Traditional and probabilistic design standards
4.2.7 From 1988 to date
Forty years after the industry was nationalised the whole process was reversed. Legislation for privatisation was enacted in the form of The Electricity Act 1989 and, in common with previous acts, its contents were primarily concerned with organisational, legal and fiscal matters.
Similarly, a new set of regulations was introduced – The Electricity Supply Regulations 1988. These regulations represented a significant change in the philo- sophy of electricity regulations. Whereas previous regulations had given specific design parameters, the 1988 regulations gave very few, in fact, within the actual design parameters, only the Schedule of Ground Clearances survived. Henceforth, the responsibility for selecting design parameters would rest squarely with the elec- tricity supplier. This new philosophy is encapsulated in Part V, Section 17 of the regulations which states:
All supplier’s works shall be sufficient for the purposes for, and the circumstances in which they are used and so constructed, installed, protected (both electrically and mechanically), used and maintained as to prevent danger or interruption of supply so far as is reasonably practicable.
In 1988, therefore, a position was reached where the design engineer, without assistance from a statutory list, must select/devise parameters and components to construct a line which is ‘fit for the purpose’ taking into account all the technical, safety, geographical, climatic, amenity and environmental conditions that might be encountered.
To the standards engineer and the line design engineer this new approach was both an added responsibility and an opportunity for innovation. However, when privatisa- tion arrived, the Electricity Council’s system of working parties more or less ceased for a period of some years. This raised the likelihood that each DNO with its newly found freedom to innovate would begin to develop its own standards. The spectre of
declining standards across the UK with some 12 versions of the same basic design could bring about consequential manufacturing and commercial difficulties. 4.2.7.1 The Electricity Safety, Quality and Continuity Regulations, 2002 These regulations (known as ESQCR 2002) were published in October 2002 and came into force from 31 January 2003. They cover:
a protection and earthing b sub-stations
c underground cables d overhead lines e generation
f supplies to other networks g design schedules.
Here, this chapter covers only items d and g. The regulations again refer to either bare or insulated wire, and therefore continue the previous regulations in treating covered conductors as bare wire, unless they have sufficient covering to be regarded as insulated. The regulations also put the responsibility of safety squarely with the owner of the overhead line. The term ‘ordinarily accessible’ is used. This means that the OHL should not be reachable by hand ‘if any scaffolding, ladder or other conducting object was erected or placed in, against or near to a building or structure’. If someone wishes to erect a building so that it may allow an OHL to be ordinarily accessible then all they have to do is give reasonable notice to the DNO. These defini- tions leave the industry open to many problems concerning permanent or temporary structures (e.g. tents) built under or near lines.
The section on anticlimbing devices (ACDs) is also woolly and open to many interpretations. It states that every support shall ‘if circumstances reasonably require’ be fitted with ACDs to prevent ‘so far as is reasonably practical’ any unauthorised person reaching a ‘position of danger’.
Schedule 2 of item g specifies the minimum height above ground of overhead lines (see Table 2.1).
4.2.7.2 Other standards
For many years the UK standards listed at the end of this chapter (section 4.4) have been the design bedrock of the UK overhead line networks. Nowadays the hierarchy of standards is on an international scale and standards should be selected and used in the following sequence dependent on their availability:
1 international standards 2 European standards 3 British standards
4 Electricity Association technical specifications 5 company standards.
One of the most important recent standards for lines at all distribution volt- ages is BS EN 50341 (a CENELEC standard) and its sister draft publication
Traditional and probabilistic design standards 57 BS EN 50423. Both these standards have a common UK national normative annex (NNA) BS EN 50423-3-9.
4.2.8
BS EN 50341 and BS EN 50423
BS EN 50341 refers to overhead electrical lines exceeding AC 45 kV and BS EN 50423 refers to overhead electrical lines exceeding AC 1 kV up to and includ- ing AC 45 kV. Essentially, the UK has to adopt these new standards and in particular their view on wood pole design using either probabilistic methods (called the general approach in these documents) or deterministic methods based on traditional factors of safety (known as the empirical approach). It is this latter empirical approach that the UK BSI standards committee has declared will be adopted for all wood pole line designs.
The general (or probabilistic) approach was seen as far too onerous for the UK in respect of wood pole designs due to the following reasons:
1 The characteristic fibre stress (the fibre stress of the bottom 5 per cent of the spread of fibre stress values found in wood poles is effectively taken as the characteristic value in the UK after the use of factors of safety) would be lower than currently used. Typically, the mean fibre stress for Scots Pine as purchased by the UK DNOs is 53.4 N/m2, whereas around 5 per cent of these actually have a fibre stress of below 21.4 N/m2due to the normal range of properties of a natural product. At the time of writing, no supplier in Europe has published an appropriate characteristic stress that should be adopted for a probabilistic design approach for each potential pole species.
2 Ice load is very large for no wind conditions, so very high line tensions need to be considered.
3 Vertical loads for intermediate poles must be considered. Overturning loads, only, have historically been considered for these structures.
4 A ten per cent deflection limit was considered very onerous for intermediate poles, especially for the taller poles.
To take account of UK experience in wood pole line designs, a NNA for the UK was agreed to be associated with the main body text of BS EN 50341/50423. Known as BS EN 50423-3-9, this part of the standard details the specific empirical design approach to be employed for wood pole lines and allows the UK to specify loading scenarios and factors of safety very similar to those included in ENATS 43-20. It should be noted that the load factored design approach employed in ENATS 43-40 Issue 1 has now been superseded by this new design approach, which has now been incorporated within Issue 2 of that technical specification.
