1、| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | BRITISH STANDARD BS 7798 : 1998 ICS 91.140
2、.65 NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW Specification for Industrial immersion heaters using metal-sheathed elementsThis British Standard, having been prepared under the direction of the Electrotechnical Sector Board, was published under the authority of the Standa
3、rds Board and comes into effect on 15 May 1998 BSI 1998 The following BSI references relate to the work on this standard: Committee reference PEL/27 Draft for comment 92/04453 DC ISBN 0 580 29536 2 BS 7798 : 1998 Amendments issued since publication Amd. No. Date Text affected Committees responsible
4、for this British Standard The preparation of this British Standard was entrusted to Technical Committee PEL/27, Electroheating, upon which the following bodies were represented: Association of Manufacturers Allied to the Electrical and Electronic Industry (BEAMA Ltd.) British Glass Manufacturers Con
5、federation British Industrial Furnace Construction Association British Lighting Association for the 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
6、Industry Council (ETHIC) Electricity Association Energy Industries Council Engineering Equipment and Materials Users Association Engineering Industries Association Health and Safety Executive Induction and Dielectric Heating Manufacturers Association Institution of Electrical Engineers Lighting Indu
7、stry Federation Ltd.BS 7798 : 1998 BSI 1998 i Contents Page Committees responsible Inside front cover Foreword ii Specification 1 Scope 1 2 References 1 3 Definitions 1 4 Heating elements 2 5 Rating 2 6 Degree of protection 2 7 Electrical insulation 2 8 Thermal insulation 3 9 Construction 3 10 Suppl
8、y connections 4 11 Terminations for external conductors 4 12 Provision for earthing 8 13 Internal wiring 9 14 Mounting of components 9 15 Screws and connections 10 16 Creepage distances, clearances, and distances through insulation 11 17 Controls 11 18 Labels and warning notices 12 19 Routine testin
9、g 12 20 Instructions 13 21 Marking 13 Annexes A (normative) Methods of test 14 B (informative) Guidance for installation of immersion heaters 16 Tables 1 Power output 2 2 Minimum thickness of metallic enclosures 3 3 Terminal sizes 5 4 Fastening strength of terminals 5 5 Dimensions of pillar terminal
10、s 6 6 Dimensions of screw terminals 7 7 Dimensions of stud terminals 7 8 Minimum spacings 10 9 Creepage distances and clearances 11 10 Temperature marking for supply connections 12 11 Symbols for marking 13 A.1 Torque values for screw torque test 15 A.2 Torque values for screw torque test, for termi
11、nals described in 11.2.3 and 11.3.3 15 List of references 18ii BSI 1998 BS 7798 : 1998 Foreword This British Standard has been prepared by Technical Committee PEL/27. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible
12、 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, pages i and ii, pages 1 to 18, an inside back cover and a back cover. BSI 1998 1 BS 7798 : 1998
13、 Specification 1 Scope This British Standard specifies requirements for industrial electric immersion heaters designed for permanent installation in cylinders or tanks, for the heating of liquids. The heaters may have fixed, replaceable or removable elements. This standard does not apply to immersio
14、n heaters for mounting over the side of tanks, nor to heaters designed for household use. 2 References 2.1 Normative references This British Standard incorporates, by dated or undated reference, provisions from other publications. These normative references are made at the appropriate places in the
15、text and the cited publications are listed on page 18. For dated references, only the edition cited applies: any subsequent amendments to or revisions of the cited publication apply to this British Standard only when incorporated in the reference by amendment or revision. For undated references, the
16、 latest edition of the cited publication applies, together with any amendments. 2.2 Informative references This British Standard refers to other publications that provide information or guidance. Editions of these publications current at the time of issue of this standard are listed on the inside ba
17、ck cover, but reference should be made to the latest editions. 3 Definitions For the purposes of this British Standard, the following definitions apply. 3.1 (metal-sheathed heating) element A heating assembly comprising a resistance alloy within, and insulated from, a tubular metal sheath. 3.2 fixed
18、 element An element which is permanently attached to its supporting body by adhesive bonding, soldering, brazing, welding or any type of mechanical joint. 3.3 replaceable element An element which can only be removed with the aid of a tool, normally after removal of the immersion heater from its cont
