The project was sub-divided into three headings as follows:
11.4.1 Low voltage underground operations including:
- Underground cable jointing.
- Metering and service work.
- LV feeder pillars jointing - inspections, live testing, fuse/link replacements, fuse mate applications.
- Link box operations - inspections & testing, linking.
- Industrial service units - inspections, live testing, fuse/link replacements.
- Bulk LV supplies to Industrial Commercial Users, provision of large single feeder ACBs. Possible regeneration effects of large induction motors.
- Application of temporary generation to systems, effects on all the above scenarios.
11.4.2 Low voltage Overhead operations including:
- Work on open 3 phase and neutral distribution lines.
- Shrouding and isolation.
- Tap off operations mains and services.
- Aerial bundled conductors (ABC) terminations.
- Live work by climbing as well as access by mobile electrical work platform.
11.4.3 High voltage Distribution Operations including:
- Testing with voltage indicator (Proving Dead).
- VT Operations.
The above work groups were designed to be representative of the majority of operations and were not meant to be exhaustive. Primary substations were not included in the calculations because of the high degree of remote operations and voltage restrictions within IEEE 1584 calculations.
Complex risk assessments were undertaken over a four-month period. The risk assessment looked at worse case scenarios, taking photographs of all the high-risk operational tasks and measuring the distance between a potential arc and a worker’s body in different positions. The measurements were collated into a report that evaluated the energy levels, arc flash risks and highlighted areas where there was opportunity to improve. The process met European Legislation (European Council Directive 89/391/EEC EU Workplace Health and Safety Directive) requiring employers to perform a risk assessment on tasks and was framed by the 4P approach, See Chapter 3: Risk Assessments and the Four P guide.
The incident energy calculations were carried out in accordance with IEEE 1584 Guide for Performing Arc Flash Hazard Calculations 2002. Two single line diagrams were produced which would give the flexibility to model most circuit configurations at both high voltage and low voltage. Large permanently connected motors and temporary generators were included to add to the complexity of various infeed fault contributions.
11.5 Modelling and Data Gathering
The Northern Powergrid training centre was used as a base for simulated conditions for high voltage and low voltage substation operations. The training centre had most types of switchgear that could be encountered in the field. In addition, the outdoor overhead lines training centre was also used to get actual conditions and body measurements for various overhead tasks. This was all backed up by visits to worksite locations and system design offices to view cable records. Body positioning, working distances, access and egress could all influence severity of injury and were taken into account as part of the assessment. The models that were developed to give circuit configurations included the following:
- Various cable types, sizes and lengths.
- Different LV boards.
- Transformers of varying sizes.
- Parallel configurations between substations.
- Temporary generators – contributions.
- Fault contributions from large LV industrial induction motors.
- Various protective devices including TLFs, fuses, electromechanical relays and electronic relays.
11.6 Assumptions and Generalisations
A great deal of discussion was had about various assumptions/generalisations that could be made and also which of these could be discounted. The following highlights some of these topics and how decisions were made.
Embedded Generation. Whilst the models simulated various temporary generators, normally installed to keep customers online during maintenance, there was no account taken of smaller embedded generators. It was recognised at that time, that in the future, micro generation may become more prevalent and could have an impact on incident energy levels.
Time Lag Fuses. TLFs were the most popular type of device in use for the protection of secondary distribution transformers and various types were modelled into the scenarios including tagged, barrel type and rewireable. Tagged TLFs were used as the final basis of the studies as they are the most popular and have a greater long-time disconnection as displayed on the time / current characteristic curve. Rewireable fuses were being replaced at the time and were non-existent in some areas.