From the outline load assessment it is possible to begin the design process, the design may be refined as the notification of electrical supply requirements forms are submitted. Once the total and projected loads have been calculated and the load locations and definitions confirmed it is possible to determine the supply required from the PES. For level 1 and level 2 supplies a single supply from the PES is required, for a level 3 supply two supplies from the PES are required. With level 3 supplies the ideal is to get two diverse supplies that is a supply from two different circuits, from different sub-stations, however at some point within the PES distribution network the two circuits will combine to become one, it is at which point that this happens which is of interest, the higher up the chain the better. At the very minimum it should be two feeds from different rings or feeders. The geographic locations of the loads will determine the number and locations of the switchrooms/sub-stations, ideally these should be no more than 200metres apart.
The outline load assessment should give enough information to determine the primary distribution and the final distribution systems, and whether there will be a secondary distribution system, generators and UPSs. It should also provide sufficient information to allow the physical sizes of equipment to be determined. The architect will have provided areas for the electricity distribution equipment, the areas must be assessed to ensure that there is adequate space and height for the equipment and the cables that will be connected to it.
In England where the assessed load is less than 35kVA the PES will probably offer a single phase LV supply above 35kVA and up to 500kVA a three phase LV supply, above 500kVA and up to 12MVA an 11kV supply, above 12MVA a 33kV supply, over 40MVA a 132kV supply. Some PESs offer a 33kV supply instead of an 11kV for supplies up to 12MVA. The PESs will normally agree to supply at a voltage level you request. PESs make a charge for the supply of a service. There is then a continuing periodic charge based on the level of capacity provided, that capacity is known as the reserve capacity.
In England the PESs have to comply with statutory instrument (SI) No. 2002-2665 The Electricity Safety, Quality and Continuity Regulations. These can be downloaded from the UK government’s website at the following address:-
http://www.legislation.gov.uk/uksi/2002/2665/contents/made
It is important to be aware of these regulations, you may be sure that the PESs are.
From the PES you should request the following information the supply impedance in r+jx form or impedance plus the x/r ratio (from which the r+jx values may be calculated), the maximum prospective short circuit current and earth fault level, the supply switchgear short circuit rating. The MV equipment installed as part of the distribution system must therefore be rated at the same level as the supply switchgear for 3 seconds. It is important to be able to determine the supply impedance in r+jx form as it allows calculation of the instantaneous short circuit level which determines the peak make rating of the circuit breakers.
It is common practice for the PESs to install neutral earthing resistors on the HV & MV supply transformers to limit the earth fault level, this will also affect the distribution design as it will vary the choice of cables and protective relays. The earth fault level cannot therefore be approximated from the short circuit level and the supply point earth fault level or earth loop impedance will be required from the PES.
Where a LV supply is given the PES normally supply an earth point, where the PES give a HV or MV supply you will need to design and install the earth point, more on earthing in the primary distribution section.
If a supply security level 2 or 3 is to be installed the PES must be informed that generators will be connected to the system, ensure that the single line diagrams issued to the PES include any generators.
The Electricity Association have a standard document covering the connection of generators to a system with an electricity supply from a PES, it is G59 get a copy of this together with their Engineering Technical Report No. 113 which gives additional information. (You will be amazed at the cost for these photocopied sheets - the great British rip-off is alive and well. The Irish version G10 and the French version GTE 2666 are free. Incidentally the Irish G10 is, except for the reverse power protection, exactly the same as G59) Within the PES there will be engineers who administer the G59 regulations, contact them as soon as it is determined that generators are to be installed. If the generators operate as an alternative to the PES the requirements are minimal, if the generators are to operate in parallel with the PES for short term paralleling there are more stringent requirements, if the generators are to be used for peak lopping or export then the requirements are even more stringent.
If a supply security level 5 or 6 is to be installed with the UPSs having bypass circuits then G59 applies, the UPSs should be considered as electronic generators and interlock systems must be incorporated to ensure that the UPSs cannot feed back onto the PES network.
The engineers who administer the G59 regulations are extremely busy people who need as much notice as possible of meetings and witness testing, in any case at least 15 working days. Prior to meeting with them have your single line diagrams, G59 circuit diagrams prepared and details of any mechanical interlocking schemes. If at all possible send them copies in advance of the meeting so that they may come to the meeting fully informed.
Where a supply security level 3 is to be installed it must not be possible to operate the two supplies in parallel, the PES will require a demonstration of the control scheme that prevents this.
For new houses requiring a domestic supply the PES will supply a meter cabinet which will need to be built in to the house where it may be accessed by the meter reader, the PES will issue their standard installation drawings with the cabinet make sure you comply with them. Its location should be near to the distribution board.
