Substations presents a significant risk to personnel, working on electrical powered equipment, particularly from electric shock. Consequently, the design of electrical substations has always had to take into account the relevant safety techniques for live bare equipment to protect people who may be building, or commissioning an equipment, operating an equipment, maintaining, or decommissioning the substation or such equipment from these risks as well as others.
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Safety Techniques for Live Bare Equipment
The safety techniques used for naked live equipment and hot conductors is generally divided into three main categories and are discussed below:
Safety by Use of Self-Protected Equipment
The category of equipment, known as “self-protected,” is equipment having an insulated or metal casing of any kind. It is constructed in such a manner so that any contact with live equipment which may even be accidental, is impossible during normal operation.
Such equipment is normally used at lower voltages, such as in distribution substations; however, with the increasing use of gas-insulated switchgear, it now extends to the highest transmission voltages.
On the basis of used technology, self-protected equipment can be divided into two main groups which are usually dependent upon the level of voltage.
• Category A
For voltages up to about 36 kV, the circuit breaker may be of a drawout type which means that if it is drawn out from the panel between the bus bar and feeder or circuit connections, then bus bar and feeder isolators are not usually needed. This is because, the disconnection of the circuit breaker is evident from its position in the cubicle.
• Category B
At higher voltages, except for special cases, the circuit breaker is fixed, and the substation is fitted with a bus bar and circuit selectors enclosed in metal casings that constitute protection for the equipment and the conductors. The verification of these disconnector’s position close or open, at any time should be possible.
This kind of safety technique is usually used in gas-insulated switchgear. Under normal operating conditions, operating these circuit breakers safely, is not an issue. However, during the maintenance work, which requires the dismantling of the protective casings, may necessitate taking additional safety measures such as checking that the equipment is dead and disconnected and also checking if the conductors involved in the work are earthed and may need the erection of screens. The specific measures will differ for each type of equipment, so it is not possible to give general rules.
Furthermore, the equipment should include the sequence controls or interlocking arrangements which are necessary for the prevention of dangerous maloperation of any kind. Where bus selectors and circuit disconnectors are used, it should be possible to verify the isolation of the circuit achieved by:
• Either directly by visual inspection of the position of the blades or spring of the circuit breaker if it is of withdrawable type and performs the function of the isolators in the withdrawn position.
• Or using discrepancy or position indicator lamps which are designed to have the impossibility of error in the indication of the position
For such indications to be effective, they must be “active”, this means that the “open” and “closed” positions must both be obtained by the transmission of a signal initiated by the effective position of the disconnector. In the application of luminous indication, two lamps must be used, one corresponding with the open position and one with the closed position.
The switchgear must be designed to allow fittings of local earthing connections or arrangement to the conductor terminal and equipment itself, if necessary by using suitably placed fixed takeoff points. The casings of the equipment should be designed to enable the fixing of any mobile protective screens that may be necessary for the carrying out of certain maintenance work.
Finally, it is strongly recommended to have a detailed operating procedure, made jointly with the manufacturer for each case concerned and a summary of the same to be provided to all supervisory staff. It is therefore very important for any work that necessitates the overhauling of any component for the maintenance purpose.
In gas-insulated switchgear, there are certain considerations that often occur when it is necessary to carry out the maintenance work within a gas chamber. Many utilities do not allow the work to be carried out on a gas chamber, adjacent to a gas barrier if the chamber on the other side of the barrier have a normal working pressure. The GIS designer must therefore explain how all the work can be carried out within a chamber without the above-mentioned condition arising.
Furthermore, as the gas-insulated switchgear has become so compact, particularly for voltages of 145 kV and below, many utilities insist upon having a buffer chamber between bays which enable safe access to all the parts of the equipment and to ease out the disassembly process when required. This buffer chamber assists in avoiding work adjacent to a barrier with safe pressure on the other side.
Safety Technique by inaccessible screens
When screens are provided, whatever their construction solid or grill, insulating or conducting, brick or prefabricated, or fixed or movable, they must constitute an effective safety technique for the protection of staff against approaching dangerously close to live equipment and conductors during simple inspection and operation and when carrying out work. This is among the safety techniques for live bare equipment used predominantly.
Protection walls and screens must be designed such that:
• They restricts the functional volumes intended to the receiving equipment related to a given operating component.
• Any normal switching operation may be carried out without opening or dismantling any of their elements, and that it is not necessary to enter into any completely closed cubicle.
• The safety measures, required to carry out the work, such as checking or making sure that the equipment is not live, fitting of short circuiting, and earthing devices and screens where necessary, can be done without danger to staff or equipment.
Safety Technique of Open Equipment Made Inaccessible by Distance
This technique is based completely upon maintaining suitable distances or clearance between the live equipment or conductors, and the operational staff movement area required for switching operations and other works.
Choice of Safety Technique as a Function of the Voltage Level
The choice of safety technique is usually based on the voltage level of the substation. The following solutions cited below may be considered which follows the custom code and practice and the technology of equipment offered by various manufacturers:
a) Up to and including 1,000 V (rms voltage value in the case of AC)
• Mostly use of self-protected equipment along with insulated connections is done. Rendering inaccessible by means of screens or by distance is also done only for special cases.
b) From 1,000 to 25,000 V
• Use of open equipment which is made inaccessible by the use of solid screens or grills normally forming functional cubicles type structures.
• Use of open equipment which is rendered inaccessible by distance.
• Use of self-protected material and equipment. This safety technique is now used popularly with metal-enclosed and metal-clad switchgear.
c) Above 25,000 V
• In the majority of cases, the safety techniques used is inaccessible by distance.
• Use of self-protected equipment, when space is limited. This is, of course, the safety technique used when gas-insulated switchgear is employed.
• Rendering inaccessible by the use of solid screens or grills is usually the safety technique reserved for indoor substations using open equipment.
