Telecommunication terminal equipment electrical safety requirements

[1] EN 60950:1992, Specification for Safety of Information Technology Equipment, including Electrical Business Equipment, 1992

[2] EN 41003:1997, Particular Safety Requirements for Equipment to be connected to Telecommunications Networks, 1997

[3] ITU-T Recommendation K.51 (Draft Edition), Safety Criteria for Telecommunication Equipment, 1999.

[4] ETSI Technical Report ETR 012, Terminal Equipment (TE) – Safety categories and protection levels at various interfaces for telecommunication equipment in customer premises, 1992

[5] TCVN 3256:1979, An toàn điện – Thuật ngữ và định nghĩa, 1979

[6] TCVN 3144:1979, Các sản phẩm kỹ thuật điện – Yêu cầu kỹ thuật

[7] TCVN 5556:1991, Thiết bị điện hạ áp – Yêu cầu chung về bảo vệ chống điện giật, 1991

[8] TCVN 5699-1:1998, An toàn đối với thiết bị điện gia dụng và các thiết bị điện tương tự - Phần 1: Yêu cầu chung, 1998.
CONTENTS

Foreword

1. Scope

2. Definitions and terms

3. Technical requirements

3.1. Telecommunication Network Voltage (TNV) circuits and electrical shock requirements

3.2. Protection of telecommunication network service personnel, and users of other equipment connected to the network, from hazards in the equipment

3.3. Protection of equipment users from overvoltage on telecommunication network

3.4. General conditions for tests

Annex A (Informative): Requirements on electrical safety for stand –alone equipment

Annex B (Informative): Measuring instrument for leakage current tests

Annex C (Informative): Impulse test generator

Annex D (Informative): Criteria for telephone ringing signals

Annex E (Informative): Tools to be used in tests

Annex F (Informative): Non-radio telecommunication terminal equipment included in the scope of the standard

Annex G (Informative): Cross – Reference table to the orginal standards

References
FOREWORD

The technical standard TCN 68-190: 2003 “Telecommunication Terminal Equipment - Electrical Safety Requirements” is based on amending the technical standard TCN 68-190: 2000 in accordance with the standard EN 41003:1996 and EN 60950:1992 (amd. 11, 1997).

The technical standard TCN 68-190: 2003 is edited by Research Institute of Posts and Telecommunications (RIPT) at proposal of the Department of Science & Technology and adopted by the Decision No 195/2003/QD-BBCVT of the Minister of Posts and Telematics dated 29/12/2003.

The technical standard TCN 68-190: 2003 is issued in a bilingual document (Vietnamese version and English version). In cases of interpretation disputes, Vietnamese version is applied.

TELECOMMUNICATION TERMINAL EQUIPMENT - ELECTRICAL SAFETY REQUIREMENT

This standard is used as the basic for type approving on electrical safety for Telecommunication Terminal Equipment (TTE).

This standard applies to TTE intended to be connected to fixed telephone network by two or more conductors.

The technical requirements specified in this standard are aimed for:

+ Protection of service personnel working on to the fixed telephone network and other users of the fixed telephone network, from hazardous conditions on the fixed telephone network resulting from the connection of the equipment;

+ Protection of equipments users from the voltages on the fixed telephone network.

The requirements on electrical safety for TTE itself are specified in annex A. These requirements apply in process of designing and producing equipment.

The requirements for the following items are not covered by this standard:

+ Functional reliability of equipment;

+ Protection of equipment or telecommunication network from damage;

+ Telecommunication facilities with remote supply.

2. Definitions and terms

2.1. Hazardous Voltage (Excessive Voltage)

A voltage exceeding 42.4 V peak, or 60 V DC, existing in a circuit which does not meet requirements for either a limited current circuit or a TNV circuit.

Insulation needed for the correct operation of the equipment.

Insulation to provide basic protection against electric shock.

Independent insulation applied in addition to basic insulation in order to ensure protection against electric shock in the event of a failure of the basic insulation.

