GB 14249.1-1993Safety requirements for electronic weighing instruments (English PDF)

4.1 Rated operating conditions and fault conditions

4.1.1 Rated operating conditions

4.1.1.1

Load cells (hereinafter referred to as sensors) shall be able to operate safely under the following conditions:

Temperature: -10 °C to +55 °C; relative humidity: 5% to 90%; atmospheric pressure: 86 kPa to 106 kPa.

4.1.1.2

Weighing indicators (hereinafter referred to as indicators) shall be able to operate safely under the following conditions:

Temperature: 10 °C to 20 °C for class I and class II weighing instruments; 0 °C to 40 °C for class III and class IIII weighing instruments and automatic weighing instruments.

Relative humidity: less than 75% for class I and class II weighing instruments; less than 90% for class III and class IIII weighing instruments and automatic weighing instruments.

Rated voltage: 220 V, variation range -15% to +10%;

Rated frequency: 50 Hz, variation range -2% to +2%.

4.1.1.3

The other parts of the electronic weighing instrument shall be able to operate safely in the range -10 °C to +40 °C.

Stricter ranges than the above conditions may be specified by agreement between the manufacturer and the user.

4.1.2 Fault conditions

a. environmental conditions exceeding the rated operating conditions;

b. parts actually used exceeding the requirements of the technical specification;

c. failure of parts or components.

4.2 High temperature and other hazards

4.2.1 Temperature rise under rated operating conditions and fault conditions

4.2.1.1 Permitted temperature rise under rated operating conditions

No accessible part of the electronic weighing instrument shall reach an unsafe temperature liable to injure persons, and no part or component shall reach a temperature liable to damage electrical insulation or reduce mechanical strength. The maximum safe temperature rises under rated operating conditions are the values specified in column 1 of Table II of GB 8898.

4.2.1.2 Temperature rise under fault conditions

The permitted temperature rises of electronic weighing instruments operating under fault conditions are the values specified in column 2 of Table II of GB 8898.

4.3 Explosion protection

Explosion-proof electronic weighing instruments operating in environments where flammable gases, vapours, dusts or other flammable or explosive substances are present shall be manufactured in accordance with GB 3836 and with the basic requirements of the Electrical Safety Regulations for Explosion Hazardous Locations of the People's Republic of China (Trial).

4.4 Protection against electric shock by contact

4.4.1 Accessible parts

Accessible parts shall not be live; even inaccessible earth terminals shall not be live.

4.4.2 Preset controls

Where the enclosure has holes through which controls can be adjusted with a tool, there shall be no danger of electric shock when the controls are adjusted.

4.4.3 Mains plugs

Electronic weighing instruments connected to the mains supply by a plug shall not give rise to a danger of electric shock when the plug is inserted or withdrawn.

4.5 Construction requirements

4.5.1 Protection against electric shock

4.5.1.1 Protective earthing terminal

Electronic weighing instruments shall have a protective earthing terminal or earthing point, and all accessible conductive parts shall be reliably connected to the protective earth.

The connection resistance between accessible parts and the protective earthing terminal shall be less than 0.5 Omega.

a. Parts of electronic weighing instruments located outdoors shall have a separate protective earthing terminal. The material of the earthing terminal shall be compatible in electrolytic behaviour with the earthing conductor, and the earthing terminal shall be such that it cannot be loosened by hand;

b. The requirements of a. also apply to electronic weighing instruments with non-detachable flexible cords or cables. In addition, every non-detachable flexible cord or cable connected to the mains shall have a green/yellow core connected to the protective earthing terminal of the indicator and, where there is a plug, to the protective earthing contact of the plug.

4.5.1.2 Protective earthing

The earthing arrangement shall be connected by specially laid conductors of suitably low impedance, so as to ensure that accessible parts are electrically safe under rated operating conditions and under fault conditions. The earthing resistance shall be less than 4 Omega.

