normal voltage plus 1000. Old windings, of course, will not stand such a test; but it may reasonably be assumed that they should withstand 1-1/2 times normal voltage for one minute. High potential tests should not be applied repeatedly, as frequent application of high potential tends to weaken the insulation. After a dielectric test, insulation resistance readings should again be taken to ascertain whether or not the insulation has been damaged by the high potential. A periodic insulation resistance check is advisable, especially on large machinery. A record of such readings should be kept. Consist- ent readings will indicate continuing satisfactory condition of the insulation. Declining readings will provide advance warning of troubles which may be corrected before breakdown occurs. PERIODIC CLEANING If top performance is to be expected from the machine, it must be kept clean. The frequency and extent of the cleanings will, of course, depend upon the conditions under which tha machine is required to operate. In most cases, the machine should be given a complete overhauling and thorough cleaning at least once a year. Suction is the most desirable method of cleaning, and is expecially recommended where metallic and abrasive dusts are encountered. If com- pressed air is used to blow out the machine, it must be dry and of moderate pressure (about 25 to 30 psi). Even lower pressure is advis- able if conducting ind abrasive dusts are present, as such dusts are extremely harmful if driven into the insulation. Wherever possible, the air should be so directed as to prevent the least possibility of such damage. 3-53