Insulation comprising both basic insulation and supplementary insulation.

A single insulation system which provides a degree of protection against electric shock equivalent to double insulation under the conditions specified in this standard.

An internal circuit which is directly connected to the external supply mains or other equivalent source (such as a motor - generator set) which supplies the electric power.

A circuit which has no direct connection to primary power and derives its power from a transformer, converter or equivalent isolation device, or from a battery.

A secondary circuit with voltages between any two conductors of the ELV circuit, and between any one such conductor and earth, not exceeding 42.4 V peak, or 60 V DC., under normal operating conditions, which is separated from hazardous voltage by at least basic insulation, and which neither meets all of the requirements for an SELV circuit nor meets all of the requirements for a limited current circuit.

A secondary circuit which is so designed and protected that, under normal and single fault conditions, its voltages do not exceed a safe value.

A circuit which is so designed and protected that, under both normal conditions and a likely fault condition, the current which can be drawn is not hazardous.

A circuit in the equipment to which the accessible area of contact is limited and that is so designed and protected that, under normal operating and single fault conditions, the voltages do not exceed specified limiting values.

TNV circuit is considered to be a secondary circuit in the meaning of this standard.

TNV circuit are classified as TNV-1, TNV-2 and TNV-3 as defined in 2.13, 2.14, 2.15.

A TNV circuit:

- Whose normal operating voltages do not exceed the limits for an SELV circuit under normal operating conditions;

- On which over voltages from telecommunication networks are possible.

A TNV circuit:

- Whose normal operating voltages exceed the limits for an SELV circuit under normal operating conditions;

- Which is not subjected overvoltages from telecommunication networks.

A TNV circuit:

- Whose normal operating voltages exceed the limits for an SELV circuit under normal operating conditions;

- On which overvoltages from telecommunication networks are possible.

Persons having appropriate technical training and experience necessary to aware of hazards to which they are exposed in performing a task and of measures to minimize the danger to themselves or other persons.

Any person, other than service personnel.

The term operator in this standard is the same as the term user and the two can be interchanged.

2.18. User

See operator (2.17).

Equipment where protection against electric shock is achieved by:

- Using basic insulation, and also;

- Providing a means of connecting to the protective earthing conductor in the building wiring those conductive parts that are otherwise capable of assuming hazardous voltages if the basic insulation fails.

Equipment in which protection against electric shock does not rely on basic insulation only, but in which additional safety precautions, such as double insulation or reinforced insulation, are provided, there being no reliance on either protective earthing or installation conditions.

An area to which, under normal operating conditions, one of the following applies:

- Access can be gained without the use of a tool, or

- The means of access is deliberately provided to the operator, or

- The operator is instructed to enter regardless of whether or not a tool, is needed to gain access.

2.22. Service Access Area

An area, other than an operator access area, where it is necessary for service personnel to have access even with the equipment switched on.

A location for equipment where both of the following dashed paragraphs apply:

- Access can only be gained by services personnel or by users who have been instructed about the reasons for the restrictions applied to the location and about any precautions that must be taken; and

- Access is through the use of a tool or lock and key, or other means of security, and is controlled by the authority responsible for the location.

Telecommunication Terminal Equipment shall be so adequately designed and protected to meet requirements on electrical safety as following:

- Meet requirements for TNV circuit and provide protection against electric-shock;

- Provide protection of service personnel working on the telecommunication network and other users of the telecommunication network, from hazardous conditions on the telecommunication network resulting from the connection of the equipment;

- Provide protection of equipment users from voltages on the telecommunication network.

3.1. Telecommunication Network Voltage (TNV) circuits and electrical shock requirements

3.1.1. Interconnection of equipment

3.1.1.1. Types of interconnection circuits

Where equipment is intended to be electrically connected to fixed telephony network, interconnection circuit shall be one of the following types:

- An SELV circuit or a limited current circuit;

- A TNV-1, TNV-2 or TNV-3 circuit.