4.5.1.3 Wiring

The insulation between live conductors in wires (or cables) and accessible components, or the insulation of wires (or cables) connecting live components to accessible metal parts, shall be able to withstand the dielectric strength test of 5.4.3, and its thickness shall provide equivalent mechanical strength.

4.5.1.4 Electrical connections

a. Short-circuiting of the insulation between live components and accessible components as a result of the loosening of connecting screws, etc. shall be prevented;

b. It shall be ensured that, if one end of any wire becomes loose, the natural movement of the loose end does not reduce the creepage distances and clearances in air below the values shown by curve A of Table II of GB 8898 (0.6 mm for 34 V, 3.0 mm for 354 V).

4.5.2 Mechanical construction

a. Sensors and indicators shall be able to withstand vibration and shock tests;

b. Sensors and load receptors shall be designed so as to minimise the possibility of mechanical injury to persons, for example injury caused by sharp edges, burrs or protruding corners;

c. The groove dimensions and deviations of welded parts shall comply with the relevant requirements of GB 985.

4.6 Insulation requirements

4.6.1 Damp heat treatment and insulation requirements

4.6.1.1 Damp heat effects and insulation requirements for load cells

The damp heat effects and the insulation resistance of sensors shall comply respectively with the relevant provisions of GB 7551.

4.6.1.2 Damp heat effects and insulation requirements for indicators

a. The damp heat effects on the indicator shall comply with the relevant provisions of GB 7724;

b. The insulation resistance of the indicator shall be not less than 5 MOmega;

c. In the dielectric strength test of the indicator, there shall be no breakdown at an a.c. voltage of 1 500 V applied for 1 min;

d. The power-frequency a.c. leakage current shall not exceed 3.5 mA.

4.7 Components

Under rated operating conditions the load on components shall not be permitted to exceed their rated values, and under fault conditions it shall as far as possible not exceed them.

4.8 Fuses

Suitable fuses shall be fitted to ensure safe and reliable operation.

4.9 Power switches

Power switches shall maintain good making and breaking capability.

4.10 Terminals for external flexible cords

Terminals for external flexible cords shall comply with the relevant provisions of Chapter 15 of GB 8898.

4.11 External flexible cords

External flexible cords shall comply with the relevant provisions of Chapter 16 of GB 8898.

4.12 Electrical connections and mechanical fixings

The electrical connections and mechanical fixings of the parts of electronic weighing instruments shall comply with the relevant provisions of Chapter 17 of GB 8898.

5.1

The temperature rise test under the rated temperature limit conditions shall be carried out in accordance with 7.1 of GB 8898.

5.2

Explosion-proof electronic weighing instruments for explosion hazardous locations may be produced and used only after they have been inspected and found acceptable, in accordance with the relevant provisions of GB 3836 and of the Electrical Safety Regulations for Explosion Hazardous Locations of the People's Republic of China (Trial), by a national explosion protection safety supervision and inspection station (explosion protection station for short) established with the approval of the Ministry of Labour.

5.3 Tests for protection against electric shock by contact

5.3.1

The test of preset controls shall be carried out in accordance with 9.1.5 of GB 8898.

5.3.2

The test of mains plugs shall be carried out in accordance with 9.1.7 of GB 8898.

5.4 Tests for protection against electric shock

5.4.1

The connection resistance between accessible parts and the protective earthing terminal shall be less than 0.5 Omega. The connecting leads may be preselected by calculation; then, using an a.c. source with a no-load voltage not exceeding 6 V, a current of 10 A is passed for 1 min in turn through the connection between each protective earthing terminal and the accessible metal parts, the voltage drop between them is measured, and the resistance is finally calculated from the current and the voltage drop:

R = U / I < 0.5 Omega

where:

R - connection resistance, Omega;

U - measured voltage drop, V;

I - applied current, 10 A.

Note 1: the resistance of the flexible cord is not included in the measured resistance.

