In this article by Eaton MEM's Ray Upton, he examines the reasons why the harmonised standard for circuit breakers, BS EN 60947-2 has just been revised for the second time, and the effect it will have:
The harmonised standard for circuit breakers, BS EN 60947-2 has just been revised for the second time. The most significant changes affect requirements for earth leakage protection, electromagnetic compatibility (EMC) and test procedures. However, the good news for contractors is that these changes should have no significant impact on the way they select and install circuit breakers.
The background to this second revision:
BS EN 60947-2 forms part of the harmonised European standard EN 60947: Specification for low voltage switchgear and controlgear which is itself based on the International standard IEC 60947: Low-voltage switchgear and controlgear. The first Edition of IEC 60947-2, published in 1989 as IEC 947-2, introduced significant changes to earlier practice, including new utilisation categories, insulation co-ordination, sequential testing procedures and the use of rated ultimate short-circuit capacity and rated service short-circuit capacity to replace the old P1 and P2 ratings.
The harmonised European standard BS EN 60947-2 was published in 1992 and the Second Edition appeared four years later in 1996. The latest Edition was approved as an International standard by the International Electrotechnical Commission in April 2003 and by CENELEC as a harmonised European Standard in June. The IEC version is a bi-lingual (French and English) 425-page publication.
The standard applies to circuit breakers up to 1000V AC or 1500V DC for industrial and similar applications. It covers both moulded-case circuit breakers (MCCBs) and air circuit-breakers (ACBs). Miniature circuit breakers (MCBs) are covered by their own standard BS EN 60898: Specification for circuit breakers for overcurrent protection for household and similar applications. However, manufacturers can test their MCBs to BS EN 60947-2 for use on industrial or commercial installations.
A new Edition has become necessary, not because of any failings in the old standard, which will not be withdrawn until 1 May 2006, but because of continuing technological change and new legislation, in particular the requirements of the European Electromagnetic Compatibility Directive.
Residual current detection:
Many circuit breakers incorporate residual current protection either as an integral function or as a standard modular option. Industrial-type circuit breakers incorporating residual current protection are known as CBRs, they perform the same functions as the RCBO (residual current circuit-breaker with overcurrent protection) but for higher current ratings. Some manufacturers also offer separate earth leakage monitoring systems that interface with circuit breakers.
A new requirement calls for the CBR to continue to trip if the supply voltage drops to 70% of normal voltage on a three-phase system or to 85V on a single-phase system. The test procedure, included in Annex 'B' to the standard, also calls for the three-phase test to be carried out under conditions of loss of one phase; the test is repeated with each phase disconnected in turn.
This requirement was introduced after studies into the possibility of failure during a voltage drop, for example when a low-impedance fault results in a serious overload which pulls down the applied voltage. It applies to both electromechanical devices (where the residual current is used to trip the circuit-breaker directly) and to voltage-dependent electronic devices (which employ an electronic amplifier to energise the tripping mechanism). It takes account of the fact that overload releases will generally trip out the circuit breaker at a fault current higher than described above.
Earlier editions of the standard were restricted to CBRs with an integral or add-on modular residual current device. The latest Edition includes a new annex covering separate "modular residual current devices" (MRCDs) which provide an output to operate the tripping function of a separate circuit-breaker. Under these conditions it is important that the MRCD provides an output that is compatible with the circuit breaker's tripping mechanism and that the overall system is itself self-protecting.
DC currents:
Residual current devices may be classified as Type AC, Type A or Type B as follows:
- Type AC - Protection against AC earth leakage currents.
- Type A - Protection against AC leakage currents that contain pulsating DC components.
- Type B - Protection against smooth DC leakage currents.
Previous Editions of IEC 60947-2 did not include Type B, but this is now included in the requirements for modular residual current devices to take account of the use of rectifying circuits downstream of the circuit-breaker.
The Type B MRCD must operate under the following conditions:
- Residual sinusoidal AC leakage currents.
- Residual pulsating direct currents.
- Residual pulsating direct currents superimposed by a smooth direct current of 6mA.
- Residual currents which may result from rectifying circuits.
Four examples of rectifying circuits are given:
- Single-phase connection with capacitive load causing smooth direct current.
