oil. In large transformers. the tanks have external tubes or external radiators through which the oil circulates by natural convection caused by the differences in oil temperature. The liquid-immersed water-cooling transformer is sometimes used where a plentiful supply of cool water is available. In this type, a coil of copper or brass pipe is installed near the top of the tank in the cooling oil. Water is circulated through this coil and carries away the heat from the oil as it rises in the tank. Insulating  liquids  have  high-insulating  qualities and serve two purposes: first. they insulate the coil, and second. they help dissipate the heat generated by the resistance of the windings and eddy currents in the iron core. If this heat were not removed. the transformer would  operate  at  excessively  high  temperatures, which. in turn, would damage or destroy the insulation on the coils. Two  common  types  of  insulating  liquids  are mineral oil and Askarel^®. Mineral oil is a nontoxic insulating liquid. It is used in different types of high- voltage electrical equipment, such as circuit breakers, switches.  and  transformers.  Mineral  oil  must  be  kept  in an  airtight  container,  or  else  sludge  will  form.  This sludge will settle in the bottom of the tank and slow the natural transfer of heat. Also the longer mineral oil is left exposed to air, the  greater the loss of insulation properties. Askarel^®  is  a  synthetic,  nonflammable  insulating liquid. It has other trade names, such as Pyranol^®, Inerteen^®,  Chlorexirol^®,  and  Asbestol^®.  This  liquid must be handled with care because of its toxic chemical properties.  Askarel^®  is  used  in  special  transformers for applications where flammable liquids must be avoided. Askarel^®  may  have  an  irritating  effect  upon  the skin. eyes, nose, and lips. It also may irritate skin abrasions   or   tender   areas   between   the   fingers. Askarel^®  may  contain  polychlorinated  biphenyls (PCBs):  a  toxic,    carcinogenic   oily   liquid. Transformers tested and found to be contaminated with PCBs should have labels on the outside of the transformer warning of this hazard. WARNING If assigned to work on a transformer known  to  be  contaminated  with  PCBs, see your supervisor for a Material Safety Data Sheet (MSDS) for hazards and 4-6 precautions.   Personal   protective equipment, such as impermeable gloves and  chemical  splash  goggles,  are mandatory. Avoid prolonged skin contact and wash thoroughly after use. Avoid  breathing  vapors. When  removing  transformer  oil, wear  respiratory  protection.  If  you discover PCB transformer oil spilled on soil,  immediately  notify  your  supervisor who   must   notify   environmental authorities  and  summons  a  trained hazardous material spill clean-up team. To protect yourself when handing Askarel^®, wear impermeable  gloves.  Also  wear  splashproof  goggles. Whenever liquid comes in contact with the skin, wash it thoroughly with warm water and soap. Ensure that the work space is properly ventilated before working on transformers containing Askarel^®. Avoid   breathing   Askarel^®   vapors.   Wear   an approved organic vapor cartridge respirator when vapors  are  present.  When  removing  Askarel^®  oil which is contaminated with PCBs, air respirators may be necessary. If   a   blueprint   of   a   particular   transformer installation  is  available  to  you,  your  job  will  be comparatively  easy.  All  construction  and  electrical specifications will be worked out for you beforehand, and all you have to do is convert this information into a finished  product.  However,  in  some  instances,  a blueprint will not be available. Then it will be up to you to determine the location and size of the transformer and install it according to the latest specifications. You should be familiar with the rules and requirements of the most current electrical codes. Be sure to carefully study   any   applicable   code   requirements   before installing a transformer. Transformers are mounted on poles in various ways, such as suspended on a bracket bolted to the pole, suspended from a crossarm with brackets, or set on a platform mounted on an H-frame. Single-phase transformers are usually hung with a through-bolt type of bracket or a cross-arm type of bracket. Figure 4-6 shows a single transformer hung with cross-arm brackets. Figure 4-7 shows a bank of three transformers of 25 kVA capacity hung the same way.