Note 2: the contact resistance between the tip of the measuring probe and the accessible metal part shall not affect the test result.

Note 3: if the rated supply voltage is relatively low, the resistance value shall be reduced.

5.4.2

The earthing resistance of electronic weighing instruments shall be determined in accordance with the relevant provisions of GBJ 65.

5.4.3

Test of the insulation of wires. A metal foil at least 10 cm long is first wrapped tightly around the insulation of the wire, and a test voltage of 1 500 V a.c. is then applied between the conductor of the wire and the metal foil. During the test the voltage is raised gradually to the specified value so as to avoid any appreciable transient, held at the specified voltage for 1 min, and then reduced smoothly to zero. No flashover or breakdown shall occur during the test, but corona effects and similar phenomena are permitted.

5.4.4

The construction tests of electronic weighing instruments cover electrical connections and mechanical performance.

a. In the construction of the electronic weighing instrument, if the measures taken to prevent short-circuiting of the insulation between live components and accessible components caused by accidentally loosened screws, etc. withstand the tests of 5.4.4c and 5.4.4d, the requirements are considered to be met;

b. The construction of the electronic weighing instrument shall ensure that, if one end of any wire becomes loose, the natural movement of the loose end does not reduce the creepage distances and clearances in air below the values shown by curve A of Table II of GB 8898; compliance is checked by inspection and measurement;

c. If the sensor passes the vibration and shock tests carried out in accordance with the relevant provisions of GB 7551, the requirements are considered to be met;

d. If the indicator passes the vibration and shock tests carried out in accordance with GB 6587.4 and GB 6587.5, the requirements are considered to be met.

5.5 Damp heat treatment and insulation requirements

5.5.1

If the damp heat effect test and the insulation resistance test of the sensor, carried out in accordance with the relevant provisions of GB 7551, are passed, the requirements are considered to be met.

5.5.2

In the damp heat test of the indicator, the indicator is placed in a damp heat chamber at a temperature of 40 °C ± 2 °C and a relative humidity of 90% to 95% and kept there for 48 h; immediately after the test, the tests of 3.1, 3.2 and 3.3 of GB 6587.7 are carried out.

5.6 Mechanical strength tests

5.6.1

The external appearance of all parts of the electronic weighing instrument shall be checked visually for compliance.

5.6.2

Welded parts are in general checked visually. Welds that require mechanical performance tests shall be indicated in the drawings or technical requirements. Mechanical performance tests of welds shall be carried out in accordance with GB 2651.

5.7

Components that have been shown to comply with the safety requirements by tests appropriate to the rated operating conditions need not be tested.

Components not falling within the above case shall be tested inside or outside the electronic weighing instrument (under conditions equivalent to the rated operating conditions of the component within the instrument).

5.8

The capacity of fuses shall be accepted in accordance with the factory product standard.

5.9

Switches shall be checked for compliance visually and by measurement.

5.10

The test of terminals for external flexible cords shall be carried out in accordance with 15.3 of GB 8898.

5.11

The test of external flexible cords shall be carried out in accordance with the relevant provisions of Chapter 16 of GB 8898.

5.12

The electrical connections and mechanical fixings of the parts of electronic weighing instruments shall be checked in accordance with Chapter 17 of GB 8898.

6.1 Safety marking

6.1.1

The number of the explosion protection certificate of explosion-proof products shall be marked on the product nameplate.

6.1.2

The symbols and letters of quantities and units shall comply with GB 3100 to 3102.

6.1.3

Graphical symbols shall comply with GB 5465.

6.2 Power supply marking

a. an a.c. supply shall be indicated by the symbol "~";

b. a d.c. supply shall be indicated by the symbol "-";

c. where safety depends on the correct mains frequency, the rated mains frequency shall be marked;

d. switches shall be clearly marked with their on and off positions;

e. fuses shall be clearly marked;

f. protective earthing shall bear an earthing symbol.