BRITISH STANDARD
Direct acting indicating
analogue electrical
measuring instruments
and their accessories —
Part 4: Specification for special
requirements for frequency meters
click below BS 89-4:1990 EN 60051-4: 1989 IEC 51-4:1984 pdf free download
click here BS 89-3:1990 EN 60051-3: 1989 IEC 51-3:1984 pdf free download