Telecommunication terminal equipment electrical safety requirements


ANNEX A (Informative) REQUIREMENTS ON ELECTRICAL SAFETY FOR STAND – ALONE EQUIPMENT A1. General requirements



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ANNEX A

(Informative)

REQUIREMENTS ON ELECTRICAL SAFETY FOR STAND – ALONE EQUIPMENT

A1. General requirements

A.1.1 Equipment design and construction

Equipment shall be so designed and constructed that, under all conditions of normal use and under a likely fault condition, it protects against risk of personal injury from electric shock and other hazards, and against serious fire originating in the equipment, within the meaning of this standard.

Where the equipment involves technologies and material or methods of construction not specifically covered, the equipment shall provide a level of safety not less than that generally afforded by this standard.

A1.2. User information

Sufficient information shall be provided the concerning any condition necessary to ensure that, when used as prescribed by the manufacturer, the equipment will not present a hazard.



A 1.3. Classification of equipment

Equipment is classified according to its protection from electric shock as:

- Class I, or

- Class II, or

- Class III.

Note: Equipment containing ELV circuits or parts at hazardous voltage is Class I or Class II. There are not requirements in this standard from protection against electric shock for Class III equipment.

A2. Power interface

A2.1. Input current

The steady state input current of the equipment shall not exceed the rated current by more than 10% under normal load.



A2.2. Voltage limit of hand-held equipment

The rated voltage of hand- held equipment shall not exceed 250 V.



A2.3. Neutral conductor

The neutral conductor, if any, shall be insulated from earth and the body throughout the equipment as if it were a phase conductor. Components connected between neural and earth shall be rated for a working voltage equal to the phase - to - neutral voltage.



A2.4. Components in equipment for IT power systems

For equipment to be connected to IT power systems, component connected between phase and earth shall be capable of withstanding the stress due to a working voltage equal to the phase - to - phase voltage. However, capacitors intended to be operated in such applications and complying with one of the following standard are permitted if they are rated for the applicable phase- to-neutral voltage.

- IEC 384-14:1981; or

- IEC 384-14:1993, subclause Y1, Y2 or Y4.



Notes: The above capacitors are endurance tested at 1.7 times at the rated voltage of the capacitor.

A2.5. Mains supply tolerance

Equipment intended to operated from the mains supply shall be designed for a minimum supply tolerance of +6% and -10%. If the rated voltage is 230 V single phase or 400 V three phase, the equipment shall operate safety within a minimum supply tolerance of +10% and -10%.



A3. Protection from hazards

A3.1. Protection from electric shock and energy hazards

A3.1.1. Access to energized parts

This standard specifies requirements for protection against electric shock from energized parts based on the principle that the operator is permitted to have access to:

- Bare parts of SELV circuits;

- Bare parts of limited current circuits;

- TNV circuits under the conditions specified in 3.1.3.

Access to other energized parts and wiring, and to their insulation, is restricted as specified in A3.1.2 and A3.1.3.

Additional requirements are specified in A3.1.4 and A3.1.5.

A3.1.2. Protection in operator access areas

The equipment shall be so constructed that in operator access areas there is adequate protection against contact with:

- Bare parts of ELV circuits or bare parts at hazardous voltages;

- Parts of ELV circuits or parts at hazardous voltages protected only by lacquer, enamel, ordinary paper, cotton, oxide film, beads or sealing compounds other than self - hardening resin;

- Operational or basic insulation of parts or wiring in ELV circuits or at hazardous voltages, except as permitted in A3.1.3;

- Unearthed conductive parts separated from ELV circuits or from parts at hazardous voltages by operational or basic insulation only.

This requirement applies for all positions of the equipment when it is wired and operated as in normal use.

Protection shall be achieved by insulation or guarding or by the use of interlocks.



A3.1.3. Access to internal wiring

A3.1.3.1. ELV circuit

It is permitted that the insulation of internal wiring in an ELV circuit is accessible to an operator, provided that the wiring:

a) Does not need to be handle by the operator;

b) Is routed and fixed so as not to touch unearthed accessible conductive parts;

c) Has distance through insulation not less than given in table A3.1;



Table A3.1: Distance through insulation of internal wiring

Working voltage

(in case of failure of basic insulation)

Minimum distance through insulation

V peak or d.c.

