IEC TR 61641
IEC TR 61641
INTERNAL ARC TEST
If you specify an assembly as requiring an internal arc fault capability in accordance with IEC/TR 61641; what can you expect, what benefit will you get, are there more effective options for the particular application. MODUTEC provides some clarity.
INTRODUCTIONAssemblies that are correctly operated, properly maintained, well designed and verified in accordance with the IEC 61439 series very rarely incur an internal arcing fault in service. However, as fault levels increase, the potential consequences when such a fault occurs become more severe. This, together with the ongoing pursuit for ever greater safety, is bringing internal arcing faults within assemblies to the fore. In the belief that their personnel will be ‘safe’, increasingly, users of low-voltage assemblies are calling for them to comply with IEC/TR 61641; Enclosed low-voltage switchgear and Controlgear assemblies — Guide for testing under conditions of arcing due to internal fault. Since the document is a TR (Technical Report) it cannot be ‘complied with’ in the way that a full Standard can. Whilst in some circumstances specifying that equipment has to ‘comply’ with IEC/TR 61641 will lead to greater operator safety and reduced down time in the event of an arcing fault within an assembly, it is not a ‘cure for all ails’. Before specifying ‘compliance’ with this requirement an objective assessment should be carried out considering the whole application to determine if ‘compliance’ with IEC/TR 61641 brings worthwhile benefits. The factors to be considered are varied and at times very subjective. They include: what enhanced safety does ‘compliance’ with IEC/TR 61641 offer for the application being considered; when is an internal arcing fault most likely to occur, and; are there better options for increasing operator safety and process security. Making the right choice in respect of an assembly ‘complying’ with IEC/TR 61641 is complex. It requires a good understanding of the intended application for the assembly, knowledge of the design and use of assemblies and a detailed understanding of IEC/TR 61641. In addition, a skill in assessing risks is necessary. MODUTEC seeks to share this guide to assist users of low-voltage assemblies in making the right choice in respect of an internal arc fault capability for their application. Causes of internal arcing faults Internal arcing faults within assemblies occur for of a number of reasons, for example;
- Insulation breakdown;
- Contamination or moisture on insulation leading to tracking;
- Faults in materials or workmanship;
- Load current arcing across a loose or deteriorating joint;
- A foreign object within the assembly, e.g. a tool;
- Lack of, or inappropriate, maintenance;
- Inappropriate operating conditions;
- Incorrect operation;
- Entry of vermin;
- Operator errors while working adjacent to live equipment.
CHARACTERISTICS:A manufacturer offering an assembly as having an arc fault classification is obliged to provide details of its capabilities. The characteristics to be declared are set out in Clause 5 of IEC/TR 61641 and in general terms include, as applicable:
- Rated operational voltage.
- Arcing Class, A, B, C, or I.
- For Arcing Class B or C, the areas to which the arc will be confined.
- Prospective short circuit current at the incoming terminals of the assembly together with any upstream current limiting requirements.
INTERNAL ARCING FAULT TESTS:The prime aim of internal arcing fault tests on an assembly is to demonstrate, as far as reasonably practical, an improved level of safety for personnel in the vicinity of an assembly, should an internal arcing occur. The tests do not guarantee personnel in the vicinity of an assembly will not be frightened or suffer some injury as a result of the fault. However, it is expected the risk of severe burns, or worse, will be much reduced with an assembly that has been shown to have an ability to withstand internal arc faults. During the test clothing of personnel is simulated by ‘indicators’ around the assembly. The indicators are made of different grades of cotton to simulate light summer clothes and workwear (unrestricted and restricted access). A second reason for conducting internal arcing fault tests on the assembly is to demonstrate the impact of the fault on the assembly itself. In some instances, and as defined by the Arcing Class, there may be a requirement to confine arc damage to a part of the assembly, and in addition, with minimal maintenance, part of the assembly can be re-energised for emergency use. The process of conducting internal arc fault tests within an assembly is relatively straightforward. In simple terms, the assembly to be tested is connected to a supply capable of delivering the required prospective short circuit current for an appropriate time. Indicators are placed a short but defined distance from all sides (front, rear, left and right) of the panel being tested. Fuse wire is used to bridge all phases of main conductors in a three phase assembly or between phase and neutral in a single phase assembly. This is placed at the point where the arc is to be initiated. The supply is then energised and the test carried out. Noting that insulation on conductors is not removed to apply the fuse wire, sufficient tests are carried out on an assessed worst case basis to cover arcs initiated anywhere on the main conductor system;
- Test panel mounted on a simulated cable trench
- Arc fault initiated on the incoming cable terminals;
- indicators for unrestricted access;
- result deemed to provide protection for ordinary persons in accordance with IEC/TR 61641e.g. incoming supply terminals, complete busbar system, load and line side of all outgoing functional units.
IEC/TR 61641 is:
- Not a standard, but a technical report
- Not a part of the IEC 60439/61439 series
- Not a compulsory test
The Standard covers Short-circuit arising from a fault between active parts of different potential and/or between active parts and other conductive parts resulting in a free burning arc within an ASSEMBLY.
It also covers personal protection under arcing conditions, defining the ability of an ASSEMBLY to limit the risk of personal injury caused by the mechanical and thermal effects of an internal arcing fault.
Please note that this applies to enclosed low-voltage switchgear and controlgear assemblies when doors and covers are closed and correctly secured and it does not cover all effects which may constitute a risk, such as toxic gases.
MODUTEC Modular Switchboards Enclosure Systems are compliant with Internal Arc requirements as per IEC TR 61641. Handling this internal arc does not get limited to designing the panel to withstand the internal pressure developed during such a fault. It also extends to the subject of disposing these off. This gets done by carrying the arc products to a much safer place away from the people. This would involve adding the arc ducts over and above the standard MV panel. This carries away the arc products and also cools and absorbs the arc energy as it travels in the arc duct. Design and supply of arc ducts to suit specific installation site conditions will be taken up based on specific customer requirements
- the probability and severity of injury to personnel in the vicinity of an assembly incurring an internal arcing fault is much reduced;
- damage to a low-voltage assembly suffering the internal arcing fault is vastly reduced;
- many of the additional assembly design measure necessary to make an assembly more robust, so as to withstand a full arcing fault, are not necessary; and
- the feasibility of returning an assembly to normal service, quickly, after an arcing fault is significantly increased.
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