Страница 4: ДСТУ EN 62305-4:2012. Protection against lightning — Part 4: Electrical and electronic systems within structures (61692)

6. Coordinated SPD system

The protection of internal systems against surges requires a systematic approach consisting of coordinated SPDs for both power and signal lines. The rules for the selection and installation of a coordinated SPD system are similar in both cases (see Annex C).

In SPM using the lightning protection zones concept with more than one inner LPZ (LPZ 1, LPZ 2 and higher), SPD(s) shall be located at the line entrance into each LPZ (see Figure 2).

In SPM using LPZ 1 only, an SPD shall be located at the line entrance into LPZ 1 at least.

In both cases, additional SPDs may be required if the distance between the location of the SPD and the equipment being protected is long (see Annex C).

The SPD’s test requirements shall comply with

• IEC 61643-1 for power systems,

• IEC 61643-21 for telecommunication and signalling systems.

Information on selection and installation of a coordinated SPD system are reported in Annex C. Selection and installation of a coordinated SPD system shall comply also with

• IEC 61643-12 and IEC 60364-5-53 for protection of power systems,

• IEC 61643-22 for protection of telecommunications and signalling systems.

Information and guidelines as to the magnitude of surges created by lightning, for the purpose of dimensioning SPDs at different installation points in the structure, is provided in Annex D of this standard and Annex E of IEC 62305-1:2010.

8. Isolating interfaces

Isolating interfaces may be used to reduce the effects of LEMP. Protection of such interfaces against overvoltages, where needed, may be achieved using SPDs. The withstand level of the isolating interface, and the voltage protection level of the SPD U_p shall be coordinated with the overvoltage categories of IEC 60664-1.

NOTE The scope of this part of IEC 62305 deals with protection of equipment within structures, and not protection of interconnected structures to which the isolation transformer may provide some benefit.

8. SPM management

   1. General

To achieve a cost-effective and efficient protection system, the design should be carried out during the building conception stage and before construction. This allows one to optimize the use of the natural components of the structure and to choose the best compromise for the cabling layout and equipment location.

For a retrofit to existing structures, the cost of SPM is generally higher than that of the cost for new structures. However, it is possible to minimize the cost by a proper choice of LPZ and by using existing installations or by upgrading them.

Proper protection can be achieved only if

• provisions are defined by a lightning protection expert,

• good coordination exists between the different experts involved in the building construction and in the SPM (e.g. civil and electrical engineers),

• the management plan of 9.2 is followed.

The SPM shall be maintained by inspection and maintenance. After relevant changes to the structure or to the protection measures, a new risk assessment should be carried out.

Planning and coordination of the SPM requires a management plan (see Table 2), which begins with an initial risk assessment (IEC 62305-2) to determine the required protection measures needed to reduce the risk to a tolerable level. To accomplish this, the lightning protection zones shall be determined.

In accordance with the LPL defined in IEC 62305-1, and the protection measures to be adopted, the following steps shall be carried out:

• an earthing system, comprising a bonding network and an earth-termination system, shall be provided;

• external metal parts and incoming services shall be bonded directly or via suitable SPDs;

• the internal system shall be integrated into the bonding network;

• spatial shielding in combination with line routing and line shielding may be implemented;

• requirements for a coordinated SPD system shall be determined;

• suitability of isolating interfaces shall be determined;

• for existing structures, special measures may be needed (see Annex B).

After this, the cost/benefit ratio of the selected protection measures should be re-evaluated and optimised using the risk assessment method again.Table 2 - SPM management plan for new buildings and for extensive changes in construction or use of buildings

The inspection comprises checking the technical documentation, visual inspections and test measurements. The object of the inspection is to verify that

• the SPM complies with the design,

• the SPM is capable of performing its design function,

• any new additional protection measure is integrated correctly into the SPM.during the installation of the SPM,

• after the installation of the SPM,

• periodically,

• after any alteration of components relevant to the SPM,

• possibly after a lightning flash to the structure (e.g. where indicated by a lightning flash counter, or where an eyewitness account of a flash to the structure is provided, or where there is visual evidence of lightning-related damage to the structure).

The frequency of the periodical inspections shall be determined with consideration to

• the local environment, such as corrosive soils and corrosive atmospheric conditions,

• the type of protection measures employed.