V r.m.s (sinusoidal)

mm

over 71, up to 350

over 350


over 50, up to 250

over 250


0.17

0.31


d) Meets the requirements for supplementary insulation.

Where wiring in an ELV circuit does not meet both conditions a) and b), the insulation shall meet the full requirements for supplementary insulation and shall pass the electric strength test in 3.3.2.2.

A3.1.3.2. Hazardous voltage circuits

The insulation of internal wiring at hazardous voltage that is operator-accessible, or that is not routed and fixed to prevent it from touching unearthed accessible conductive parts, shall meet the requirements for double and reinforced insulation.



A3.1.4. Protection in service access areas and restricted access locations

A3.1.4.1. Protection in service access areas

In a service access area, the following requirements apply:

- Bare parts operating at hazardous voltages shall be so located or guarded that unintentional contact with such parts is unlikely during servicing operations involving other parts of the equipment.

- No requirement is specified regarding contact with ELV circuits or with TNV circuits.

- In deciding whether or not unintentional contact with bare parts would be likely, account shall be taken of the way service personnel need to gain access past, or near to, the bare parts in order to service other parts.

- Bare parts that involve an energy hazard (see A3.1.5) shall be so located or guarded that unintentional bridging by conductive materials that might be present is unlikely during service operations involving other parts of the equipment.

Any guards for compliance with this subclause shall be easily removable and replaceable if removal is necessary for servicing.

A3.1.4.2. Protection in restricted access locations

For equipment to be installed in a restricted access location, the requirements for operator access areas apply except as permitted in 3.1.3 and in the following two requirements:

- If a secondary circuit at a hazardous voltage is used to supply a ringing signal generator that complies with 3.1.2.1 b), contact with bare parts of the circuit is permitted with the test finger (annex E). However, such parts shall be so located or guarded that unintentional contact is unlikely. In deciding whether or not unintentional contacted would be likely, account shall be taken of the need to gain access past, or near to, the bare parts at hazardous voltage.

- Bare parts that involve an energy hazard (see A3.1.5) parts shall be so located or guarded that unintentional bridging by conductive materials that might be present is unlikely.



A3.1.5. Energy hazards in operator access areas

There shall be no energy hazard in operator access areas.



A3.1.6. Clearances behind earthed or unearthed conductive enclosures shall not be reduced to a level that would result in energy hazard arising during the relevant tests to which the test is applicable.

A3.1.7. Shafts of operating knobs, handles, levers and the like shall not be connected to a circuit at hazardous voltage nor to an ELV circuit.

A3.1.8. Conductive handles, levers, control knobs and the like which are manually moved in normal use and which are earthed only through a pivot or bearing shall be either:

- Separated from hazardous voltage within the component or elsewhere by creepage distances and clearances of double or reinforced insulation, or

- Covered by supplementary insulation over accessible parts.

A3.1.9. Conductive casings of capacitors operating in ELV circuits or circuits at hazardous voltages shall not be connected to unearthed conductive parts in operator access areas and shall be separated from these parts by supplementary insulation or earthed metal.

A3.1.10. Equipment shall be so designed that at an external point of disconnection of the mains supply, there is no risk of electric shock from stored charge on capacitors connected to the main circuit.



A3.2. Insulation

A3.2.1. Methods for insulation

Electrical insulation shall be achieved by provision one of the following, or a combination of the two:

- Solid or laminated insulating materials having adequate thickness and adequate creepage distances over their surface;

- Adequate clearance through the air.



A3.2.2. Properties of insulating materials

The choice and application of insulation materials shall take into account the needs for electrical, thermal and mechanical strength, frequency of working voltage, and the working environment (temperature, pressure, humidity and pollution).

Neither natural rubber nor materials containing asbestos shall be used as insulation.

A3.2.3. Requirements for insulation

Insulation in equipment shall comply with the heating requirements in working condition and, except where A3.1.3 applies, with:

- The electric strength requirements, and

- The creepage distance, clearance and distance through insulation.



A3.2.4. Insulation parameters

For the purpose of determining the test voltages, creepage distances, clearances and distance through insulation for a given piece of insulation, two parameters shall be considers:

- Application (see A3.2.5);

- Working voltage (see A3.2.6).