Interconnection circuits shall be selected to provide continued conformance with the requirements for SELV circuits and TNV circuits.

3.1.1.2. ELV circuit as interconnection circuits

Where additional equipment is specifically complementary to the host (first) equipment, ELV circuits are permitted between the equipment, provided that the equipment continue to meet the requirements of this standard when connected together.

3.1.1.3. Safety statements

The safety status (SELV circuit, TNV circuit, limited current circuit, ELV circuit and excessive voltage) of interconnection points for the connection of other equipment shall be stated in the manufacturer’s documentation supplied with the equipment.

3.1.2.1. Limits

In a single TNV circuit or interconnected TNV circuits, the voltage between any two conductors of the TNV circuit or circuits and between any one of such conductor and earth shall comply with the following:

(a) TNV-1 circuits

The voltage do not exceed the following:

- The limits of 42.4 V peak or 60 V DC. under normal operating conditions;

- The limits of figure 1 measured across a 5 k ± 2% resistor in the event of a single failure of insulation or of a component (excluding components with double or reinforced insulation) within the equipment.

Note: In the even of a single insulation or component failure, the limit after 200 ms is the limit for a TNV-2 or TNV-3 circuit for normal operating conditions.

- For telephone ringing signals, voltages such that the signal complies with the criteria of annex D.

(b) TNV-2 and TNV-3 circuits

For voltages exceed the limits of 42.4 V peak or 60 V DC, but other than telephone ringing signals:

The voltage do not exceed the following:

- The limits of 70.7 V peak or 120 V DC under normal operating conditions;

- The limits of figure 1 in the event of a single failure of insulation or of a component (excluding components with double or reinforced insulation) within the equipment.



Figure 1: Maximum voltage of TNV circuit after a single fault

3.1.2.2. Separation from other circuits and from accessible parts

Separation of SELV CIRCUITS, TNV-1 CIRCUITS and accessible conductive parts from TNV-2 and TNV-3 CIRCUITS, shall be such that in the event of a single insulation fault, the limits specified in 3.1.2.1 for TNV-2 and TNV-3 circuits under normal operating condition are not exceeded on the SELV circuits, TNV-1 circuits and accessible conductive parts.

Notes:

- See also 3.2.3 and 3.3.

- Under normal conditions, the limits of A3.3.2 always apply to each SELV circuit and accessible conductive part.

- The limits of 3.1.2.1 always apply to each TNV circuit.

The separation requirements will be met if BASIC INSULATION is provided as indicated in table 1, which also shows where 3.3.1 applies; other solutions are not included.

- The SELV circuit, TNV-1 circuit or accessible conductive part to the protective earthing terminal, and

- The insulation instructions specify that the protective earthing terminal shall have a permanent connection to earth, and

- The test of 3.1.2.3 is carried out if the TNV-2 or TNV-3 circuits is intended to receive, during normal operation, signal or power at a voltage in excess of the limit of 42.4 V peak or 60 V DC for an SELV circuit, generated externally and connected to the equipment (e.g. from a telecommunication network).

At the choice of the manufacturer, it is permitted to treat a TNV-1 circuit or TNV-2 circuit as a TNV-3 circuit. In this case the TNV-1 or TNV-2 circuit shall meet all the separation requirements for a TNV-3 circuit.

Compliance is checked by inspection and measurement and, where necessary, by simulation of failure of components and insulation such as are likely to occur in the equipment.

3.1.2.3. Test of separation between TNV circuits and earthed SELV circuits

To check the separation between TNV circuits and earthed SELV circuits. This test is carried out if specified in 3.1.2.2.

A test generator specified by the manufacturer is used, representing the maximum normal operating voltage expected to be received from the external source. In the absence of such a specification, a test generator is used that provides 120 V ± 2 V AC. at 50 Hz or 60 Hz and has an internal impedance of 1200 ± 2%.

The test generator is connected between the telecommunication network terminals of the equipment. One pole of the test generator is also connected to the earthing terminal of the equipment, see figure 2. The test voltage is applied for a maximum of 30 min. If it is clear that no further deterioration will take place, the test will terminated earlier.

