Single Bus Configuration

The simplest configuration is the single bus arrangement shown in Figure 14.6, drawn to represent an HV aerial bus where each circuit breaker is provided with the isolation switches necessary to permit circuit breaker maintenance. While this is the least expensive arrangement, as mentioned above, it suffers from the distinct disadvantage that a bus fault will lead to the loss of the entire bus. Individual circuits must also be removed from service in order to maintain the associated circuit breaker or the protection relay. On the other hand, this configuration is easy to operate and expand, while bus zone and feeder protection is straightforward to implement.

Sectionalised Bus

Figure 14.7 shows a modification to the HV single bus arrangement with the inclusion of a bus‐coupler circuit breaker and its associated isolation switches. A sectionalised bus can preserve half the load in the event of a bus fault, since only the faulty portion need be shed. Alternatively, if the bus coupler is normally run open, then in the event that one incomer feeder trips, the coupler can be closed, allowing the entire load to run from the remaining feeder. This arrangement is frequently used in small substations supplied from two distribution transformers (each able to carry the full load), where the combined fault level is beyond the interruption capacity of the station’s switchgear, thus providing (n − 1) redundancy.

Double Bus Configuration

A much more flexible double bus configuration appears in Figure 14.8a. This schematic is appropriate to medium voltage metal clad switchgear, where each breaker can be withdrawn from its cubicle to complete the isolation, thus removing the need for isolation switches. Two independent buses are provided; these run through separate chambers in each cubicle. Each feeder can be connected to either bus, depending on where the circuit breaker is inserted into its cubicle. The arrangement shown in Figure 14.8b has top and bottom buses arranged in separate chambers, with the outgoing circuit termination located in between. This may be coupled to the top bus by inserting the breaker in the upper position, thereby connecting the circuit side to the top bus, or to the bottom bus by inserting it in the lower position and connecting the circuit side to the lower bus. Figure 14.8b shows such an arrangement in which some loads are connected to the top bus with others on the bottom. In this case the circuit breakers can be racked up and down on a permanent carriage inserted into the cubicle. Other configurations use circuit breakers similar to those shown in Figure 14.5 and are placed on a temporary carriage for connection to either the top or bottom bus.

The double bus arrangement permits critical loads to be quarantined from non‐critical, thereby preventing a fault in a non‐critical piece of equipment from disturbing a critical load. Frequently, a critical bus is provided with a degree of feeder redundancy so that in the event that one incomer trips, no load will be lost. In addition, a sectionalised double bus provides the advantages described above, so that in the unlikely event of a bus fault, the bus zone protection will only trip that portion of the load on the faulted bus. This configuration also permits the load to be transferred to one bus, should the other require maintenance or extension. MV busbars are segregated from one another in separate chambers, so that access to one is not compromised by the presence of the other.

Figure 14.9 shows the side view of a suite of double bus GIS switchgear. The circular bus chambers are located one above the other at the front, and run the length of the suite. The switchgear is divided into individually pressurised bays, one for each incoming or outgoing circuit. These are connected via HV cables which enter from the basement, at the rear of each bay. Each circuit is supplied with a voltage transformer in addition to a chamber containing the necessary protection and metering current transformers. Each incoming or outgoing circuit passes through a circuit breaker before being routed to two isolators, one for the top bus and one for the bottom. Sectionalising breakers are inserted at intervals along each bus in order to minimise the load loss in the event of a fault.

This particular equipment has a rated SF₆ pressure of 6.3 bar, although it can safely operate at pressures as low as 5.5 bar (by which time low gas pressure alarms will certainly have been generated). The SF₆ gas provides dielectric insulation for the busbars and associated switchgear, while acting as the arc quenching medium for the circuit breakers as well.

Figure 14.10 shows a sectional view of bays A and B in a double bus aerial switchyard together with the corresponding portion of the power circuit one line diagram, which shows only the primary devices in the circuit. Not shown in this figure are six additional bays that accommodate two local generators, two transformers, two additional transmission lines, and two bus section circuit breakers and a bus coupler breaker.

Bay A terminates an incoming transmission line via a dead tank breaker A52, and a combination isolation/earth switch, A29. It can be coupled to either bus via bus‐selection isolators A29, AB29 and B29. Then A29 and B29 are pantograph isolators, which operate vertically on a scissor principle and can be conveniently located beneath a busbar when space is limited; AB29 is a conventional double break isolator. This bay is also equipped with three single‐phase capacitive voltage transformers (A97) for synchronising, protection and metering purposes. Bay B supplies a 30 MVA, 110 kV:22 kV distribution transformer, via dead tank circuit breaker B52.

