As part of the 18th Edition IET wiring regulations an onus has been put on electrical designers to consider surge protection (against transient overvoltages due to atmospheric or switching conditions) as part of the electrical installation. To fulfil their obligations this can require them to seek information from the network operator.
In the case of new connections, this information may be obtained from the design proposal, but as the electrical network is continually changing it would be almost impossible as a network operator to give the exact information for each property and to continually revise this in the event of a local network change. Therefore, any information provided has to be at a point in time and be general to the area.
To complete the necessary designs the designer requires information on the connecting conductor arrangement for the first kilometre. E.g. LV, HV, overhead, underground, or to the nearest installed overvoltage protective device.
Therefore the worst condition should be assumed of LPAL = 1km
Section 443 of BS7671 – Protection Against Transient Overvoltages Of Atmospheric Origin Or Due To Switching
This states the requirement for overvoltage control and makes reference to performing a risk assessment based on the first kilometre of the electrical supply network or the nearest installed overvoltage protective device in the power network from the premises.
Sub section 443.5 Risk assessment method
- The Calculated Risk Level is given by the following formula CRL=Fenv/(Lp x Ng)
- Fenv is an environmental factor selected according to Table 443.1 in BS7671.
- Ng is the lightning ground flash density(flashes per km2 per year) relevant to the location of the power line and connected structure (See Figure 44.2 shown in BS7671)
- Lp is the risk assessment length in km. Lp = 2 LPAL + LPCL + 0.4 LPAH + 0.2 LPCH (km)
- LPAL is the length (km) of low voltage overhead line. (Assume LPAL = 1)
- LPCL is the length (km) of low voltage underground cable. (Assume LPCL =0)
- LPAH is the length (km) of high voltage overhead line. (Assume LPAH =0)
- LPCH is the length (km) of high voltage underground cable. (Assume LPCH =0)
In relation to the network operator it is impractical to maintain these records and update the relevant parties whenever the network changes. Therefore the worst condition should be assumed of LPAL = 1km and this will lead to a worst condition Lp of 2.
As the main driver for the formula in Northern Ireland will be whether the premises are located in a rural/suburban location or in an urban location. The assumption LPAL = 1km will ensure that the designer and the end user are best served.
References BS7671:2018 © The The Institution of Engineering and Technology and BSI.