Circuit breaker opening times. Northern Powergrid operation manuals listed high voltage circuit breaker times as between 40ms for Vacuum up to 75ms for Oil filled equipment. A conservative figure of 83ms was used in the final models. It was assumed that there was a high level of confidence in switchgear operating times and therefore no further safety margin was built in.
Generators. Most generator sets for temporary supplies were hired in and sizes varied from 200kVA to 1200kVA. Information about the generator impedance data was obtained from the main hire company. The models were adjusted for generator sizes which were appropriate for the transformer capacity so an LV generator of 800kVA would be used where the transformer size was 800kVA.
Low Voltage Fuses. The majority of low voltage protection was by BS88 J-Type Links and there were some small variations found between manufacturers. In the end all time/current characteristics were based upon Lawson stated data for total clearance time.
Low Voltage Networks. The vast majority of low voltage circuits within the south of the region was solidly connected with no sectioning of smaller feeders or services. In the north however, it had been the norm in recent years to install downstream fuses for tee offs to achieve grading and automatic disconnection of supplies in no more than 30 seconds. As it happened the models were finally based upon solidly connected LV networks to give conservative outputs.
Secondary High Voltage Distribution Fault Levels. The maximum fault level was taken as 250MVA at 11kV and 350MVA at 20kV. The system models create a number of scenarios in increments from 25MVA up to the maximum fault level. It was experienced that arcing faults on transformer tails and LV Boards would struggle to clear at very low fault levels particularly on 20kV systems.
Two Second Rule. There was a good deal of discussion about long clearance times for protective devices and many engineers would argue that an individual would recoil from an arcing fault of long duration. This will, however, be subject to them having sufficient working space to move away from the arc. In the IEEE 1584 guide there is reference to this which says:
“If the time is longer than two seconds, consider how long a person is likely to remain in the location of the arc flash. It is likely that a person exposed to an arc flash will move away quickly if physically possible and two seconds is a reasonable maximum time for calculations. A person in a bucket truck or a person who has crawled into equipment will need more time to move away.”
This is often quoted as the “Two Second Rule” but is really saying that judgement should be used in determining how long a person will stay in a fixed position in relation to an arc. When calculated to the 2002 version of IEEE 1584, incident energy roughly follows an inverse square law so 20cals/cm2 at 300mm will become 5cals/cm2 at 600mm. However, this relationship between working distance and incident energy is rather more complex with the 2018 version of IEEE 1584 so this will need to be taken into account.
Working space and ability to move away in the event of an arcing fault should therefore be key factors in any risk assessment prior to live work. In the context of UK Legislation this is expressed in regulation 15 of the Electricity at Work Regulations 1989 which states: “For the purposes of enabling injury to be prevented, adequate working space, adequate means of access, and adequate lighting shall be provided at all electrical equipment on which or near which work is being done in circumstances which may give rise to danger.”
Furthermore, the HSE guidance on Regulation 15 from HSR 25 Memorandum of Guidance says that: “Where there are dangerous exposed live conductors within reach, the working space dimensions should be adequate to allow people to pull back from the conductors without hazard.”
11.7 Results and Conclusions.
Low Voltage Operations. Incident energy tables were produced for low voltage boards that were connected directly to a transformer and via low voltage tails. Scenarios were reported for; six separate primary fault levels, four working distances and four transformer sizes. These scenarios were run at 11kV and 20kV. As was expected, the incident energy levels were at their highest in transformer low voltage boxes and associated low voltage boards. This was particularly the case when reduced fault levels were experienced on the primary side of the largest transformers that were modelled. Tables were produced to show the incident energy levels at a typical industrial service unit for various transformer sizes, primary fault levels and working distances on the same basis as the low voltage boards. Industrial Service Units were modelled with a 200kW motor connected and contributing to fault current.
Low Voltage Jointing. The computer model was used to predict the incident energy along a 300mm2 aluminium conductor wave form cable. A two second disconnection cut off was factored in once more. Incident energy tables were produced to show the results for a main supplied from a 400 ampere and a 500 ampere, J type BS88 Fuse. The table produced reported incident energy levels at various distances from the substation up to 700 metres for a 400-ampere fuse and 500 metres for a 500-ampere fuse. Also reported were four different working distances.
Public Lighting Cut Outs. Incident Energy levels at public lighting and street furniture cut outs are very often fed directly off a main and the incident energy levels can be assumed to be the same for a three-phase fault will be reduced as they are almost always connected at single phase. The IEEE 1584 calculations are valid only for three phase faults and to use them for single phase events will be very conservative.