1、BRITISH STANDARD BS7351:1990 Specification for Metal-sheathed heating elements for industrial useBS 7351:1990 This British Standard, having been prepared under the directionof the Power ElectricalEngineering StandardsPolicy Committee, waspublished under the authorityof the Board ofBSIandcomes intoef
2、fect on 30September1990 BSI07-1999 The following BSI references relate to the work on this standard: Committee referencePEL/87 Draft for comment87/26023 DC ISBN 0 580 18861 2 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Power Electric
3、al Engineering Standards Policy Committee (PEL/-) to Technical Committee PEL/87, upon which the following bodies were represented: Association of Manufacturers Allied to the Electrical and Electronic Industry (BEAMA Ltd.) British Glass Manufacturers Confederation British Lighting Association for the
4、 Preparation of Standards (BRITLAPS) British National Committee for Electroheat British Steel plc Committee for Electrical Equipment for Use in Flammable Atmosphere (BEAMA Ltd.) Electric Trace Heating Industry Council (ETHIC) Electricity Association Energy Industries Council Engineering Equipment an
5、d Materials Users Association Engineering Industries Association Health and Safety Executive Induction and Dielectric Heating Manufacturers Association Institution of Electrical Engineers Lighting Industry Federation Ltd. Ministry of Defence Amendments issued since publication Amd. No. Date Comments
6、BS7351:1990 BSI 07-1999 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1 2 Definitions 1 3 General requirements 1 4 General notes on tests 1 5 Rating 1 6 Marking 1 7 Rated power 2 8 Electrical insulation and leakage current at operating temperature 2 9 Electrical insul
7、ation and leakage current in humid conditions 2 10 Routine tests 3 11 Construction 3 12 Supply connections 3 13 Screws and connections 3 14 Creepage distances, clearances and distances through insulation 3 Figure 1 Diagram for leakage current measurement at operating temperature for single-phase con
8、nection of element 5 Figure 2 Diagram for electrical strength test at operating temperature 5 Table 1 Relationship of sheath diameter of element to circular cross section of the former 4 Table 2 Torque values for screwed connections 4 Table 3 Creepage distances and clearances 4 Publication(s) referr
9、ed to Inside back coverBS 7351:1990 ii BSI 07-1999 Foreword This British Standard has been prepared under the direction of the Power Electrical Engineering Standards Policy Committee. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standard
10、s are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pagesi andii, pages1 to6, an inside back cover and a back cover. This standard
11、 has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.BS7351:1990 BSI 07-1999 1 1 Scope This British Standard specifies general requirements together with performance and constructional requirements an
12、d tests for individual metal-sheathed heating elements for industrial use. This British Standard does not apply to elements designed for household use. NOTEThe titles of the publications referred to in this standard are listed on the inside back cover. 2 Definitions For the purposes of this British
13、Standard the following definitions apply. 2.1 metal sheathed heating element (hereinafter called an element) an assembly comprising a resistance alloy within, and insulated from, a tubular metal sheath which is particularly resistant to mechanical stress 2.2 rated voltage the voltage assigned to the
14、 element by the manufacturer 2.3 rated voltage range the range of voltage assigned to the element by the manufacturer, expressed by its lower and upper limits 2.4 design voltage the voltage at which the stated characteristics are obtained. In the case of a rated voltage range, design voltage is the
15、mean of the upper and lower voltage limits 2.5 rated power the wattage assigned to the element by the manufacturer at rated or design voltage 2.6 operating temperature the temperature at which the element sheath operates under service conditions 2.7 maximum sheath temperature the maximum temperature
16、 attained by any part of the element sheath 2.8 maximum permissible temperature the maximum temperature that any part is permitted to attain under service conditions 2.9 basic insulation the insulation contained within the element to provide basic protection against electric shock NOTEBasic insulati
17、on does not necessarily include insulation used exclusively for functional purposes. 2.10 creepage distance the path between two conductive parts, or between a conductive part and the bounding surface of the element, measured across the surface of the insulating material 2.11 clearance the distance