NOTE Where no specific requirements are identified by the authority having jurisdiction, the values of Table E.2 of IEC 62305-3:2010 are recommended.

After the installation of new SPM measures, the technical documentation shall be checked for compliance with the relevant standards, and for completeness. Consequently, the technical documentation shall be continuously updated, e.g. after any alteration or extension of the SPM.

Visual inspection shall be carried out to verify that

• there are no loose connections nor any accidental breaks in conductors and joints,

• no part of the system has been weakened due to corrosion, especially at ground level,

• bonding conductors and cable shields are intact and interconnected,

• there are no additions or alterations which require further protection measures,

• there is no indication of damage to the SPDs and their fuses or disconnectors,

• appropriate line routings are maintained,

• safety distances to the spatial shields are maintained.

A measurement of electrical continuity should be performed on those parts of an earthing and bonding system that are not visible for inspection.

NOTE If an SPD does not have a visual indicator (flag), measurements shall be performed in accordance with the manufacturer’s instructions to confirm its operating status, when so required.

An inspection guide should be prepared to facilitate the process. The guide should contain sufficient information to assist the inspector with his task, so that all aspects of the installation and its components, tests methods and test data which is recorded, can be documented.

The inspector shall prepare a report, which shall be attached to the technical documentation and the previous inspection reports. The inspection report shall contain information covering

• the general status of the SPM,

• any deviation(s) from the technical documentation,

• the result of any measurements performed.

After inspection, all defects noted shall be corrected without delay. If necessary, the technical documentation shall be updated.Annex A
(informative)

Basis of electromagnetic environment evaluation in an LPZ

A.1 General

Annex A provides information for the evaluation of the electromagnetic environment inside an LPZ, that can be used for protection against LEMP. It is also suitable for protection against electromagnetic interference.

A.2 Damaging effects on electrical and electronic systems due to lightning

A.2.1 The source of damage

The primary source of damage is the lightning current and its associated magnetic field, which have the same waveshape as the lightning current.

NOTE For protection considerations the influence of the lightning electric field is usually of minor interest.

A.2.2 Object of damage

Internal systems installed in or on a structure having only a limited withstand level to surges and to magnetic fields, may be damaged or operate incorrectly when subjected to the effects of lightning and its subsequent magnetic fields.

Systems mounted outside a structure can be at risk due to the unattenuated magnetic field and, if positioned in an exposed location, due to surges up to the full lightning current of a direct lightning strike.

Systems installed inside a structure can be at risk due to the remaining attenuated magnetic field, due to the conducted or induced internal surges and due to external surges conducted by incoming lines.

For details concerning equipment withstand levels the following standards are of relevance:

• the rated impulse voltage level of the power installation is defined in Table F.1 of IEC 60664-1:2007. The withstand level is defined by the rated impulse withstand voltage 1,5 kV - 2,5 kV - 4 kV and 6 kV for 230/400V and 277/480V systems;

• The withstand level of telecommunication equipment is defined in ITU-T K.20 ^[3J, K.21 ^[41 and K.45 ^[5].

The withstand level of equipment is generally defined in the accompanying product specification sheet, or can be tested

• against conducted surges using IEC 61000-4-5 with test levels for voltage: 0,5 kV - 1 kV - 2 kV and 4 kV at 1,2/50 ps waveshape and with test levels for current: 0,25 kV - 0,5 kV - 1 kV and 2 kA at 8/20 ps waveshape,

NOTE In order for certain equipment to meet the requirements of the above standard, it may incorporate internal SPDs. The characteristics of these internal SPDs may affect the coordination requirements.

• against magnetic fields using IEC 61000-4-9 with test levels: 100 A/m - 300 A/m - 1 000 A/m at 8/20 ps waveshape and IEC 61000-4-10 with test levels: 10 A/m - 30 A/m - 100 A/m at 1 MHz.

Equipment not complying with radio frequency (RF) radiated emission and immunity tests, as defined by the relevant EMC product standards, can be at risk due to directly radiated magnetic fields into it. On the other hand, the failure of equipment complying with these standards can be neglected.

A.2.3 Coupling mechanisms between the object of damage and the source of damage

The equipment’s withstand level needs to be compatible with the source of damage. To achieve this, the coupling mechanisms need to be adequately controlled by the appropriate creation of lightning protection zones (LPZs).