The test is repeated after reversing the connections to the telecommunication network terminals of the equipment.

(c) Compliance criteria

During the test, the voltage between any two conductors and between any one such conductor and the equipment protective earthing terminal in SELV circuits, TNV circuits and accessible conductive parts, shall not exceed 42.4 V peak, or 60 V DC, under normal operating conditions.

3.1.2.4. Separation from hazardous voltages

Except as permitted in 3.1.2.5, TNV circuits shall be separated from circuits at hazardous voltage by one or both of the following methods:

- By double or reinforced insulation;

- By basic insulation, together with protective screening connected to the protective earthing terminal.

Compliance is checked by inspection and by measurement.

3.1.2.5. Connection of TNV circuits to other circuits

A TNV circuits is permitted to be connected to other circuits, provided that it is separated from any primary circuit (including the neutral) within equipment, except as permitted in A3.2.7.

Note: The limits of 3.1.2.1 always apply to TNV circuit.

If a TNV circuit is connected to one or more other circuits, the TNV circuits that part which complies with 3.1.2.1.

If a TNV circuit obtains its supply conductively from a secondary circuit is separated from a hazardous voltage circuit by:

- Double or reinforced insulation, or by

- The use of an earthed conductive screen that is separated from a hazardous voltage circuit by basic insulation, the TNV circuit shall be considered screen as being separated from a hazardous voltage circuit by the same method.

Compliance is checked by inspection, and by simulation of failures of components and insulation such as are likely to occur in the equipment.

3.1.3.1. Accessibility

Equipment shall be provided with adequate protection against contact with bare conductive TNV circuit (TNV-2 or TNV-3 in particular) parts that carry voltage which exceed 42.4 V peak or 60 V d.c., under normal operating conditions.

Exempt from this requirement are:

- Contacts of connectors which can not be touched by the test probe (annex E);

- Equipment intended for installation in a restricted access location;

- Bare conductive parts in the interior of a battery compartment that complies with 3.1.3.2;

- Bare conductive parts in service access areas.

3.1.3.2. Battery compartments

Access to bare conductive parts of TNV-2 and TNV-3 circuits within a dedicated battery compartment in the equipment is permitted if all of the following conditions are met:

- The compartment has a door that requires a deliberate technique to open, such as use of a tool or latching device;

- The TNV- 2 or TNV- 3 circuit is not accessible when the door is closed;

- There is a marking next to the door, or on the door if the door is secured to the equipment, with instructions for protection of the user once the door is opened.

3.2. Protection of telecommunication network service personnel, and users of other equipment connected to the network, from hazards in the equipment.

3.2.1. Protection from hazardous voltages

Circuitry intended to be directly connected to a telecommunication network shall comply with the requirements for an SELV circuit or a TNV circuit.

Compliance is checked by inspection and measurement.

3.2.2. Use of protective earthing

Protective earthing of class I equipment shall not rely on the telecommunication network.

Where protection of the telecommunication network relies on the protective earthing of the equipment, the equipment installation instructions and other relevant literature shall state that integrity of protective earthing must be ensured.

Compliance is checked by inspection.

3.2.3.1. Requirements

Except as specified in 3.2.3.3, there shall be insulation between circuitry intended to be connected to a telecommunication network and any parts or circuitry that will be earthed in some applications, either within the equipment under test or via other equipment. Surge suppressors that bridge the insulation shall have a minimum DC sparkover voltage of 1.6 times the rated voltage or 1.6 times the upper voltage of the rated voltage range of the equipment. If left in place during electric strength testing of the insulation, they shall not be damaged.

Compliance is checked by inspection and by the following tests.

3.2.3.2. Test of separation of the telecommunication network from earth

(a) Objective

To check the separation of the telecommunication network from earth.