A3.2.5. Categories of insulation

Insulation shall be considered to be operational, basic, supplementary, reinforced or double insulation.

The application of insulation in many common situations is described in table A3.2 and illustrated in figure A3.1. In certain cases, insulation may be bridged by a conductive path, e.g. where A3.2.7, A3.3.5, A3.4.6 or 3.1.2.5 applies, provided that the level of safety is maintained.

For double insulation it is permitted to interchange the basic and supplementary elements. Where double insulation is used, ELV circuits or unearthed conductive parts are permitted between the basic insulation and the supplementary insulation provided that the overall level of insulation is maintained.



Table A3.2: Examples of application of insulation

Grade of insulation

Insulation

Key to figure A3.1

between:

and:

1. Operational

SELV circuit

- Earthed conductive parts

OP1

- Double - insulated conductive part

OP2

- Another SELV circuit

OP1

ELV circuit

- Earthed conductive parts

OP3

- Earthed SELV circuit

OP3

- Basic - insulated conductive part

OP4

- Another ELV circuit

OP1

Earthed hazardous voltage secondary circuit

- Another earthed hazardous voltage secondary circuit

OP5

TNV circuit

- Earthed conductive parts




- Earthed SELV circuit




- Another TNV circuit of the same classification

OP1

OP6


Series/ parallel sections of a transformer winding







2. Basic

Primary circuit

- Earthed or unearthed hazardous voltage secondary circuit

B1

- Earthed conductive parts

B2

- Earthed SELV circuit

B2

- Basic - insulated conductive part

B3

- ELV circuit

B3

Earthed or unearthed hazardous voltage secondary circuit

- Unearthed hazardous voltage secondary circuit

B4

- Earthed conductive parts

B5

- Earthed SELV circuit

B5

- Basic - insulated conductive part

B6

- ELV circuit

B6

TNV circuit

- Double - insulated conductive part

B7

- Unearthed SELV circuit

B7

- Earthed conductive parts

B8

- Earthed SELV circuit

B8

3. Supplementary

Basic-insulated conductive part or ELV circuit

- Double- insulated conductive part

S1

- Unearthed SELV circuit

S1

TNV circuit

- Basic - insulated conductive part

S2

- ELV circuit

S2

4. Supplementary or reinforced

Unearthed hazardous voltage secondary circuit

- Double- insulated conductive part

S/R

- Unearthed SELV circuit

S/R

- TNV circuit

S/R

5. Reinforced

Primary circuit

- Double- insulated conductive part

R1

- Unearthed SELV circuit

R1

- TNV circuit

R2

Earthed hazardous voltage secondary circuit

- Double - insulated conductive part

R3

- Unearthed SELV circuit

R3

- TNV circuit

R4




OP - Operational R - Reinforced

S - Supplementary B - Basic

Figure A3.1: Examples of application of insulation

A3.2.6. Determination of working voltage

For the purposes of determining working voltage the rules of A3.2.6.1, and, where relevant, those of A3.2.6.2, A3.2.6.3, A3.2.6.4 and A3.2.6.5 shall be applied.



Note: Working voltage in switch mode power supplies are best determined h\ measurement.

A3.2.6.1. General rules

Where the working voltage between a primary circuit and either a secondary circuit or earth is to be determined, the value of the rated voltage or the maximum value of the rated voltage range shall be used.

Unearthed accessible conductive parts shall be assumed to be earthed.

Where a transformer winding or other part is floating, i.e., not connected to a circuit which establishes its potential relative to earth, it shall be assumed to be earthed at the point by which the highest working is obtained.

Where double insulation is used, the working voltage across the basic insulation shall be determined by imagining a short circuit across the supplementary insulation, and vice versa. For insulation between transformer windings, the short circuit shall be assumed to take place at the point by which the highest working voltage is produced in the other insulation.

For insulation between two transformer windings, the highest voltage between any two points in the two windings shall be used, taking into account external voltages to which the windings may be connected.

For insulation between a transformer winding and another part, the highest voltage between any point on the winding and other part shall be used.

A3.2.6.2. Clearances in primary circuits

For a working voltage to be used in determining clearances for primary circuits:

- For DC voltages, the peak value of any superimposed ripple shall be included;

- Non- repetitive transients (due, for example, to atmospheric disturbances) shall be disregarded;



Note: It is assumed that any such transient in a secondary circuit will not exceed the transient rating of the primary circuit.