- Two-pulse line-to-line bridge connection.
- Three-pulse star connection.
- Six-pulse bridge connection.
These may, or may not, involve phase angle control and may be suddenly applied or slowly rising fault currents.
Electromagnetic compatibility (EMC):
Requirements for electromagnetic compatibility - the ability of the circuit-breakers to function satisfactorily in the electromagnetic environments in which they are installed, without introducing intolerable electromagnetic disturbance to anything in that environment - have been rationalised in the new Edition of IEC 60947-2.
The European Electromagnetic Compatibility Directive, 89/336/EEC was first published in May 1989. Compliance became mandatory on 1st January 1996. Consequently, EMC requirements have been incorporated into earlier editions of IEC 60947-2 piecemeal as they were formulated. The Third Edition seeks to bring these requirements and tests together on a more rational footing through a new Annex J Electromagnetic compatibility (EMC) - Requirements and test methods for circuit breakers prepared in collaboration with CENELEC, The European Committee for Electrotechnical Standardisation.
The immunity requirements in IEC 60947-2, that is the requirements for the impact of the electromagnetic environment upon the circuit breaker, are more severe than those for other low voltage equipment. This is because the role of the circuit breaker is to protect the installation, making it necessary to apply additional safety factors.
Yet another annex, Annex N, gives additional requirements and test methods for the accessories - shunt trips, undervoltage releases, closing coils, etc., which are not covered by the EMC requirements of other parts of the standard.
Other changes:
Other changes to the standard relate largely to test procedures and are aimed at clarification and streamlining of these procedures; they have little or no impact upon either the quality of the product or its application. For example a new 'resetting time' has been introduced to give guidance to the test house when carrying out short-circuit tests. This indicates the time that should be left, to allow the thermal trip mechanism to recover after a test, before resetting the breaker ready for the next test.
Another, relatively minor, change is the introduction of a special symbol to be attached to three-phase circuit breakers with RCD function which are not suitable for single-phase applications. This symbol (the legend "1Ph" in a circle, crossed out by a diagonal bar) applies only to the residual current function where the 85V minimum operating voltage requirement for single-phase devices could not be satisfied by devices intended for three-phase operation.
Where to go from here?
BS EN 60947-2 is identical to the International Standard IEC 60947-2. This was prepared by Working Group 5 of IEC Technical Committee 17B Low-voltage switchgear and controlgear. Under IEC procedures, this working group has now been wound up and replaced by a 'maintenance team' MT15 which has to monitor the standard and prepare amendments and any future Editions.
It is a characteristic of standards work that activity frequently commences on the next Edition of a standard, or amendments to the latest Edition, almost as soon as a standard is published. This is true of IEC 60947-2, where work has already started.
A major element of the next stage will relate to communications. Modern circuit breakers increasingly form part of a system where people want remote monitoring, remote operation, tripping and load-shedding operations.
BS EN 60947-2 and the electrical contractor:
It should be apparent from the foregoing that the implications for the contractor are positive rather than negative. The fundamental requirements for circuit breakers and the way in which they are specified remain unchanged. The changes largely affect manufacture and testing, particularly in relation to residual current protection, and should give the installer even greater confidence in the quality and suitability of the product.
Unlike other standards, re-certification of circuit breakers to this new Edition will not be necessary. This has been built into the document by WG5. Indeed specifiers and installers can generally continue to specify, purchase and install circuit breakers to BS EN 60947-2 without concern as to whether they have been manufactured to the Second or Third Edition.
After 1st May 2006 they can assume that any circuit breaker which they purchase complies with the new Edition. Only where there are concerns about DC components in the waveform may it be necessary to look more carefully into the characteristics of the residual current device.
Biography:
Ray Upton has been a UK Expert on IEC Technical Committee 17B, Working Group 5, working on all aspects of low voltage switchgear but particularly circuit breakers. He worked on the First, Second and Third Editions of IEC 60947-2 and has recently been appointed convenor of the maintenance team, MT15. He has worked for many years in the UK electrical industry, mainly in test and design, and joined MEM 30 years ago as a specialist in switchgear and motor control gear.
Contact: Richard Hunt
Eaton Electric Ltd.,
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