Insulation is subjected to the electric strength test of 3.3.2.2. It is permitted to remove components that bridge the insulation, other than capacitors, during the electric strength testing. If this option is chosen, an additional test with a test circuit according to figure 3 is performed with all the components in place. The test is performed with a voltage equal to either the rated voltage, or to the upper voltage of the rated voltage range, of the equipment.

- There shall be no breakdown of insulation during electric strength testing, and

- Components bridging the insulation that are left in place during electric-strength testing shall not be damaged, and

- The current flowing in the test circuit of figure 3 shall not exceed 10 mA.

3.2.3.3. Exclusions

The requirements of 3.2.3.1 do not apply to:

- Permanently connected equipment or pluggable equipment type B; or

- Equipment that is intended to be installed by service personnel and has installation instructions that require the equipment to be connected to a socket -outlet with a protective earthing connection; or

- Equipment that has provision for a permanently connected protective earthing conductor and is provided with instructions for the installation of that conductor.

3.2.4. Leakage current to telecommunication network

3.2.4.1. Limits

The leakage current to a telecommunication network originating from a mains powered equipment shall not exceed the values in table A4.1 (for equipment intended to be connected to TT or TN power system) or in table A5.1 (for equipment intended to be connected to IT power system).

This requirement does not apply to equipment where the circuit to be connected to a telecommunication network is connected to an earthing terminal in the equipment.

Compliance is checked by the tests of 3.2.4.2 using the measuring instrument described in annex B, or any other circuit giving the same results, and preferably using an isolating supply transformer as shown.

3.2.4.2. Leakage current test

To test the leakage current from equipment to telecommunication network.

- For equipment having more than one circuit to be connected to a telecommunication network, the test is applied to only one example of each type of circuit.

- For single - phase equipment, the test circuit of figure 4 is used. The test is made in all the combinations of the normal and reverse polarity of the supply circuit (switch S1) and of the circuit to be connected to a telecommunication network (switch S2).

- For three - phase equipment, the test circuit of figure 5 is used. The test is made in the normal and reverse polarity of the circuit to be connected to a telecommunication network (switch S2).

- For class II equipment the dotted line shown in figure 4 and 5, if it exists, is not included in the test circuit.

The leakage current read in measuring instrument shall not exceed the values in table A4.1 (for equipment intended to be connected to TT or TN power system) or in table A5.1 (for equipment intended to be connected to IT power system).

Equipment shall provide adequate electrical separation between a TNV-1 circuit or TNV-3 circuit and certain parts of the equipment. These parts are:

(a) Unearthed conductive parts and non - conductive of the equipment expected to be held or touched during normal use, e.g. a telephone handset or a keyboard;

(b) Parts or circuitry that can be touched by the test finger (annex E), except contacts of connectors that cannot be touched by the test probe (annex E);

(c) Circuitry which is provided for connection of other equipment. This applies whether or not this circuitry is accessible. It does not apply to circuitry intended to be connected to another equipment that is itself in compliance with 3.3.

These requirements do not apply where circuit analysis and equipment investigation indicate that safety is provided by other means, for example between two circuits each of which has a permanent connection to protective earth.

Compliance is checked by the tests of 3.3.2.

3.3.2. Test procedure

Compliance with 3.3.1 is checked by the test of either 3.3.2.1 or 3.3.2.2.

As an alternative to testing the complete equipment, it is permitted to apply the test to a component (for example a signal transformer) which is clearly intended to provide the separation required, in such case, the component shall not be bypassed by other components, mounting devices or wiring, unless these components or wiring also meet the separation requirements of 3.3.1.

The choice of the tests:

- Between those of 3.3.2.1 or 3.3.2.2, and

- Between testing the complete equipment or a component

Is specified by the manufacturer.

Test circuit shown in figure 6 is applied for the test of 3.3.2.1 and 3.3.2.2.

For the test, all conductor intended to be connected to the telecommunication network (see figure 6), including any conductors required by the telecommunication network authority to be connected to earth. Similarly, all conductors intended to be connected to other equipment are connected together in case (c).