A.3 Spatial shielding, line routing and line shielding

A.3.1 General

The magnetic field caused inside an LPZ by lightning flashes to the structure or the nearby ground, may be reduced by spatial shielding of the LPZ only. Surges induced into the electronic system can be minimised either by spatial shielding, or by line routing and shielding, or by a combination of both methods.

Figure A.1 provides an example of the LEMP in the case of lightning strike to the structure showing the lightning protection zones LPZ 0, LPZ 1 and LPZ 2. The electronic system to be protected is installed inside LPZ 2.

Figure A.1 - LEMP situation due to lightning strike

In Table A.1 points 1, 2 and 3 define the parameters /₀, H_o, and U_w of Figure A.1; suitable test parameters, to assure that equipment is able to withstand the expected stress in its installation location, are given in points 4 and 5.Table А.1 - Parameters relevant to source of harm and equipment

The primary electromagnetic sources of harm to the electronic system are the lightning current /₀ and the magnetic field H_o. Partial lightning currents flow through the incoming services. These currents as well as the magnetic fields have approximately the same waveshape. The lightning current to be considered here consists of a first positive stroke /_F (typically with a long tail 10/350 ps waveshape) and first negative stroke /_FN (1/200 ps waveshape) and subsequent strokes /_s (0,25/100 ps waveshape). The current of the first positive stroke /_F generates the magnetic field H_F, the current of the first negative stroke /_FN generate the magnetic field H_FN, and the currents of the subsequent strokes /_s generate the magnetic fields H_s.

The magnetic induction effects are mainly caused by the rising front of the magnetic field. As shown in Figure A.2, the rising front of H_F can be characterized by a damped oscillating field of 25 kHz with maximum value H_FIMAX and time to maximum value T_p/F of 10 ps. In the same way, the rising front of /7_S can be characterized by a damped oscillating field of 1 MHz with maximum value H_S/MAX and time to maximum value T_p/S of 0,25 ps. Similarly the rising front of /7_fn can be characterised by a damped oscillating field of 250 kHz with maximum value ^hfn/max ^and ^{time to} maximum value T_P/FN of 1 ps.

It follows that the magnetic field of the first positive stroke can be characterized by a typical frequency of 25 kHz, the magnetic field of the first negative stroke by a typical frequency of 250 kHz, and the magnetic field of the subsequent strokes by a typical frequency of 1 MHz. Damped oscillating magnetic fields of these frequencies are defined for test purposes in IEC 61000-4-9 and IEC 61000-4-10.

By installing magnetic shields and SPDs at the interfaces of the LPZ, the effect of the unattenuated lightning defined by /₀ and H_o should be reduced to or under the withstand level of the equipment. As shown in Figure A.1, the equipment should withstand the surrounding magnetic field H₂ and the conducted lightning currents l₂ and voltages U₂.

The reduction of /₁ to /₂ and of U₁ to U₂ is the subject of Annex C, whereas the reduction of H_Q to a sufficiently low value of H₂ is considered here as follows:

In the case of a grid-like spatial shield, it may be assumed that the waveshape of the magnetic field inside the LPZs {H_vH₂) is the same as the waveshape of the magnetic field outside (/-/₀).

The damped oscillating waveforms shown in Figure A.2 comply with the tests defined in IEC 61000-4-9 and IEC 61000-4-10 and can be used to determine the equipment’s withstand level to magnetic fields created by the rise of the magnetic field of the first positive stroke H_F and of the subsequent strokes H_s.

The induced surges caused by the magnetic field coupled into the induction loop (see Clause A.5), should be lower than, or equal to, the equipment’s withstand level.

I Basic standard: IEC 61000-4-9 |

Figure A.2a - Simulation of the rise of the field of the first positive stroke (10/350 ps) by a single impulse 8/20 ps (damped 25 kHz oscillation)Basic standard: ІЕС 61000-4-10

Figure A.2b - Simulation of the rise of the field of the subsequent stroke
(0,25/100 ps) by damped 1MHz oscillations (multiple impulses 0,2/0,5 ps)

NOTE 1 Although the definitions of the time to the maximum value T_p and the front time are different, for a convenient approach, their numerical values are taken as equal here.

NOTE 2 The ratio of the maximum values I I = 4:2:1 F/MAX FN/MAX S/MAX

Figure A.2 - Simulation of the rise of magnetic field by damped oscillations