18、between two conductive parts, or between a conductive part and the bounding surface of the element, measured through air 3 General requirements Elements shall be so designed and constructed that, in normal use, they function safely so as to cause no danger to persons or surroundings. TESTS. In gener
19、al, compliance shall be checked by carrying out all the tests specified. 4 General notes on tests 4.1 Tests made in accordance with this standard are type tests, except for those made in accordance with clause 10, which are routine tests. 4.2 Unless otherwise specified, the test shall be made on a n
20、ormal sample which shall withstand all the relevant tests. 4.3 The tests shall be carried out in the order of the clauses of this standard. NOTE 1Before testing is started, the element should be operated at rated voltage in order to verify that it is in working order. NOTE 2It is important to ensure
21、 that the tests are carried out in such a manner that the maximum permissible sheath temperature is not exceeded. 4.4 If the test results are influenced by the temperature of the ambient air, the room temperature shall be maintained at20 5 C. 5 Rating The maximum rated voltage shall be440V. TEST. Co
22、mpliance shall be checked by inspection of the marking. 6 Marking 6.1 Elements shall be durably and legibly marked with the following: a) rated voltage, in volts (V);BS 7351:1990 2 BSI 07-1999 b) rated power, in watts (W); c) manufacturers name, identification, trade mark or responsible vendor; d) m
23、anufacturers model or type reference; e) manufacturers date code. TEST. Compliance shall be checked by inspection. NOTE 1Additional markings are allowed, provided that they do not give rise to misunderstanding. NOTE 2It is recommended that the following additional information be made available by th
24、e manufacturer: a) maximum permissible temperature, in degrees Celsius( C); b) environmental conditions which may adversely affect safety; c) operating position/application limitations; d) requirements for storage including the method of restoring the insulation should the element be subject to ingr
25、ess of moisture; e) maximum permissible sheath temperature, in degrees Celsius ( C); f) details of unheated and heated zones on a particular design of element; g) recommended method of forming, where applicable; h) the K value for calculating resistance, where K is a conversion factor for a change i
26、n resistance at operating temperature. 6.2 When symbols are used they shall be as follows: TEST. Compliance shall be checked by inspection. 7 Rated power Permissible variations in rated power shall not differ from the rated wattage by more than 7.5% or10W, whichever is the greater, when measured at
27、rated voltage. Compliance shall be checked by calculating the resistance of the element in cold condition R (in 7) from the following equation: 8 Electrical insulation and leakage current at operating temperature 8.1 The electrical insulation of elements at operating temperature shall be checked by
28、the tests specified in8.2 and8.3. 8.2 TEST. Leakage. The test shall be carried out with the element connected to the design voltage and at no less than its operating temperature. Having reached a steady state temperature a voltage equal to1.06times the design voltage is applied and the leakage curre
29、nt is measured between each pole of the supply and metal element sheathing within5s. The measuring circuit shall be as shown in Figure 1. The resistance of the measuring circuit shall be2000 1007. The test shall be made with a.c. The leakage current from the element shall not exceed0.75mA or0.75mA p
30、er1000W rated power, whichever is the greater, with a maximum of5mA. 8.3 TEST. Insulation. The insulation shall be subjected for1min to a voltage of substantially sine-wave form, having a frequency of50Hz or60Hz. The connections shall be as shown in Figure 2 under the following conditions. The test
31、voltage shall be applied between parts intended for connection to the supply mains and the metal sheath enclosing the element and metal fittings. The value of the test voltage shall be1000V at the maximum operating temperature as specified in8.2. Initially not more than half of the specified voltage
32、 shall be applied, and then it shall be raised rapidly to the full value. No flashover or breakdown shall occur. 9 Electrical insulation and leakage current in humid conditions 9.1 Where the element is to be used in humid conditions, the cold element shall be tested as specified in9.2 to9.4. 9.2 TES
33、T. Set-up. The humidity treatment shall be carried out in a humidity cabinet containing air with a relative humidity of93 2%. The temperature of the air is maintained within1 C of any convenient value between20 C and30 C. The sample shall be kept in a cabinet for48h. NOTE 1The sample may be brought
34、to the specified temperature and maintained in a steady state condition for a period of4h prior to the humidity test. NOTE 2A relative humidity of93 2% can be obtained by placing in the humidity cabinet a saturated solution of sodium sulphate (Na 2 SO 4 ) or potassium nitrate (KNO 3 ) in water, havi