- The voltage of any ELV circuit, SELV circuit, or TNV circuit shall be regarded as zero;

- For repetitive peak voltages exceeding the peak values of the mains supply voltage, the maximum repetitive peak value shall be used.

A3.2.6.3. Clearances in secondary circuits

For a working voltage to be used in determining clearances for secondary circuits:

- For DC voltages, the peak value of any superimposed ripple shall be included;

- For non - sinusoidal waveforms, the peak value shall be used.

A3.2.6.4. Creepage distances

For a working voltage to be used in determining creepage distances:

- The actual r.m.s or DC value shall be used;

- If the DC value is used, any superimposed ripple shall be ignored;

- Short - term conditions (e.g. in cadenced ringing signals in TNV circuits) shall be disregarded.

A3.2.6.5. Electric strength tests

For a working voltage to be used in determining the electric strength test voltages of 3.3.2.2, DC values shall be used for DC voltage and peak values for other voltages.



A3.2.7. Double or reinforced insulation bridged by components

A3.2.7.1. Bridging capacitors

It is permitted to bridge double or reinforced insulation by:

- A single capacitor complying with IEC 384-14:1993, subclause Y1; or

- Two capacitors in series, each complying with IEC 384-14:1981, class U or Y; or IEC384-14:1993, class Y2 or Y4.

Where two capacitor are used in series, they shall each be rated for the total working voltage across the pair and shall have the same nominal capacitance value.

A3.2.7.2. Bridging resistors

It is permitted to bridge double or reinforced insulation by two resistors in series. They shall have the same nominal resistance value.

A3.2.7.3. Accessible parts

Where accessible conductive parts or circuits are separated from other parts by double or reinforced insulation that is bridged by components in accordance with A3.2.7.1 and A3.2.7.2, the accessible parts shall comply with the requirements in A3.4. These requirements shall apply after electric strength testing of the insulation has been carried out.



A3.3. SELV circuits

A3.3.1. General requirements

SELV circuit shall exhibit voltages safe to touch both under normal operating conditions and after a single fault, such as breakdown of a layer of basic insulation or failure of a single component.

If an SELV circuit is intended to be connected to a telecommunication network, consideration shall be given both to normal operating voltages generated internally in the equipment and to those generated externally, including ringing signals. Each potential rises and included voltages from power lines and from electric traction lines, that may received from telecommunication network, shall not be considered.

A3.3.2. Voltages under normal conditions

In a single SELV circuit or in interconnected SELV circuits, the voltage between any two conductors of the SELV circuit or circuits and, for class I equipment, between any one such conductor and the equipment protective earthing terminal, shall not exceed 42.4 V peak, or 60 V DC, under normal operating conditions.



Note: A circuit that meets the above requirements but that is subject to overvoltages from a telecommunication network is a TNV-1 circuit.

A3.3.3. Voltages under fault conditions

Except as permitted in 3.1.2.2, in the event of a single failure of basic or supplementary insulation, or of a component (excluding components with double or reinforced insulation), the voltages in an SELV circuit shall not exceed 42.4 V peak, or 60 V DC, for longer than 0.2 s. Moreover, a limit of 71 V peak, or 120 V DC, shall not be exceeded.

Except as permitted in A3.3.5, one of the methods specified in A3.3.3.1, A3.3.3.2 or A3.3.3.3 shall be used.

In a single circuit (e.g. transformer - rectifier circuit), it is permitted for some parts to comply with all of the requirements for SELV circuits and to be operator accessible, while other parts of the same circuit do not comply with all the requirements for SELV circuits and are therefore not permitted to be operator -accessible.



Note:

- Different parts of the same SELV circuit may be protected by different methods, for example:

+ Method 2 within power transformer feeding bridge rectifier;

+ Method 1 for the AC secondary circuit;

+ Method 3 at the output of the bridge rectifier.

- For normal conditions the SELV circuit voltage limit is the same for an SELV circuit; an SELV circuit may be regarded as an ELV circuit with additional protection under fault conditions.

