Requirements. Each utility shall have the following standard watt-hour meters for the purposes indicated: Secondary Standard. Each utility shall have at least one portable secondary standard watt-hour meter to be used to check the accuracy of the working standard watt-hour meters. Working Standard. Each utility shall have at least one portable working standard watt-hour meter to be used to check the accuracy of watt-hour meters. Accuracy. Each portable secondary and working standard watt-hour meter shall have a correction of not more than 0.5 percent at commonly used loads. If the correction percentage varies between successive tests by more than 0.25, a complete check shall be made to determine the cause of the variation. If the cause of variation cannot be corrected, the use of the instrument should be discontinued. Calibration. The accuracy of the standard watt-hour meters shall be checked as indicated below: Secondary Standards. Secondary standards shall be checked at intervals not exceeding 12 months at the National Institute of Standards and Technology (NIST) or at a laboratory acceptable to the Commission. Working Standards. Working standards shall be checked at intervals not to exceed 12 months by comparison with a secondary standard in the utility's meter shop or other meter shop approved by the Commission. Certificates. Each standard shall be accompanied at all times by a certificate or calibration card, duly signed and dated, on which are recorded the corrections required to compensate for errors found at the customary test points at the time of the last previous test. Care In Handling. Extreme care shall be exercised in the use and handling of standards to assure that their accuracy is not disturbed.