Practicalities When Applying AS/NZS4853 for AC Low Frequency Induction Risk Assessment
Annual Conference
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AS/NZS 4853 Electrical hazards on metallic pipelines prescribes limits for steady state induced voltages from neighbouring electrical sources. Consequently, low frequency induction (LFI) risk assessments need to be undertaken. These assessments contribute to the overall asset management and life expectancy of the pipeline.
When AC voltages exceed the steady state induced voltage limit, high current densities at small coating defects appear causing AC corrosion. This is the result in part from improved coating quality and increasing interaction between pipeline and power transmission infrastructure. Induced voltage limits are provided for soil electrical resistivity ranges: (a) 4 V AC for soil resistivity ≤25 Ω.m; and (b) 10 V AC for soils whose resistivity is >25 Ω.m. Depending on the risk-based assessment outcome, the requirement for risk treatment of LFI for control of pipeline corrosion may be more severe than the requirement for human safety.
This paper reviews the risk assessment process and the practical limitations for calculating steady state impressed voltages. It also proposes an alternative to calculating these voltages. The paper presents a practical case study along the Watercare (Auckland) Huia No.1 & Niho No.1 replacement watermain. Included in the paper is a discussion regarding the limitations when applying AC measurements as a replacement for calculations.
Practical limitations for calculating steady state impressed voltages are more prevalent to urban type areas when compared to rural based areas. The reason for this is that there are usually multiple urban distribution overhead and underground electrical sources along the pipeline route that influence the pipeline. For computer simulated calculations, it is challenging to obtain accurate operating currents at a specific time/day/season as well as the circuit phasing for all electrical sources. As an alternative, for accurate assessments it is adequate to obtain real-time AC voltage test point recordings.
It can be expected that there are differences between measured & calculated AC steady state impressed voltages. These can be attributed either to a single, or a combination of the factors. These factors and an applied project are discussed in the paper.