A3.3.3.1. Separation by double or reinforced insulation (method 1)

Where an SELV circuit is separated from other circuit by double or reinforced insulation only, one of the following construction shall be employed:

- Provide permanent separation by barriers;

- Provide insulation of all adjacent wiring involved that is rated for the highest working voltage present;

- Provide insulation on either the wiring of the SELV circuit or that of the other circuits that meets the insulation requirements for supplementary or reinforced insulation, as appropriate, for the highest working voltage present;

- Provide an additional layer of insulation, where required, over either the wiring of the SELV circuit;

- Provide two separated transformers in tandem, where one transformer provides basic insulation and the other transformer provides supplementary insulation;

- Use any other means providing equivalent insulation.

A3.3.3.2. Separation by earthed screen (method 2)

Where SELV circuits are separated from parts at hazardous voltage by an earthed screen or other earthed conductive parts, the parts at hazardous voltage shall be separated from the earthed parts by at least basic insulation. The earthed parts shall comply with A3.5.

A3.3.3.3. Protection by earthing of the SELV circuit (method 3)

Parts of SELV circuits by earthing shall be connected to the protective earthing terminal in such a way that the requirements of A3.3.3 are met by relative circuit impedances or by the operation of a protective device or both. They shall also be separated from parts of other non - SELV circuits by at least basic insulation. The SELV circuit shall be have adequate fault current carrying capacity to ensure operation of the protective device, if any, and to ensure that the fault current path to earth will not open.



A3.3.4. Additional constructional requirements

The equipment shall also be constructed as follows:

- Ring - tongue and similar terminations shall be prevented from any pivoting that would reduce creepage distances and clearances between SELV circuits and parts at hazardous voltage below specified minimum values;

- In multiway plugs and sockets, and wherever shorting could otherwise occur, means shall be provided to prevent contact between SELV circuits and parts at hazardous voltage by loosening of a terminal or breaking of a wire at a termination;

- Uninsulated parts at hazardous voltage shall be so located or guarded as to avoid accidental shorting to SELV circuits, for example by tools or test probes used by service personnel;

- SELV circuits shall not use connectors compatible with those specified in IEC83 or IEC320.



A3.3.5. Connection of SELV circuits to other circuits

SELV circuits are permitted to be supplied from or connected to other circuits provided that all of the following conditions are met:

- Except as permitted by A3.2.7 and A3.4.6, the SELV circuit is separated by at least basic insulation from any primary circuit (including the neutral) within the equipment;

- SELV circuit meets the limits of A3.3.2 under normal operating conditions;

- Except as specified in 3.1.2.2, the SELV circuit meets the limits of A3.3.3 in the event of a single failure of any component or insulation of the secondary circuit to which it is connected.

If an SELV circuit is connected to one or more other circuits, the SELV circuit is that part which complies with the requirements of the A3.3.2 and A3.3.3.

Where an SELV obtains its supply conductively from a secondary circuit which is separated from a hazardous voltage circuit by:

- Double insulation or reinforced insulation, or by

- The use of an earthed conductive screen that is separated from a hazardous voltage circuit by basic insulation,

the SELV circuit shall be considered as being separated from the primary circuit or other hazardous voltage circuit by the same method.



A3.4. Limited current circuits

A3.4.1. Limited current circuits shall be so designed that the limits specified in A3.4.2, A3.4.3, A3.4.4 and A3.4.5 are not exceed under normal operating conditions and in the event of breakdown of any basic insulation or a single component failure, together with any faults which are the direct consequence of such breakdown or failure.

Except as permitted in A3.4.6, segregation of parts of limited current circuits from other circuits shall be as described in A3.3 for SELV circuits.



A3.4.2. For frequencies not exceeding 1 kHz, the steady-state current drawn through a non-inductive resistor of 2000 Ω connected between any two parts of limited current circuit, or between any such part and the equipment protective earthing terminal, shall not exceed 0.7 mA peak AC, or 2 mA DC. For frequencies above 1 kHz, the limit of 0.7 mA is multiplied by the value of the frequency in kilohertz but shall not exceed 70 mA peak.

A3.4.3. For parts not exceeding 450 V peak or DC, the circuit capacitance shall not exceed 0.1 μF.

A3.4.4. For parts exceeding 450 V peak or DC, but not exceeding 15000 V peak or DC, the available stored charge shall not exceed 45 μC.



