All instruments and meters shall be fitted with glasses of low reflectivity and shall not cause pointer deflection due to electro-static charging through friction.

All indicating instruments shall be of the flush mounted pattern with dust and moisture proof cases complying with BS 2011, Classification 00/50/04, and shall comply with BS 89:1977 or IEC 51.

Unless otherwise specified, all indicating instruments shall have 95mm square cases to DIN standard or equivalent circular cases.

Instrument dials in general should be white with black markings and should preferably be reversible where double scale instruments are specified.

Scales shall be of such material that no peeling or discoloration will take place with age under humid tropical conditions.

The movements of all instruments shall be of the dead beat type.

Instruments shall be provided with a readily accessible zero adjustments.

The mounting height of the centre of all indicating instruments shall not exceed 2000mm.

AC ammeters for transformer, feeder or inter connector circuits, and DC ammeter for all load circuits except motors, shall have linear scales commencing at zero.

AC and DC ammeters for motor circuits shall have scales commencing at zero and with a compressed overload portion for reading of the associated minor starting current.

DC ammeters for the main battery circuit of DC systems shall have scales with positive and negative ranges, labeled charge and discharge respectively.

Voltmeters for feeders and transformer circuits shall have expanded scales to display the nominal service voltage ± 20%.

Wattmeter for feeders shall have linear positive and negative reading scales to be approved.

VAR meters for all circuits shall have linear positive and negative reading scales to be approved.

Frequency meters shall be of the pointer type, scaled approximately 45-55Hz, and biased to swing to one end of the scale on loss of voltage.
Synchronizing voltmeters shall be scaled in per unit values only, to correspond to the above expanded voltmeter scales.

Synchroscopes should be continuously rated, but if not then at least 30 minute rated with an individual on/off switch.

The synchroscopes shall indicate synchronism between two circuits with the pointer at the “12’0’clock” position only, and shall have arrows on the face to show that the frequency of the “incoming” supply is fast or slow with respect to the “running” supply. The synchroscope switch shall disconnect both supplies to the instrument and the instrument pointer shall move at least 450 from the vertical position when either or both supplies are removed and shall then remain stationary without any tendency to creep.

In addition all synchroscopes shall include two synchronizing lamps. These lamps shall be “bright” for synchronized conditions, but are to act solely as a supplementary indication that may be used by the operation with caution in the event of failure of the synchroscope.

Integrating metering shall be provided where indicated on the specification drawings. These meters shall be of the withdrawable flush mounted type and comply with the relevant parts of IEC 521 and BS 5685, Class 1.0 accuracy and BS 37, Part 9. The meters shall include cyclometer dial type registers.

Approved test terminal blocks of the three-phase type shall be provided for connecting in circuit with each meter a portable testing meter.

Recording instruments shall be of an approved type, and unless otherwise specified, shall have two chart speeds of 25 mm and 50 mm per hour available for selection by means other than changing connections. They shall be complete with sufficient charts and inks for two years’ working.

All instruments, meters, recorders and apparatus shall be capable of carrying their full load currents without undue heating. They shall not be damaged by the passage of fault currents within the rating of the associated switchgear through the primaries of their corresponding instrument transformers.

All instruments, motors and apparatus shall be back connected and the metal cases shall be earthed.

All voltage circuits to instruments shall be protected by a fuse in each unearthed phase of the circuit placed as close as practicable to the main connection.

All power-factor indicators in 3-phase circuits shall have the star point of their current coils brought out to a separate terminal which shall be connected to the star point of the instrument current transformer secondary windings.

All instruments and meters associated with multi-ratio CT’s shall be provided with sets of scales etc. appropriate to each CT ratio. It shall be possible to replace the scales of instruments without dismantling the instruments or interfering with any tropicalization finish.

The Contractor shall provide electrical instrument and meter schedules to include, manufacturer, type, designation, current and voltage rating, accuracy class and circuit designation.


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