The protection and metering current transformers required for each bay reside within the associated dead tank circuit breaker, while the CTs associated with the transformer’s low voltage winding are contained within the transformer tank.

A variation of the double bus arrangement is sometimes used in HV aerial switchyards where outgoing circuits are supplied by two circuit breakers, one from each bus, as shown in Figure 14.11. While this provides a very flexible and secure supply (since any bus fault can be isolated without loss of load), it is particularly expensive since two circuit breakers are required per circuit.

Breaker‐and‐a‐Half Configuration

Figure 14.12 shows the breaker‐and‐a‐half configuration, so called because three circuit breakers are required for every two incoming or outgoing circuits. It uses two normally energised busses, and allows any circuit breaker to be removed for maintenance without disturbing the associated loads. It provides a highly flexible and secure arrangement and allows bus faults to be isolated without loss of load.

In the event of a circuit breaker failure, however, some load will be lost. Should the centre breaker fail then depending on the position of the associated isolators, both circuits may be lost, although each can be readily restored. On the other hand, if either of the outside breakers fails only one circuit will be lost. The protection arrangements are somewhat more complex, however: each circuit requires its own voltage transformer as well as duplicate current transformer(s) in each adjacent circuit breaker.

Ring Bus Configuration

The ring bus, shown in Figure 14.13, is often used in MV cable networks where supply must be distributed between several small substations, for example around a large hospital or throughout a university campus. It operates as a closed loop with two or more incoming feeders and multiple out‐goers, each of which effectively has two sources of supply. A fault in any part of the ring can be isolated with minimal load disruption, and circuit breakers can be removed at any time for maintenance, without loss of load. The ring bus protection requirements are similar to those for the breaker and a half scheme, and in the case of aerial substations, it can be readily adapted to a breaker and a half configuration.

Main and Transfer Bus Configuration

In the case where the circuit breaker maintenance must be performed without a loss of load the main and transfer bus arrangement can be used, as depicted in Figure 14.14. Under normal operation the main bus supplies the load and the transfer bus is de‐energised. When a circuit breaker must be removed for maintenance the transfer breaker supplies the load from the transfer bus, without loss of load. As with the single bus configuration, a main bus fault will result in the loss of the entire load.

Since the transfer circuit breaker is capable of supplying any incoming or outgoing circuit, its protection relay must first be programmed with the appropriate settings before any switching can take place.

Reactive Voltage Drop

The distribution of any reactive power sources (capacitors) or heavily reactive loads on a busbar should be carefully considered. Any busbar has a small resistive and inductive impedance, the inductive component of which is generally dominant and becomes particularly important when considering the reactive power flowing along the bus. The inductive impedance of medium voltage busbars is typically around 0.15 milliohms/m and the small voltage drops that arise when loads are adversely arranged can easily upset the current balance between incoming feeders. This is illustrated in Figure 14.15a, where three inductive loads are supplied from an MV bus, in which the inductive reactance is shown. In order to improve the power factor and suppress harmonics, two harmonic filters have also been installed at the opposite end of the bus. The reactive power flowing from these to each load generates small reactive drops (δV) along the bus, in such a way that the voltage at point A is slightly higher than that at point B. This means that there is less voltage dropped across incomer 2 and, as a result, it supplies less current than incomer 1. This arrangement means that it is not possible to fully load feeder 2 without overloading feeder 1. A better configuration is shown in Figure 14.15b, where by interspersing the filters and loads there is little net reactive drop between the incomers, and therefore both share the current equally.

For loads without capacitive support, the reactive demand of each will still generate reactive voltage drops, and there are still advantages in arranging them symmetrically with respect to the incomers. If load 2 has the lowest power factor, for example, it is best installed at the centre of the bus, as shown in Figure 14.15b.

Bus Overload

Critical loads are frequently provided with supply redundancy, so that should an incoming feeder trip unexpectedly (an n − 1 situation) those remaining can carry the load. Consideration must be given to the resulting current distribution in such a situation, to ensure that sections of the bus do not become overloaded. Figure 14.16a depicts an arrangement in which the 1600 A bus only operates within its current rating when all feeders are connected. Should the left‐hand feeder trip or be taken out of service (OOS), then although those remaining are capable of supporting the load, the bus will become overloaded at points A and B, as shown in Figure 14.16b. A better arrangement would see an incomer positioned between each pair of outgoing circuits.