Non - conductive parts are tested with metal foil in contact with the surface. Where adhesive metal foil is used, the adhesive shall be conductive.

3.3.2.1. Impulse test

To test the withstanding of the electrical separation between TNV circuit and parts of the equipment against voltage impulses.

The electrical separation is subjected to ten impulses of alternating polarity (see figure 6), using the impulse test generator of annex C. The interval between successive impulses 60 s and the initial voltage, U_c, is:

- In case (a) of 3.3.1 : 2.5 kV;

- In case (b) and (c): 1.5 kV.

- During the test there shall be no breakdown of insulation. Insulation breakdown is considered to have occurred when the current which flows as a result of the application of the test voltage rapidly increases in an uncontrolled manner, i.e. the insulation does not restrict the flow of current.

- If a surge suppressor operates (or sparkover occurs within a gas discharge tube) during the test:

+ In case (a) of 3.3.1 such operation represents a failure;

+ In cases (b) and (c) such operation is permitted.

Damage to insulation may be checked by an insulation resistance test. The test voltage is 500 V d.c. or, where surge suppressors are present, a d.c. voltage that is 10% less than the surge suppressor operating or striking voltage. The insulation resistance shall not be less than 2 MΩ. Disconnection of surge suppressors is permitted while insulation is being measured.

3.3.2.2. Electric strength test

To check the electric strength of the insulation between TNV circuits and parts of the equipment.

The electrical separation is subjected for 60 s to a substantially sinusoidal voltage having a frequency of 50 Hz or 60 Hz, or to a DC voltage equal to the peak value of the prescribed AC voltage.

The AC test voltage is:

- In case (a) of 3.3.1: 1.5 kV;

- In cases (b) and (c): 1.0 kV.

The voltage is gradually raised from zero to the prescribed voltage and then held at that value for 60 s.

In cases (b) and (c), it is permitted to remove surge suppressors, provided that such devices pass the impulse test of 3.3.2.1 for cases b) and c) when tested as components outside the equipment..

(c) Compliance criteria

- During the test there shall be no breakdown of insulation. Insulation breakdown is considered to have occurred when the current which flows as a result of the application of the test voltage rapidly increases in an uncontrolled manner, i.e. the insulation does not restrict the flow of current.

- If a surge suppressor operates (or sparkover occurs within a gas discharge tube) during the test:

+ In case a) of 3.3.1 such operation represents a failure;

+ In cases b) and c) such operation (by any surge suppressor left in place) represents a failure.

Note: Alternatively, surge suppressor operation or breakdown through insulation may be judged from the shape of an oscillogram.

3.4. General conditions for tests

3.4.1. The requirements and tests detailed in this standard shall be applied only if safety is involved. If it is evident from the design and construction of the equipment that a particular test is not applicable, the test shall not be made.

In order to establish whether or not safety is involved, the circuits and construction shall be carefully investigated to take into account the consequences of possible failures.

As an alternative to carrying out the tests on the complete equipment, tests may be carried out separately on circuits, components or sub - assemblies outside the equipment, provided that inspection of the requirement and circuit arrangements ensures that such testing indicate that the assembled equipment would conform to the requirements of the standard. If any such test indicates a likelihood of non - conformance in the complete equipment, the test shall be repeated in the equipment.

If a test specified in this standard could be destructive, it is permitted to use a model to represent the condition to be evaluated.

Notes:

1) The tests should be carried out in the following order:

- Component or material pre - selection;

- Component or sub - assembly bench tests:

- Tests where the equipment is not energized;

- Live tests:

+ Under normal operating conditions;

+ Under abnormal operating conditions:

+ Involving like destruction.

2) It is recommended that all parties concerned jointly consider the test programme, the test samples and the test sequence.

3.4.4. Except where specific test conditions are stated elsewhere in the standard and where it is clear that there is a significant impact on the results of the test, the tests shall be carried out under the most unfavorable combination within the manufacturer’s operating specifications of the following parameters:

- Supply voltage,

- Supply frequency,

- Physical location of equipment and position of movable parts,

- Operating mode,

- Adjustment of thermostats, regulating devices or similar controls in operator access areas, which are:

+ Adjustable without the use of a tool, or

+ Adjustable using a means, such as a key or a tool, deliberately provided for the operator.

