tems
The hazardous area of touch and step voltages for persons outside of a building is within the distance of 3 m to the building and up to a height of 3 m. This height of the area to be protected corres- ponds to the level which a person can reach with his hand plus an additional separation distance s (Figure 5.7.1.1).
Special measures of protection are required, for example, for the entrances or canopies of struc- tures highly frequented such as theatres, cinemas, shopping centres, kindergartens where non-insu- lated down conductors and earth electrodes are nearby.
Structures which are particularly exposed (at risk of lightning strikes) and freely accessible to mem- bers of the public, for example mountain huts, may also be required to have measures preventing intolerably high touch voltages. Moreover life haz- ard is considered as parameter L1 (injury or death of persons) in the risk analyse of a structure according to IEC 62305-2 (EN 62305-2).
The following measures can reduce the risk of touch voltage:
⇒ The down conductor is sheathed in insulating material (min. 3 mm polymerised polyethylene
with an impulse withstand voltage of 100 kV (1.2/50 μs).
⇒ The position of the down conductors is changed, (e.g. down conductors are not installed in the entrance of the structure). ⇒ The specific resistance of the surface layer of
the earth at a distance of up to 3 m around the down conductor is at least 5 kΩm.
⇒ The probability of people accumulating can be reduced by information or prohibition signs; barriers can also be used.
The measures of protection against touch voltage may be insufficient with regard to an effective pro- tection of people. The required high-voltage resistant coating of an exposed down conductor, for example is not enough if there are no addition- al measures of protection against creep-flashovers at the surface of the insulation. This is particularly important if environmental influences such as rain (humidity) are to be considered.
Just like at a bare down conductor, high voltages occurs at an insulated down conductor in case of a lightning strike. This voltage, however, is separat- ed from people by the insulation. The human body being a very good conductor compared with the insulator, the insulating layer is stressed by almost the whole touch voltage. If the insulation does not cope with the voltage, part of the lightning cur- rent might flow to the earth via the human body as in case of the bare down conductor. Safe protec- tion against life hazard due to touch voltage requires to prevent from flashover through the insulation and from creep-flashovers along the insulation.
A balanced system solution as provided by the CUI conductor meets these requirements of electric s
2.50 m
copper conductor
PEX insulation
PE coating
strength and creep-flashover insulation to protect against touch voltage.
Structure of the CUI conductor
A copper conductor with a cross section of 50 mm2 is coated with an insulating layer of surge proof cross-linked polyethylene (PEX) of approx. 6 mm thickness (Figure 5.7.1.2).
The insulated conductor has an additional thin polyethylene (PE) layer for protection against external influences. The insulated down conductor CUI is installed vertically in the whole hazard area,
i.e. from the earth surface level up to a height of 3 m. The upper end of the conductor is connected to the down conductor coming from the air-termi- nation system, the lower end to the earth-termina- tion system.
Not only the electric strength of the insulation but also the risk of creep-flashovers between the ter- minal point at the bare down conductor and the hand of the touching person has to be considered. This problem of creeping discharges, well-known in high voltage engineering, is getting worse in case of rain, for example. Tests have shown that under sprinkling the flashover distance can be more than 1 m at an insulated down conductor without additional measures. A suitable shield on the insulated down conductor keeps the CUI con- ductor dry enough to avoid a creep-flashover along the insulating surface. The operating safety of the CUI conductor with regard to the electric strength and the resistance against creep-flash- overs at impulse voltages up to 100 kV (1.2/50 μs) has been tried and tested in withstand voltage tests under sprinkling conditions according to IEC 60060-1. At these sprinkling tests water of a cer- tain conductivity and quantity is sprinkled on the conductor in an angle of approx. 45 ° (Figure
5.7.1.3).
The CUI conductor is prefabricated with connec- tion element to be connected to the down conduc- tor (inspection joint) and can be shortened on site if necessary for being connected to the earth-ter- mination system. The product is available in lengths of 3.5 m or 5 m and with the necessary plastic or metal conductor holders (Figure 5.7.1.4). By the special CUI conductor the touch voltage at down conductors can be controlled with easy measures and little installation work. Hence the damage risk for persons in special areas will be considerably reduced.
Inductive coupling at a very great steepness of current
Regarding the damage risk for persons also the magnetic field of the arrangement with its influ- ence on the closer surrounding of the down con- ductor has to be considered. In extended installa- tion loops, for example, voltages of several 100 kV can occur near the down conductor which can result in high economic losses. Also the human body, due to its conductivity, together with the down conductor and the conductive earth, forms a connection
element
shield conductor
holder
Fig. 5.7.1.4 CUI conductor
loop having a mutual inductance of M where high voltages Uican be induced (Figures 5.7.1.5a and
5.7.1.5b). In this case the system
arrester-person has the effect of a transformer.
This coupled voltage arises at the insulation, the human body and the earth being primarily consid- ered as conductive. The voltage load becoming too high it results in a puncture or creeping flash- over. The induced voltage then drives a current through this loop, the magnitude of which depends on the resistances and the self- inductance of the loop and means life hazard for the person con- cerned. Hence the insulation must withstand this voltage load. The normative specification of 100 kV at 1.2/50 μs includes the high but very short voltage impulses which are only applied as long as the cur- rent rises (0.25 μs in case of a neg- ative subsequent lightning strike). The deeper the insulated down conductors are buried, the greater is the loop and thus the mutual inductance. Hence the induced voltage and the loading of the insulation increases correspond- ingly which also has to be taken into account with regard to the inductive coupling. h a ∆i/∆t a) ∆i/∆t b) M Ui U M i t i= ⋅ Δ Δ M h a rconductor = ⋅ ⋅ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 0 2. ln
Fig. 5.7.1.5 (a) Loop formed by conductor and person (b) Mutual inductance M and induced voltage Ui
6.1 Equipotential bonding for metal
installations
Equipotential bonding according to IEC 60364-4- 41 and IEC 60364-5-54
Equipotential bonding is required for all newly installed electrical power consumer’s installations. Equipotential bonding according to IEC 60364 series removes potential differences, i.e. prevents hazardous touch voltages between the protective conductor of the low voltage electrical power con- sumer’s installations and metal, water, gas and heating pipes, for example.
According to IEC 60364-4-41, equipotential bond- ing consists of the
main equipotential bonding (in future: protective equipotential bonding)
and the
supplementary equipotential bonding (in future: supplementary protective equipotential bonding)
Every building must be given a main equipotential bonding in accordance with the standards stated above (Figure 6.1.1).
The supplementary equipotential bonding is intended for those cases where the conditions for disconnection from supply cannot be met, or for special areas which conform to the IEC 60364 series Part 7.