35、ng a sufficiently large contact surface with the air. V volts A amperes W watts h hours min minutes N newtons s seconds where K is a conversion factor for the change in resistance at operating temperature; V is the rated voltage as given in6.1 (inV); W is the rated wattage as given in6.1 (inW). R KV
36、 2 W - =BS7351:1990 BSI 07-1999 3 NOTE 3In order to achieve the specified conditions it is necessary to ensure constant circulation of the air within the cabinet and to use a cabinet which is thermally insulated. 9.3 TEST. Leakage. The element is removed from the humidity cabinet and connected to a
37、supply voltage equal to1.06 times the design voltage and the earth leakage current is measured within5s of connecting to the supply. The leakage current shall not exceed0.75mA or0.75mA per kilowatt rated power, whichever is the greater, with a maximum of5mA. The measuring circuit shall be as shown i
38、n Figure 1. 9.4 TEST. Insulation. Immediately after the test specified in9.3 the insulation shall be subjected for1min to a voltage of substantially sine-wave form of50Hz or60Hz. This voltage shall be applied between terminals and the element sheath. The value of the voltage shall be twice the line
39、to earth voltage plus1000V. Initially not more than half the prescribed voltage shall be applied, and then it is raised rapidly to full value. No flashover or breakdown shall occur during the test. 10 Routine tests NOTE 1Routine tests are tests that are carried out after manufacture and prior to dis
40、patch. NOTE 2Insulation resistance of an element should be checked in the cold condition using a500Vd.c. insulation resistance tester. After manufacture the insulation resistance should not be less than100M7. 10.1 Electrical strength An electrical strength test on the insulation shall be carried out
41、 for2s with a voltage of substantially sine-wave form of50Hz to60Hz. This voltage shall be applied between one terminal and the element sheath. The value of the voltage shall be twice the line to earth voltage plus1000V. No flashover or breakdown shall occur. 10.2 Cold element resistance Cold elemen
42、t resistance in the cold condition shall be calculated using the equation given in clause7. Permissible variations in rated power shall not differ from the rated wattage by more than 7.5% or10W, whichever is the greater. 11 Construction 11.1 Electrical connectors and their insulation shall withstand
43、 the maximum temperatures as specified by the manufacturer. 11.2 The element shall be so constructed that unless otherwise specified a uniform heat dissipation is attained over the heated length. 11.3 When the element is supplied in an annealed condition it shall be capable of being formed cold arou
44、nd a former (see Table 1). Forming shall not take place within15mm of either side of the cold junction. TEST. The element is formed through a180 hairpin around a former with a diameter of circular cross section as given in Table 1, at ambient temperature between15 C and30 C. The element shall be for
45、med as many times as possible at150mm intervals within the permitted forming length. After forming there shall be no visible evidence of fracture or deformation of the metal sheath and the element shall comply with clause8. NOTEWhen other materials are used for sheath construction, the manufacturers
46、 guidance should be obtained. 12 Supply connections 12.1 Elements shall be provided with means of termination for connection to an incoming supply. 12.2 TEST. Compliance shall be checked by inspection. 13 Screws and connections 13.1 Screwed connections shall withstand the mechanical stresses occurri
47、ng in normal use. TEST. Compliance shall be checked by inspection. Screws and nuts that transmit contact pressure shall be tightened and loosened five times. The test shall be made by means of a suitable test screwdriver, spanner or key, applying a torque as given in Table 2. 13.2 Connecting leads (
48、tails) and terminations shall be securely attached to the element. TEST. The connecting leads shall withstand a sudden longitudinal pull of80N. 14 Creepage distances, clearances and distances through insulation Creepage distances and clearances shall be not less than the values given in Table 3. BS
49、7351:1990 4 BSI 07-1999 Table 1 Relationship of sheath diameter of element to circular cross section of the former Table 2 Torque values for screwed connections Table 3 Creepage distances and clearances Sheath diameter of element Diameter of circular cross section of former mm 6.5 8 9.5 11 12.5 mm 19 21 23 25 32 NOTEThis table is for materials in general use, such as copper, mild steel, stainless steel and nickel alloy to BS3072, BS3073, BS3074, BS3075 and BS3076. Nominal diameter of screw Torque mm Up to and including2.8 Over2.8