Switching Plans

From time to time all electrical installations must be reconfigured in order to cater for changing load requirements or perhaps to provide access to part of the network for maintenance or augmentation purposes. Before any switching operation can be commenced a switching plan must be written. This document is prepared with reference to the one line diagram and identifies the reason for the switching, the equipment concerned, together with all isolation and earthing points. It includes detailed and numbered step‐by‐step instructions for the switching process, in the exact sequence in which they will be carried out.

The aim of any switching plan is to safely achieve the desired outcome without unexpected loss of load or overloading any particular items of equipment. This frequently involves altering the normal bus arrangement to accommodate the required change before the subject equipment is switched or removed from service. For example, it may be necessary to rearrange the loads connected to a double bus when an incoming feeder is to be taken out of service. As a result, the switching process may become quite complex and the sequence in which the steps must occur often becomes critical.

A switching plan will be prepared by staff familiar with the plant concerned, to the extent that all eventualities can be foreseen in advance and accommodated as required. It will generally be written by one person and checked by another (often the switching operators who will execute the plan). Switching plans generally include provision for the operator to record the time that each step was completed. Where a particular switching operation occurs frequently, it is common for organisations to prepare standard switching plans that can be executed by suitably qualified operators.

Isolation Procedures

In order to safely isolate an item of equipment, fundamental switching practices must be used. These are generally quite simple and logical, and will form part of the switching plan. Any piece of equipment that is in service must first be de‐energised. This requires switching off the main circuit breaker as well as every other associated energy source. This may include sources of mechanical energy; for example, sources of compressed air, pressurised hydraulic oil or water must also be isolated, so that no part of the equipment can operate unexpectedly.

The steps that must be followed for the complete electrical isolation of a piece of equipment can be summarised as follows:

1. Correctly identify the circuit to be isolated, as outlined in the switching plan.
2. De‐energise each energy source associated with this equipment. This step requires opening the main circuit breaker as well as any auxiliary sources of electrical or mechanical energy.
3. Isolate each energy source so that it cannot easily be unexpectedly brought back into service. This requires removing MV circuit breakers from the busbars and opening the isolators associated with HV circuit breakers. The tripping and closing supply fuses associated with the circuit breaker is also generally removed. Some LV circuit breakers can also be withdrawn from the busbars into an isolated position, as shown in Figure 14.17. In the case of LV circuit breakers which cannot always be removed from the bus, it is usual to withdraw the isolation fuses provided specifically for this purpose.
4. Prove dead. In the case of HV and MV circuits this is generally achieved with a test stick or a similar device. The MV test stick shown in Figure 17.18a is essentially a resistive voltage divider incorporating an analogue indicator that is used to prove an MV circuit dead. This device requires physical contact with the bus conductor concerned, usually achieved by lifting the circuit side shutters in the cubicle, to expose the busbar stubs. The length of the stick keeps the operator at a safe distance from a live busbar. HV aerial circuits can be proven dead by bringing a voltage detection device like that in Figure 14.18b (fixed to a long fibreglass pole) into close proximity with the bus conductors. This device emits an audible tone when close to any live conductor. (Its operation can therefore be checked by bringing it very close to any LV circuit). LV circuits are usually proven dead with a multimeter.

   All circuits should be treated as live until proven dead, and therefore the necessary personal protective equipment must be worn during all such tests.

   In circuits where substantial capacitance exists, such as harmonic filters or power factor correction equipment, a suitable period of time must be allowed to elapse for the capacitors to discharge, otherwise hazardous DC potentials may still exist in the circuit. Internal discharge resistors are usually built in to both MV and LV capacitors to absorb stored energy after switch‐off, a process that usually requires about 10 minutes. It will be necessary to test the capacitor potentials at the end of this period to be sure that discharge has indeed taken place.

   In the case of LV circuits, care should be taken to ensure that the circuit isolated is in fact the one to be worked on. While this may sound obvious, the main circuit breaker is frequently located in a switchboard remote from the equipment itself, and therefore if the wrong circuit breaker is opened in error, the subject equipment will still be alive. The need to prove dead at the equipment itself is therefore paramount. To obviate this problem, industrial LV equipment is frequently equipped with a motor safety station (MSS). This consists of an isolation switch and a set of line fuses, adjacent to the equipment concerned. Thus proving dead can be performed at the worksite where there is minimal risk of misidentification.

   In the case of mechanical systems, gauges should be checked to ensure that pressure vessels have discharged to zero. Similarly, any mechanism capable of storing energy must be operated and subsequently tested to ensure that all stored energy has been dissipated.

