pre-condition for storage and evaluation of all data received from machinery and systems in the sense of industry 4.0. At the same time, they are the basis of solutions on the score of »condition monitoring« and »predictive maintenance«. Intelligent systems help improve transparency and consistency of the entire system.
In addition we have supply modules for further potential line entries. There are two different approaches. The supply modules connect either all DC 24 V supply terminals or serve as + DC 24 V disconnect terminals for supply of battery-buffered or safety voltage potentials.
The REX12 electronic circuit protector combines flexibility with a compact design. It is available as a single-channel or double-channel model, can be designed conventionally or suitable for communication via IO link or Modbus-RTU. REX12 ensures space-saving and reliable protection of primary pulsed DC 24 V switch mode power supplies. The focus is on the stable operation of switch mode power supplies, on easy trouble-shooting and an unimpaired machine uptime. And what is more: no additional accessories are needed to connect the individual components electrically and mechanically. The REX12 exactly meets the technical and economic requirements of the machine construction industry. The single-channelled circuit protector is available in all standard fixed current ratings from 1 A to 10 A. The double channel models are available in the product versions with fixed current ratings 1 A, 2 A , 3 A, 4 A and 6 A.
Electronic protection of DC24V switch mode power supplies in compliance with the relevant standards
When using electronic protection devices, it has to be clarified if DC 24 V protection with physically isolating components is mandatory. The following standards are relevant: DIN EN 602042; EN 0113-1:2007-06 Safety of machinery – electrical equipment of machines - part 1: general requirements. Users in the machine building industry have to strictly observe the harmonised European standard DIN EN 60204-1 (VDE 0113-1).
The subject of overcurrent protection is dealt with in para 7.2. Regarding overcurrent protection for conductors, the following stipulations are listed in appendix D.3.:
»The use of fuses with trip characteristics gG or gM (see IEC 60269-1) and of power switches with the trip characteristics B and C e.g. To series IEC 60898 (industrial use: IEC 60947-2) ensures that the temperature limits of table D.5 are not exceeded provided the current rating In was selected to table 6, where In ≤ Iz. In practice the requirements to section 7.2 are met when the protective element interrupts the current within 5 s. This does not exclude that, particularly in the event of control circuitries, also other protective elements may be used which meet the conditions (cf. 7.2.4, 12.4 D.3). If the protective element meets the requested limits for the protection of the existing cable cross section, ampacity of conductor, possible overcurrent at installation site, trip characteristics, rupture capacity etc., it meets the requirements.«
Circuits must be protected against overcurrent. The section for the overcurrent protection of control circuits (see section 7.2.4 of DIN VDE 0113-1) says that these circuits have to be protected by overcurrent protection equipment. It is not described there how exactly this has to be done. The IEC 60364-5-53 is referred to.
In Germany, this international installation standard is replaced by the standard »Errichten von Niederspannungsanlagen – Auswahl und Errichtung elektrischer Betriebsmittel –Schalt- und Steuergeräte« (DIN VDE 0100-530). Section 533 of the DIN VDE 0100-530 describes the overcurrent protection equipment which may be used, e.g. blade fuses and MCBs. All protective devices listed there are designed so as to be able to physically isolate these circuits. Electronic fuses are not mentioned explicitly.
When can we use electronic protection nevertheless?
As this is about the protection of a control circuit with safety extra-low voltage, physical isolation is not required for protection against personal injury, but it is mandatory for cable protection. This requirement is indicated in section 533 of the DIN VDE 0100-530. This standard is referred to in the DIN VDE 0113-1.
But this requirement does not entirely prohibit the use of electronic fuses. If electronic overcurrent protection is used, overcurrent protection equipment to DIN VDE 0100-530 needs to be connected in series in addition to the electronic circuitry. It can also be included as a fail-safe element within the electronic fuse. It is compulsory to rate the cable cross section in accordance with the current rating of the fail-safe element and not in accordance with the rating of the electronic fuse.
The internal fail-safe element in the shape of a blade fuse in E-T-A’s REX12 circuit protector is directly tailored to the rating of the circuit protector’s electronic circuitry. It ensures an easy adjustment to the cable cross section. Concretely, this means that the current rating of the protector and the rating of the fail-safe element are identical. Thus the REX12 rated 4 A holds a 4 A blade fuse to IEC 60127-4/2 and to UL248-14. Besides the UL508listed approval and NEC Class2, the REX12 exclusively meets the requirements of cable protection to EN60204-1 (fig. 2).
The power distribution concept of the REX system holds two main groups: In the same system, the user can easily realise not only the + DC 24 V distribution, but also the minus distribution 0 V (GND). The new PM12-T distribution modules for the + DC 24 V distribution are mounted side by side with REX12-T and can electrically be connected with them. This increases the number of terminals, saves space and conventional distribution terminals are no longer required.
The 0 V potential is connected to the EM12-T supply module for GND and is then multiplied for DC 0 V (GND) by means of the PM12-T module. These components can also conveniently be connected and wired up. The distribution solution for DC 0 V is suitable for 40 A rated load. Rating of components is made easy for the design engineers. Complex design solutions to reduce cable cross sections from 10 mm² to 2.5 mm² are a thing of the past.
The solution for use in practice: REX12
The internal fail-safe element in the shape of a blade fuse is adapted directly to the current rating of the corresponding circuit protector, thus ensuring ease of adjustment to the cable cross section
Concretely, this means that the current rating of the protector and the rating of the fail-safe element are identical. The REX12 rated 4 A holds a 4 A blade fuse to IEC 60127.
REX12 with a fixed current rating allows standard-compliant cable protection to EN60204-1 even with cable cross sections smaller than 1.5 mm².
E-T-A’s REX12 exclusively meets the requirements of cable protection to EN60204-1.