For small LV supplies to small shops and business premises check with the PES as they will have standard drawings of the arrangements for their equipment that may be used to site the PES equipment and your main distribution equipment. If in doubt ask the PES they are usually happy to advise, and it is far better than designing the installation only to have a supply refused when the installation has been completed due to non-compliance with some obscure regulation.
For larger LV supplies and all MV supplies the PES will usually require a switchroom or area for their equipment, again they will have standard drawings of the arrangement of their equipment, and for the access to it. Where ever possible site their equipment either on an outside wall or if a room site the room with at least one wall being an outside wall. Where a supply security level 3 is to be installed the two supplies should come into separate PES switchrooms, each room should be sited with at least one wall being an outside wall.
The reason for locating the PES area on an outside wall or if a room or rooms with one wall being an outside wall is that their supply cables will be able to penetrate the wall and connect to their switchgear without passing through any client areas. Where this is not possible check with the PES for their requirements for the protection of their cables, and with the fire risk insurers as the PES cable insulation will not be of the LSF type.
Where the PES rooms are in a basement try to arrange separate access to ground level so that the PES have access to their equipment without having to pass through client areas. The access door from the street to the basement should not only be to the requirements of the PES it should also be watertight so that any flood in the street does not penetrate into the basement area. The ventilation to the room(s) should also be designed with air inlets and outlets being above any possible flood level or the door lintel level whichever is the higher. Where a supply security level 1 is to be installed and your switchroom/sub-station is either in the basement or is internal to the building request from the PES a quotation for a single phase LV supply of 80A, that is a standard domestic supply, and then show a change-over unit with the 'normal' building supply to the sub-station/switchroom distribution board, so that when the main switchgear is being maintained, as it must be on an annual basis, or there is a fault you will have lighting and small power in the sub-station/switchroom the cost is minimal when compared to the cost of hiring generators and temporary lighting each year. If there is a fault which causes the power to fail there will be light to work by, adequate light to work by is a requirement of the Electricity at Work Regulations 1989; regulation 15 applies. Make sure that the sub-station/switchroom distribution board also supplies the lighting in the access route from the outside world to the sub-station/switchroom. This additional LV supply will need to be approved if it is not included on the tender documentation, the risks and dangers involved in providing temporary lighting and small power or working with torches can be totally negated by the provision of this additional supply, this should be highlighted in writing to those who can authorise the necessary variation, as if they reject the additional supply any investigations and subsequent claims due to accidents can be passed directly to them and not left with you. Therefore put the authorisation request and response in the safety file.
To enable sensible discussion with the PES prepare the single line diagrams even if only preliminary, and if the supply requested is at MV the protection grading chart. PESs invariably give the maximum prospective short circuit current at the supply terminals as the supply switchgear short circuit rating, for the calculations of the rating of the consumer’s equipment that figure must be used, however, for protection grading that figure is invariably high; therefore if the PES does not give an accurate figure on which to base the protection grading chart then a figure of 65% of the maximum prospective short circuit current should be used. Similarly for volt drop calculations where an accurate figure is not available a figure of 50% of the maximum prospective short circuit current should be used. An example of this would be where for an 11kV supply the PES gives the maximum prospective short circuit current as 13.1kA which roughly equates to 250MVA, the figure of 13.1kA is a standard rating of 11kV switchgear and should be use to determine the fault levels around the system. For the protection grading the fault level for discrimination should be taken as 8.515kA and for volt drop calculations 6.55kA. On the protection grading chart show both the stated maximum prospective short circuit current and the fault level for discrimination; to this should be added the protection settings of the PES equipment as overcurrent 150% with a 0.15 time multiplier (tms) and inverse definite minimum time (IDMT) curve and earth fault 50% with a 0.15 tms and IDMT curve.
Add to the chart as a minimum the curves for the incomer(s) and the circuit breakers/fuses in the distribution system downstream of it as far as the largest outgoing feeder on the main LV switchboards. The settings for the incoming circuit breakers should be calculated to give a time discrimination of between 0.25 and 0.4 seconds between them and the PES supply circuit breakers. A similar time discrimination is required at each level down the distribution chain, but at each level the of the supply chain the fault current will get lower, if there is a problem with discrimination demonstrate this on the grading chart and ask the PES to review their settings.
The pdf version of this file includes a typical grading chart which has an original size of A1. It is a separate file entitled protection grading.pdf with an original paper size of A1 (597mm x 841mm) printing on to an A1 sheet will give a scale of 1:1, on an A2 sheet a scale of 0.707:1 and on an A3 sheet a scale of 0.5:1. To be able to read the drawing a recommended minimum paper size of A3 should be used, at the stated scaling the general text on the drawing has a minimum height of 1.125mm.