5. Earthing. In the case of HV and MV apparatus, an earth must be applied at the point of supply, so that staff working on it, do so downstream of the earthed point. This will generally be on the circuit side (load side) of the circuit breaker. Where possible two earths should be applied, a functional earth at the supply circuit breaker itself, and an operational earth near the actual work site, so that maintenance staff work between earths. The operational earths frequently consist of a set of trailing earth conductors, connected at one end to the line conductors and to an earth point provided for this task at the other. Dual earthing minimises the risk of induced voltages in earthed transmission lines and switchyard circuits.

   LV systems generally need not be earthed as part of the isolation process. This is because, due to the relatively low potentials and the small circuit lengths involved, it is not possible for de‐energised conductors to have dangerous potentials induced within them from adjacent live conductors (either through capacitive coupling or magnetic induction). So long as an LV circuit has been isolated, proven dead and tagged, work can generally commence.

6. Tagging each isolation point. So that others can see that the equipment is out of service, an out of service tag is placed at each isolation point. This identifies the equipment concerned; it states the reason that it has been taken out of service and by whom, as well as the time and date of the isolation. Finally, the isolation boundary around the equipment is usually delineated with rope or barrier tape so that maintenance staff do not accidentally stray into live areas. While isolation procedures appear straightforward, they are often fraught with dangers, particularly when a group of people are involved with a maintenance task on equipment that has been removed from service. Problems occasionally arise either through lack of communication or miscommunication within a work group, and they usually occur when the equipment is initially taken out of service or when it is returned to service. For example, people may attempt to commence work before it is safe to do so, or through work group confusion the equipment may be returned to service before all maintenance tasks have been completed. Either event can prove disastrous for the people concerned. To avoid such risks permit to work procedures have been devised that ensure that no work commences prior to the isolation being completed, and preventing the equipment from being returned to service until everyone has left the worksite.

Permit to Work

In order to prevent accidents from occurring permit to work procedures are used throughout all industries, the details of which tend to vary slightly from industry to industry and frequently from site to site. Accordingly everyone working on a particular site must be trained in the permit to work procedure used at that site. The following is a general discussion only and does not reflect the particular requirements that may be applied at different sites.

Permit to work procedures are used any time maintenance or construction activities are undertaken and when a work crew must be given access to a piece of equipment or to a portion of a site. The permit to work document applies to the crew conducting the work and it is used to ensure the safe isolation and de‐isolation of equipment.

Generally all isolations will be performed by one or perhaps two people, as specified in the switching plan. They must be familiar with the site and appropriately trained to perform the required isolation(s). In general, they will not be part of the maintenance work crew. Once the isolation procedure is complete, the isolation officer will demonstrate to the work crew’s representative that each piece of equipment has been correctly isolated, and when satisfied they will sign the isolation sheet, formally accepting that the worksite is safe and ready for work to commence. That person then effectively becomes in charge of the maintenance task and is responsible for communicating with the entire work crew.

Personal Danger Tags

Before work can commence, each member of the work group must sign on to the job attendance register and place their token at every isolation point in order to prevent the equipment from being re‐energised before the work has been completed. Some industries use a personal danger tag as this token. In such cases, each isolation point contains a personal danger tag from everyone working on the job. The equipment cannot be returned to service until everyone in the work crew has signed off the attendance register and removed their danger tag from each and every isolation point.

The personal danger tag includes the name and in some cases a photograph of its owner, and effectively states that this person is still working on the equipment, and until this tag has been removed by its owner, the equipment cannot be returned to service. Should members of the work group leave the site without removing their personal danger tag(s) they must return and do so before the equipment can be returned to service; it is usually a dismissible offence to remove someone else’s personal danger tag.

Personal danger tags can be a little inconvenient when there are many isolation points. A more recent alternative is the concept of a personal safety lock. This system is a little simpler and does not require each member of the work group to visit every isolation point.

The safety lock concept requires that the isolation officer places a safety lock at each isolation point, in such a way that the equipment concerned cannot be returned to service without removing the lock. The keys to each lock are then placed in a lock box similar to that in Figure 14.19. The isolation officer places his lock on the tag, together with one from every member of the workgroup concerned. In this way, de‐isolation is physically not possible until every lock has been removed by its owner.

Limits of Approach

The limited approach boundary represents the limit of approach for an unqualified person. Should there be a need for such a person to cross this boundary, they must be accompanied by a qualified person who will advise of the hazards specific to the site.

