Requirements. Each utility shall have the following voltmeters for the purposes indicated: Secondary Standard. Each utility shall have at least one secondary standard indicating voltmeter to be used in checking the accuracy of working standard voltmeters. Working Standards. Each utility shall have at least two working standard indicating voltmeters to be used in checking the voltage on the utility's electric system and in checking the accuracy of recording voltmeters. Recording Voltmeters. Each utility shall have at least two portable recording voltmeters to be used to record the voltage on the utility's electric system. Accuracy. The voltmeters shall be of sufficient quality to meet the accuracy standards prescribed below: Secondary Standard. The secondary standard indicating voltmeter shall have a stated accuracy within 0.25 percent of full scale. This instrument shall be maintained within its stated accuracy. Working Standards. The working standard indicating voltmeters shall have a stated accuracy within 0.75 percent of full scale. Recording Voltmeters. The portable recording voltmeters shall have a stated accuracy within 1.5 percent of full scale. Calibration. The accuracy of the voltmeters shall be checked as indicated below: Secondary Standard. Secondary standards shall be checked at intervals not exceeding 3 years at the National Bureau of Standards, or at a laboratory acceptable to the Commission. Working Standards. Working standards shall be checked at intervals not exceeding 3 years by comparison with a secondary standard in the utility's meter shop. 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 calibration test. Care In Handling. Extreme care shall be exercised in the handling of standards and other instruments to assure that their accuracy is not disturbed.