Electrical protection and lightning protection
I have a query on earthing system design. One designer has submitted me an earthing plan which interconnects the earth electrodes meant for equipment earthing and those for lightning protection purposes. By combining the earth electrodes he achieves overall earth resistance below one ohm required by IS 3043 (Indian standard). But my question is whether this is an acceptable practice followed by industry or the earth electrodes of the two systems (protective earthing and lightning protection) can not be combined this way?
Would appreciate feedback. thanks in advance.
D. Sunil
Answer from Didier Mignardot: When Lightning Protection System (L.P.S.) is implemented, all metal conductors (including the electricity distribution network) must be interconnected.
Everything is described in IEC standard 62305-3 (Protection against Lightning: Physical damage to structures and life hazard) and in particular in the following chapters (5.4 & 6.1 extracts):
Earth-termination system
Internal lightning protection system
Please refer to this IEC standard 62305-3 and also other part like :
- IEC 62305-1: Protection against Lightning: General principles
- IEC 62305-2 : Protection against Lightning: Risk assessment
- IEC 62305-4 : Protection against Lightning : Electrical and electronic systems within structures
Though making agrid helps in achieign low value of resistance, lighting protetionsystem earthing and power earthing needs to be kept separate.also, sensitive equipment earthinglike CNC m/c etc needs to be separate.
Earth resistance value depends on soil resisitivity,the same needs to get tested.
Alos now there are prducts like – pipe in technology based earth electrodes available to help achieve lower values and requirng no maintenance.
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In order to complete the article of D. Sunil and D. Mignardot , here is some further information:
Series of standards IEC 60364 provides requirements for low voltage electrical installation for all applications (dwelling, commercial, industry)
Existing standard IEC 60364-5-54 provides requirements for the selection and erection of electrical equipment – earthing arrangements, protective conductors and protective bonding conductors. These requirements only cover protection of persons against electric chocks.
Series of standards IEC 62305 provides requirements for protection against lightning.
Existing standard IEC 62305- 3 provides requirement for physical damage to structures and life hazards. These requirements only cover protection in case of lightning.
To answer the question, it is possible to say that it is possible to erect only one single earthing arrangement, or to interconnect both types of earthing arrangements. IEC 60364-5-54 will be probably revised this way.
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EARTH pits can be interconnected.
min resistance is 10 ohms combined as per IS 3043
As per NEC standards/IEEE recommendations, we need to provide bonding between lightning earth grid and equipment earthing at least at two points.
We can have a common grid also for lightning and equipment if both are adjacent.
Check your design requirements accordingly.
Dear Anand
Can I have a discussion with u. My e mai ID is anandan.cholams@hotmail.com
Earth pits should kept seperate for power and lighting .Earting rods are inserted in 8 ft hole and is filled by charcoal and salt becoz charcoal and salt are good conductor of electricity provides a good path for current in ground when fault occurs and hence resistance reduces ie less than 10 ohms.
It was an illusion to beleive that grounding can be isolated from each other when lightning comes. It is the rule in IEC 62305 to interconnect all earthing either direct or by an isolating spark gap as per EN 50164-3. Of course this means that surge protective (SPDs) need to be used on all connected lines and high frequency equipotential bonding of metallic structures is needed. In addition, the value of 10 ohms requested in the past is no more requested in IEC 62305-3 what is important is to have a low high frequency value
hi
Iwant to ask you about iec 60071
for the test the transformer
Sunil’s query comes up from the assumption that
Set 1) The two earthing sets (or may be three, if there is a separate earthing set for the data & communication systems) and the earthing sets are not bonded above the ground level.
In the event of a a heavy discharge on the lightining protection system, the lightining protection system will (at the time and for the duration of the surge) have a rather high voltage for the trancient period. If the lightning stroke produces a peak current of 1kA, then with a 2 ohm earthing resistance, the voltage at the top of the earthing set will be 2000 volts! And the stroke current could and in all probability will be far higher than the assumed 1 kA. This produces a peculier situation of a potential difference of 2kV between the distribution system as well as signal system earthing sets (assuming that they are separate earthing sets) and the earthing set of the lightining conductor system.
The temporary potential difference between the two (or three) earthing sets can cause different nominal earth coints can cause shock (touch voltage between two surfaces or objects, which are technically supposed to be at the earth or zero potential)
Set 2) The two earthing sets (or three) have a bonding above the ground level:
In this case the entire earthing sets will have a transcient elevation of potential to 2kV with reference to the actual earth or zero potential body.
Now such a temporary elevation of potential can cause breakdown of insulation at vulnerable points.
Set 3) The earthing sets are physically separated by a signifcant distance and are bonded at the ground level.
In such a case the while the lightning system earth will be elevated to 2 kV, the distribution earth set (and the communication earth set) will be elevated by a much smaller voltage, due to the impedance of the bonding link between the lightning system earth and the power distribution earth.
As such the best practice would be to provide the three earth sets distributed in the space available as far or distant from each other as possible and also to bind them at the ground level so that all systems get the benefit of a lower effective earth resistance and the impact of a lightining stroke with a high transcient current and consequent short time potential elevation is reduced.
Communication engineers quite often complain and request that the earthing sets for their systems be provided as far as possible physically from the distribution system earth as well as the earth set for the lightning so that the interference is attenuated.
The best practice would be to keep the earth sets distributed at locations as distant from each other as is practicable from the site conditions and bond them at the earth level, with test links in the bonding line as well as in the earthing line, so that we do not become complacent during periodic maintenance check.
These distances is depends on the length of the Earth electrode. i.e. the distance between the earth electrodes should genarally be two times of length of the electrode as genaral.
for Ex. if of the electrode say 3mtr long, then the distance of electrodes will be 6mtrs.
as well as if required we can refer IEEE80 for the standard practice of earthing in substation area for step and mesh potentials factors
please give any further clarification/comments regards this.
Good analysis. In my opinion it would be better to have common grid for electrical distribution system ( body ) and lightning conductor by interconnecting them through earthing grid below ground level . All earth electrodes (pits) shall be interconnected through this grid. But the conductors interconnecting the lightning conductor/ arrester body to earth grid above ground surface should be through insulated casing / pipe to avoid shock due to potential diff. arises during the time of discharge of lightning stroke current which may last for few cycle.