- Multiple rated voltages,

- Extremes of rated voltage ranges,

- Tolerance on rated voltage as declared by the manufacturer.

If no tolerance is declared by the manufacturer, it shall be taken as +6% and -10%. If the rated voltage is 230 V single phase or 400 V three phase, the tolerance shall not be less than +10% and -10%.

When testing equipment designed for d.c. only, the possible influence of polarity shall be taken into account.

It is not necessary to maintain the ambient temperature (T_amb) at a specific-value during tests, but it shall be monitored and recorded.

Temperatures measured on the equipment shall conform with one of the following conditions, all temperatures being in °C:

If T_max is specified: (T - T_amb) ≤ (T_max - T_mra)

If T_max is specified: (T - T_amb) ≤ (∆T_max + 25 - T_mra)

where

T_mra - the maximum room ambient temperature permitted by the manufacturer’s specification or 25°C, whichever is greater.

During the test, the room ambient temperature should not exceed T_mra unless agreed by all parties involved.

3.4.8. Unless a particular method is specified, the temperatures of windings shall be determined either by the thermocouple method or by the resistance method. The temperatures of parts other than windings shall be determined by the thermocouple method. Any other suitable method of temperature measurement which does not noticeably influence in the thermal balance and which achieves an accuracy sufficient to show compliance is also permitted. The choice of and position of temperature sensors shall be made so that they have minimum effect on the temperature of the part under test.

3.4.9. In determining of die input current, and where other test results could be affected, the following variables shall be considered and adjusted to give the most unfavorable results:

- Loads due to optional features, offered or provided by the manufacturer for inclusion in or with the equipment under test;

- Loads due to other units of equipment intended by the manufacturer to draw power from the equipment under test;

- Loads which could be connected to any standard supply outlets in operator access areas on the equipment.

It is permitted to use artificial loads to simulate such loads during testing.

3.4.10. For the electrical requirements of this standard, conductive liquids shall be treated as conductive parts.

3.4.11. Electrical measuring instruments shall have adequate bandwidth to provide accurate readings, taking into account all components (dc, mains supply frequency, high frequency and harmonic content) of the parameter being measured. If the r.m.s, value is being measured, care shall be taken that measuring instruments give true r.m.s readings of non-sinusoidal waveforms as well as sinusoidal waveforms.

3.4.12. Where it is required to apply simulated faults or abnormal operating conditions, these shall be applied in turn and one at a time. Faults which are the direct consequence of the deliberate fault or abnormal operating condition are considered to be part of that deliberate fault or abnormal operating condition.

The equipment, circuit diagrams and component specifications are examined to determine those fault conditions that might reasonably be expected to occur. Examples include:

- Short circuits and open circuits of semiconductor devices and capacitors;

- Faults causing continuous dissipation in resistors designed for intermittent dissipation;

- Internal faults in integrated circuits causing excessive dissipation;

- Failure of basic insulation between current-carrying parts of the primary circuit and

+ Accessible conductive parts,

+ Earthed conductive screens,

+ Parts of SELV circuits,

+ Parts of limited current circuits.

- The protective earthing terminal (if any); and

- Any other conductive part required to be connected to protective earth; and

- Any conductive part that is earthed within the equipment for functional reasons.

Parts that will be earthed in the application by connection to other equipment, but are unearthed (floating) in the equipment as tested, shall be connected to earth at the point by which the highest voltage is obtained. Voltage drop in the protective earthing conductor of the power supply cord, or in an earthed conductor in other external wiring, is not included in the measurements.

3.4.14. Where the standard specifies basic or supplementary insulation, the use of a better grade of insulation is permitted. Similarly, where the standard requires material of a particular flammability class, the use of a better material is permitted.