There now follows a brief description of BS EN 50341/50423 and the UK NNA BS EN 50423-3-9 as it applies to UK wood pole OHL at voltages up to and including 132 kV (trident design). It does not apply to LV lines (<1000 V AC). The standard need only apply to new OHL and not to the maintenance, re-conductoring, tee-offs, extensions or diversions to existing lines.
Table 4.3 Wind pressures and aerodynamic drag factors
Load condition Wind pressure Aerodynamic (N/m2) drag factors
qx qc Cx Cc
High wind (no ice) 1740 1740 0.8 1.0 Combined wind and ice (normal altitude) 380 380 1.0 1.0 Combined wind and ice (high altitude) 570 570 1.0 1.0 Wind only (no ice) 0 760 – 1.0 Security (broken wire) 380 380 1.0 1.0
For the leeward (shielded) pole, a shielding factor of 0.5 shall be assumed.
Wind loads
The wind pressures and aerodynamic drag factors are given in Table 4.3. The span factor, G, is assumed to be 1.0 for wind spans≤200 m and (0.75L + 30)/L for wind span lengths (L) >200 m.
The specification also defines high and normal altitude as:
normal altitude for all GB except Scotland for site altitudes 300 m in Scotland <200 m
high altitude all other locations up to 500 m.
The standard does not apply to lines on land above 500 m in the UK.
Wind and ice loads
For conductors not exceeding 35 mm2copper (60 mm2aluminium-based) conductors, the wind only case in Table 4.3 may be used. For all other lines the radial ice thicknesses are:
normal altitude 9.5 mm high altitude 12.5 mm
The ice density is taken as 913 kg/m3, although other ice densities of 500 kg/m3 (for rime ice) and 850 kg/m3 (for wet snow) may be specified. The minimum temperature is as traditionally used in the UK, i.e.−5.6◦C.
Conductor loads
Conductor load cases are specified in Table 4.4.
Factors of safety
EN 50423-3-9 includes partial factors which are essentially the UK’s factors of safety (FOS). A list of the FOS is given in Tables 4.5 and 4.6.
Traditional and probabilistic design standards 59 It is important to note that Table 4.6 was derived to simplify the effect of vertical loads that continue to be ignored for intermediate wood pole structures. OHL components also have partial factors applied, known as gamma-m factors, as given in Table 4.7.
Table 4.4 Conductor load cases
Load Temperature Load condition Notes cases (◦C)
1 −5.6 high wind (no ice) as detailed in the project specification 2 −5.6 combined wind and ice
(normal altitude)
normal altitude: conductor >35 mm2 copper
3 −5.6 combined wind and ice (high altitude)
high altitude: conductor >35 mm2 copper
4 −5.6 wind only (all altitudes – no ice)
conductor up to 35 mm2copper equivalent area
5 −5.6 security (broken wire) as detailed in the project specification 6 −5.6 construction and
maintenance
see project specification
Table 4.5 Partial factors for actions, ultimate limit state
Action (load) Partial factor Normal load cases – variable actions
Climatic loads and conductor tension
high wind (load case 1) 1.1 combined wind and ice (load cases 2 and 3) 2.52 wind only (load case 4) 2.51, 2 Permanent actions
Self weight
high wind (load case 1) 1.1 combined wind and ice (load cases 2 and 3) 2.52 wind only (load case 4) 2.51, 2 static cantilever loads (all load cases) 1.0 Exceptional load cases – security (broken wire) loads (load case 5) 1.3
Construction and maintenance (load case 6) 1.5 on static loads 2.0 on dynamic loads
1For timber pole supports, wind on the pole is ignored.
2Higher partial factors may be specified in the project specification, particularly for intermediate poles. See also Table 4.6.
Table 4.6 Partial factors for actions, inter- mediate pole declination
Action (load) FOS Declination gradient – climatic loads
Level – 1:10 (load cases 2, 3 and 4) 2.5
>1:10–1:7.5 (load cases 2, 3 and 4) 3.0
>1:7.5–1:5 (load cases 2, 3 and 4) 3.5
4.2.9
Clashing
Bare phase conductors need to be spaced to avoid mechanical and electrical damage due to clashing. Covered conductors do not, as their sheaths are easily capable of several million clashes without damage. In the case of bare conductors, for wood pole lines at normal altitudes, the minimum recommended phase separation is defined by weather zone 2B, and, for lines at high altitude, the minimum recommended phase separation is defined by weather zone 3C. Greater phase separations may be required due to the effect of funnelling or for altitudes greater than 500 m.
ENATR 111 defines the weather zone applicable to an area. This is where the likely mean wind pressure and absolute maximum ice accretion thickness may be described by a numeral and letter, respectively. The wind co-ordinate is described in 190 N/m2 increments, and the ice co-ordinate is measured in 10 mm diametric thickness increments for each letter increment (A= 10 mm, B = 20 mm etc.).
The gust and lull wind pressures are 1.832 and 0.546 times the mean wind pressure, respectively. The minimum spacing to avoid conductor clash is based on the worst combination of wind and ice.
Maps of weather zones are shown in 100 m increments of elevation above mean sea level in ENATR 111. However, as far as the NNA is concerned, 2B represents a wind pressure of 380 N/m2with 10 mm of radial ice load, whereas 3C represents 570 N/m2and 15 mm of radial ice.