The restricted approach boundary may only be crossed by qualified personnel, provided that steps are taken to mitigate the shock risk. This includes the wearing of full body cover, closed footwear and insulated gloves. It may also require that the energised electrical conductors be insulated from the qualified person, as well as from any other conductive object. Finally, the qualified person shall be insulated from any conductive object at a different potential.

Table 14.3 defines generic limited and restricted approach boundaries for a range of AC potentials. While these boundaries may change with jurisdictional requirements, they are broadly representative of those used worldwide.

Arc Rated PPE

The energy density rating of each arc flash PPE category, also called arc thermal performance value (ATPV), is the incident energy in cal/cm² that will result in a heat transfer through the material to the wearer of 1.2 cal/cm², sufficient to cause a second degree (curable) burn within about 100 ms. Arc rated clothing is assigned an ATPV, based on the results of extensive fabric testing. The ATPV of a garment increases with the weight of the fabric (g/m²) and the number of layers used in its manufacture.

Arc rated fabrics are proven to be flame resistant and will self‐extinguish once the initial heat source has been removed. They may be either produced from man‐made synthetic materials engineered in such a way to have flame resistant properties, or from natural fibres such as cotton or wool, chemically treated to achieve the required flame resistant characteristics.

Two Category Approach to Arc Flash PPE

NFPA 70E® includes a provision for a simplified two category approach to arc flash protection. For routine work and any tasks that require category 1 and 2 arc flash protection, technical staff wear a long‐sleeved shirt and trousers having an arc flash rating of at least 8 cal/cm², often in the form of a corporate uniform. When hazardous tasks must be performed on live equipment requiring arc flash category 3 or 4 protection, the collective arc rating of the PPE worn must be equivalent to category 4, having an arc flash rating of at least 40 cal/cm². Such tasks include switching operations on some types of older metal clad switchgear, inspections of and measurements in live low voltage cubicles such as metering chambers and motor safety stations, and in proving dead LV and MV circuits. The arc flash suit shown in Figure 14.23 provides head‐to‐toe protection and is worn over the everyday PPE described above. The collective arc flash rating of all this PPE is equivalent to category 4.

Arc Flash in Low Voltage Cubicles

Low voltage cubicles deserve special mention with respect to arc flash injury. Because the low voltage restricted approach boundary lies within arm’s reach, it has become accepted practice to conduct measurements, fault finding and inspection activities on live LV cubicles. Since LV arc flash injuries are relatively rare, the general impression is that live LV work is quite safe, and provided the appropriate precautions are taken and the correct PPE is used, this may be so. However, when something goes wrong and an arc is created in the confines of an LV cubicle (usually as the result of the work in progress), there is enormous scope for severe arc flash and/or pressure wave injury. Practitioners must accept that live LV work is potentially very dangerous and therefore the appropriate PPE (like that in Figure 14.23), must be worn, even for the smallest of tasks.

Manufacturers are now designing safer LV switchboards, by covering busbars with coloured insulation and shrouding them behind clear plastic panels. The LV cubicle in Figure 14.24 is an example of a well‐designed metering low voltage CT chamber. Metering personnel frequently work in such cubicles, measuring the connected burden at the CT secondary terminals and occasionally comparing the current in the primary busbar with that flowing in the CT secondary. The risk to an unprotected person conducting such activities is extreme, despite the relatively low probability of an arc fault occurring.

A careful examination of Figure 14.24 will reveal that a Perspex panel has been close fitted over the front of the entire cubicle. The only items protruding through this are the potential fuses, the CT secondary terminal covers and the neutral terminal cover. These items can be accessed live without the risk of arc flash. Unfortunately, shrouded construction of this type is rare; many LV cubicles and switchboards in service contain exposed busbars or phase terminals, and collectively they represent some of the most hazardous electrical items. This situation is made worse because, as mentioned above, electrical practitioners have for years routinely carried out live work in such locations with minimal ill effects. Few appreciate that a dropped screwdriver or sealing wire, falling across exposed busbars has the potential to initiate a powerful arc with devastating results, particularly in a chamber enclosed on three sides.

The further from the supply transformer the switchboard is located, the lower will be the fault current, but there will usually be sufficient energy in both the arc and the accompanying pressure wave, to cause severe injury to an unprotected person standing in front of an open LV cubicle.

In contrast, at the low voltage appliance level the potential fault currents are much lower, reduced by the impedance of building wiring and that of the appliance flex itself. But perhaps the most significant reduction in sustained fault level is due to the presence of fault current limiting devices such as fuses or circuit breakers. These will clear a fault very rapidly, limiting the duration of an arc fault, should one occur.