19、ainment vessel. 3.4 removable element An element which can be extracted without the need for removing the immersion heater from its containment vessel. 3.5 element head The screwed member to which the heating element(s) is(are) attached. 3.6 element plate The non-screwed member to which the heating
20、element(s) is(are) attached. 3.7 element plate thickness The distance between the two faces of the element plate, or, if the bolt holes are spot faced, the distance between the front face and the spot facing. 3.8 immersed length The total distance by which the immersion heater projects beyond the jo
21、int face of the element head or plate into the containment vessel. 3.9 thermostat A temperature-sensing device, the operating temperature of which may be either fixed or adjustable and which, during normal operation, keeps the temperature of the liquid between certain limits by automatically opening
22、 or closing a circuit. 3.10 temperature limiter A temperature-sensing device, the operating value of which may be either fixed or adjustable and which during normal use, functions by opening or closing a circuit when the temperature reaches a predetermined value. NOTE. A temperature limiter does not
23、 make the reverse operation during the normal duty cycle of the heater. It may or may not require manual resetting. 3.11 thermal cut-out A device which, during abnormal operation, limits the temperature of a heater, or of parts of it, by automatically opening the circuit or by reducing the current,
24、and which is so constructed that its setting cannot be altered by the user. 3.12 self-resetting thermal cut-out A thermal cut-out that automatically restores the current after the relevant part of the appliance has cooled down sufficiently. 3.13 non-self-resetting thermal cut-out A thermal cut-out t
25、hat requires resetting by hand, or replacement of a part, in order to restore the current. 3.14 metal conducting surface The metal surface interface between the heater and the liquid, excluding any cold or dead areas. 3.15 watts density The power (in watts) dissipated per unit surface area of the me
26、tal conducting surface in contact with liquid. 3.16 basic insulation Insulation applied to live parts to provide basic protection against electric shock. NOTE. Basic insulation does not necessarily include insulation used exclusively for functional purposes.2 BSI 1998 BS 7798 : 1998 3.17 supplementa
27、ry insulation Independent insulation applied in addition to basic insulation, for protection against electric shock in the event of a failure of the basic insulation. 3.18 double insulation Insulation comprising both basic and supplementary insulation. 3.19 reinforced insulation A single insulation
28、system applied to live parts that provides a degree of protection against electric shock equivalent to double insulation. NOTE. The term insulation system does not imply that the insulation is one homogeneous piece. It may comprise several layers that cannot be tested singly. 3.20 stud terminal Scre
29、w-type terminal in which the conductor is clamped under one or two nuts. The clamping pressure may be applied directly by a suitably shaped nut or through an intermediate part, such as a washer, clamping plate or anti-spread device. 3.21 pillar terminal Screw-type terminal in which the conductor is
30、inserted into a hole or cavity, where it is clamped under the shank of the screw or screws. The clamping pressure may be applied directly by the shank of the screw or through an intermediate part to which pressure is applied by the shank of the screw. 3.22 thermal insulation A material or system tha
31、t has the property of resisting the transfer of heat. 3.23 rated voltage The voltage assigned to the heater by the manufacturer. 3.24 rated power The wattage, at rated voltage, assigned to the heater by the manufacturer. 3.25 screw terminal Screw-type terminal in which the conductor is clamped under
32、 a bolt and washer, the bolt passing through a fixed nut or threaded bush. 3.26 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. 3.27 clearance The distance between t
33、wo conductive parts, or between a conductive part and the bounding surface of the element, measured through air. 4 Heating elements All the heating elements shall conform to BS 7351 : 1990. 5 Rating 5.1 Rated voltage Heaters shall be rated for a line voltage not greater than 440 V. NOTE. The require
34、ments of this standard are based on the assumption that, in normal use, the voltage between the supply lines and neutral does not exceed 254 V. 5.2 Rated power The actual power output from the heater at the operating temperature, on application of the rated voltage, shall not vary from the rated pow
