1. British Standards Institution, British Standard No. 5501: Part 5: 1977.
2. CENELEC, Standard No. 50 018, 1977.
3. International Electrotechnical Commission, IEC Publication 79-1, 2nd edn., 1971.
4. International Electrotechnical Commission, IEC Document SC 32 A (Central Office) 23, 1977.