A3.4.5. For parts exceeding 15000 V peak or DC, the available energy shall not exceed 350 mJ.

A3.4.6. Limited current circuits are permitted to be supplied from or connected to other circuits, provided that the following conditions are met:

- The limited current circuit meets the limits of A3.4.2, A3.4.3, A3.4.4 and A3.4.5 under normal operating conditions;

- The limited current circuit continues to meet the limits of A3.4.2, A3.4.3, A3.4.4 and A3.4.5 in the event of a single failure of any component or insulation in the limited current circuit, or of any component or insulation in the other circuit to which it is connected.

If a limited current circuit is connected to one or more other circuits, the limited current circuit is that part which complies with the requirements of A3.4.1.



A3.5. Provisions for earthing

A3.5.1. Class I equipment

Accessible conductive parts of class I equipment which might assume a hazardous voltage in the event of single insulation fault shall be reliably connected to a protective earthing terminal within equipment.

In service access areas, where conductive parts such as motor frames, electronic chassis, etc, might assume a hazardous voltage in the event of a single insulation fault, either these conductive parts shall be connected to the protective earthing terminal or, if this is impossible or impracticable, a suitable warning label shall indicate to service personnel that such parts are not earthed and should be checked for hazardous voltage before being touched.

This requirement does not apply to accessible conductive parts that are separated from parts at hazardous voltage by:

- Earthed metal parts, or

- Solid insulation or an air gap, or a combination of the two, meeting the requirements for double or reinforced insulation.



A3.5.2. Class II equipment shall have no provision for protective earthing except that it may provided with a means for maintaining the continuity of protective earthing circuits to other equipment in a system. Such a means shall be separated from parts at hazardous voltage by double or reinforced insulation.

If class II equipment has an earth connection for functional purposes, the functional earth circuit shall be separated from parts at hazardous voltage by double or reinforced insulation.



A3.5.3. Protective earthing conductors shall not contain switches or fuses.

A3.5.4. If a system comprises class I equipment and class II equipment, interconnection of the equipment shall be such that earthing connection is assured for class I equipment regardless of the arrangement of equipment in the system.

A3.5.5. Protective earthing conductors are permitted to be bare or insulated. If used, insulation shall be green/ yellow except in the following two cases:

- For earthing braids, the insulation shall be either green/yellow or transparent;

- For internal protective conductors in assemblies such as ribbon cables, busbars, flexible printed wiring, etc., any color is permitted provided that no misinterpretation of the use of the conductor is likely arise.

A3.5.6. Protective earth connections shall be such that disconnection of a protective earth at one assembly does not break the protective earthing connection to other assemblies, unless hazardous voltages are removed from other assemblies at the same time.

A3.5.7. Protective earthing connections shall make earlier and break later than the supply connections in each of the following:

- The connector of an operator- removable part that has a protective earthing connection;

- A plug on a power supply cord;

- An appliance couple.



A3.5.8. Protective earthing connections shall be so designed that they do not have to be disconnected for servicing other than for the removal of the part which they protect unless hazardous voltage is removed from part at the same time. A3.5.9. Protective earthing terminals for fixed supply conductors or for non-detachable power supply cords shall comply with the requirements for wiring terminals for external primary power supply conductor.

The clamping means, if any, of such terminals shall prevent accidental loosening of the conductor. In general, the designs commonly used for current-carrying terminals, other than some terminals of the pillar type, provide sufficient resilience to comply with the latter requirement; for other designs, special provisions, such as the use of an adequately resilient part which is not likely to be removed inadvertently, shall be used.

A3.5.10. Corrosion resistance

Conductive parts in contact at protective earth connections shall be subject to significant corrosion due electro - chemical action in any working, storage or transport environment conditions as specified in the manufacturer’s instructions.

The protective earthing terminal shall be resistant to significant corrosion. Corrosion resistance can be achieved by a suitable plating or coating process.

A3.5.11. Resistance of protective earthing conductors

The resistance of the connection between the protective earthing terminal or earthing contact and parts required to be earthed shall not exceed 0.1 Ω.

A3.6. Disconnection from primary power

A disconnect device shall be provided to disconnect the equipment from the supply for servicing.



A3.7. Safety interlock

Safety interlock shall be provided where operator access involves areas normally presenting hazards.



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