Factors influencing substation design, part 3, pollution level and corrosion. Design of substation and its equipment shall be such that their performance and availability is not hampered due to environmental pollution like dust, fog, or smog. Design and operation of a substation shall in no way affect the neighboring environment by way of noise, soil contamination, or pose risks due to electrical hazards and electromagnetic hazards. This topic covers how equipment are protected against pollutants. All the electrical equipment enclosures should be protected against the ingress of dust, vermin, and water. Merely stating that the equipment is dust proof and waterproof is only subjective and not verifiable. IEC 60529 defines various degrees of protection against solids and liquids. A sample ingress protection classification for an enclosure or cabinet is denoted as IP 42. IP stands for international protection rating. It is also interpreted as ingress protection. First numeral 0-6, indicates protection classification against ingress of foreign solids. State X, if not applicable. Second numeral 0-8 indicates protection classification against ingress of liquids. State X, if not applicable. Third and fourth letters are optional. Let us first understand the first two digits and then we shall come back to understand the letters C and W. [MUSIC] The first digit of IP code is for protection against solids. If the first digit is a zero, it means that the enclosure is not at all protected from any solids. All live parts inside the enclosure are accessible for all. First digit, one, means a solid object of diameter more than 50mm cannot penetrate inside the enclosure. Similarly, first digit as 2, 3, 4 mean that solid objects having a diameter greater than 12.5mm, 2.5mm, and one mm cannot penetrate inside the enclosure respectively. These digits symbolize respectively that finger, screwdriver, and screws or wire can not be inserted into enclosure in closed condition. So an electrician holding any kind of tool cannot get access to touch any life part accidentally if the first digit is four. The first digit as five means that a limited amount of dust can enter inside the enclosure, but it will not hinder the functioning of any component located inside the enclosure. Enclosure assigned first digit of six will not see any ingress of dust. The table shows the protection against liquids. Second digit of IP code is for protection against liquids. If the digit is zero, it means that the enclosure is not at all protected from any liquid. Falling water or droplets find way easily into live parts inside the enclosure. Digit 1 means vertically falling drops cannot enter inside the enclosure. Digit 2 means drops falling at an inclination of 15 degrees cannot enter inside the enclosure. Digit 3 means that if liquid is sprayed up to an angle of inclination 60 degrees, cannot enter inside the enclosure. Digit 4 means that liquids splashing from any direction cannot penetrate inside the enclosure. This symbolizes that rain water splashing from any direction cannot enter the enclosure. Digit as five means jet of liquid from any direction cannot enter inside the enclosure. Enclosure assigned second digit of six will not see any ingress if powerful jets of liquid are directed from any direction. Digit 7 prevents entry of liquids if immersed temporarily under water head up to one meter for not exceeding 30 minutes duration. Digit 8 allows any enclosure to be continuously emerged in liquid without any ingress. Remember, slight moist is allowed at the periphery without hindering the functionality of the enclosure. Recall the discussion on first two letters of IP code. Let us now understand the meaning of third and fourth letter of IP code. Optional third letter of IP code indicates protection against axes with A, back-off hand, B, finger. C, tool D, wire. Optional fourth letter of IP code indicates H, HV apparatus. M, tested for ingress of water when rotary equipment is in motion, S, in stationary position.\, W, also with additional measures or processes. Consider using the ingress protection code of IP 41 for indoor equipment as it is sufficient in most cases. IP 54 is not preferred for indoor application. Following IP54 affects self-cooling capability of the enclosure. Following IP54 or IP 55 may be necessary for outdoor installed equipment. Food for thought. Dust and fine sand settle on the bushings and insulators installed outdoor, effectively reducing dielectric values. Excessive deposit of sand coupled with moisture over a period of time forms a cement-like covering on the bushings and insulators which could result in short-circuit due to tracking. Creepage is the distance measured along the surface of the bushings from one end to another end. You can use a sticky tape to measure the creepage distance by sticking it over the path along the surface of the bushing. IEC 60815 standard recommends nominal Greek page distance for four levels of pollution. IEC 60815 specifies the creepage distance required in terms of millimeter per KV for line to line voltage. The table displays the recommended creepage distance for various pollution levels. For lightly and medium polluted areas, 16 and 20 millimeters per kilovolt are adopted respectively. Similarly, for heavily and very heavily polluted areas, 25 and 31 millimeters per kilowatt are adopted respectively. See from the table which area is falling under different levels of pollution. Less noble metals corrode due to several reasons. Let us look at some of the key reasons. Corrosion due to corrosive atmosphere such as coastal areas, salt, and humidity laden areas, petrochemical plant areas. Equipment buried in soil or contact with soil, galvanic reactions between materials such as mild steel and aluminum or copper and aluminum. Some of the methods to prevent corrosion are use of non-corrosive materials such as a stainless steel, painting, galvanizing, that is zinc coating of mild steel parts, passivation, tinning of copper; PVC jacketing, using sacrificial anodes or cathodic protection methods. Use of bimetallic strips at the joints of dissimilar materials. Remember, mild steel earthing conductor if buried underground, you have to include sufficient design margin to allow for corrosion of outer surface. Here is a quick recap of what you have learned this far; Performance of electrical equipment may be affected by ingress of solids and liquids. Adoption of correct IP code ensures proper design of equipment. Performance of outdoor electrical equipment deteriorates due to environmental pollution. Adoption of correct creepage distance based on level of pollution foreseen prevents equipment failure due to tracking on insulators. Reasons for corrosion of less noble metals used in electrical equipment and mitigation measures.