35、er output by more than the tolerances given in table 1, when verified in accordance with A.1. Table 1. Power output Rated power kW Tolerance % 0t o5 0 7.5 $ 51 5.5 5.3 Watts density NOTE. During the construction of the immersion heater and/or its selection for a particular application, care should b
36、e taken to ensure that the watts density is compatible with the liquid to be heated. 6 Degree of protection The minimum degree of protection provided by heater enclosures shall be in accordance with IP 40 of BS EN 60529 : 1992. 7 Electrical insulation 7.1 The mounting of electrically insulating part
37、s shall be designed so as to prevent breakage or damage due to expansion and contraction. 7.2 Electrically insulating washers, bushings and the like, which are integral parts of the heater, and bases or supports of the mounting of current-carrying parts shall be of a moisture-resistant material whic
38、h will not be damaged by the temperatures to which they will be subjected under conditions of actual use. 7.3 The insulation between any parts operating at extra-low voltage and other live parts, and between the core of the isolating transformer and other metal parts, shall be as specified in BS 353
39、5 : Part 1 and Part 2 . BSI 1998 3 BS 7798 : 1998 Table 2. Minimum thickness of metallic enclosures Material Reinforced flat surfaces Surfaces to which a wiring system is to be connected on site Unreinforced flat surfaces mm mm mm Die-cast steels 1.20 2.00 Cast malleable iron 1.60 2.40 Other cast me
40、tal 2.40 3.20 Uncoated sheet steel 0.66 0.81 0.66 Galvanized sheet steel 0.74 0.86 0.74 Non-ferrous sheet steel 0.91 1.14 0.91 7.4 Reinforced insulation shall only be used when it is manifestly impracticable to provide separate functional insulation and supplementary insulation. NOTE 1. Appliance in
41、lets and switches are examples of where reinforced insulation may be used. NOTE 2. Insulating material in which heating conductors are embedded is deemed to be functional insulation. 7.5 Parts which serve as supplementary insulation or reinforced insulation shall either: a) be fixed in such a way th
42、at they cannot be removed without being seriously damaged; or b) be designed so that they cannot be replaced in an incorrect position, and so that, if they are omitted, the appliance is rendered inoperable or manifestly incomplete. NOTE 1. Sleeving may be used as supplementary insulation on internal
43、 wiring, if it can only be removed by breaking or cutting, or if it is clamped at both ends. NOTE 2. Lining metal enclosures with a coating of lacquer, or with other material in the form of a coating which can be easily removed by scraping, is not deemed to meet this requirement. 7.6 Inside the term
44、inal enclosure, the sheath of a flexible cable or cord shall only be used as supplementary insulation where it is not subject to undue mechanical or thermal stresses and if its insulating properties are not less than those specified for the sheaths of flexible cables and cords in BS 6004, BS 6007, o
45、r BS 6500. 8 Thermal insulation 8.1 Thermal insulation materials shall not make contact with the heated part of the element sheath or pocket. 8.2 Where terminal enclosures are stood off from the element head or element plate to give cooling via natural convection, the stand-off areas shall not be th
46、ermally insulated. 9 Construction NOTE 1. Heaters should be so constructed that they operate in all positions to be expected in normal use. Any limitations of use should be specified by the manufacturer. NOTE 2. The cold length of the element should be sufficient to minimize the risk of local overhe
47、ating caused by an air lock under the element head or elememt plate. 9.1 Heaters shall be constructed so that, after being removed for servicing or maintenance, they can be refitted correctly with normal tools. 9.2 A thermal cut-out or temperature limiter shall be provided. The sensor shall be mount
48、ed so as to protect the complete heater unit. 9.3 If element surface temperature monitors are fitted, they shall be attached to the element sheath by brazing, welding or mechanical clamping. 9.4 Heaters shall be provided with an enclosure. 9.5 The minimum thickness of metallic enclosures shall be as
49、 stated in table 2. 9.6 All threaded bushes, plugs or caps shall engage to the extent of at least five full threads. 9.7 Non-metallic enclosures shall conform to the appropriate glow-wire test requirements of BS 6458 : Section 2.1. NOTE. Special consideration should be given to any non-metallic enclosure to ensure that it has sufficient physical strength, and that it has adequate resistance to impact, moisture absorption, corrosion and to distortion with respect to thermal ageing. 9.8 All parts of the heater in contact with liquids shall be manufactured from materials
