scope:

This specification covers high accuracy, shockproof switchboard mounted, expanded scale frequency meters for Naval shipboard use.

The type designation of frequency meters shall be in the following form and as specified (see table IV and 6.2):

Frequency meters covered by this specification shall be designated by the two letter symbol "MR".

The style shall be designated by a two digit number which signifies the characteristics specified in 1.2.1.2.1 and 1.2.1.2.2.

The first digit shall designate the normal size and shape of flange as follows:

3 - 3-½ inch round flange, sealed.

4 - 4-½ inch rectangular flange.

The second digit shall designate the type mounting and the normal scale length in angular degrees as follows:

8 - Flush mounting, 90 degree nominal scale.

9 - Flush mounting, 250 degree nominal scale.

The color scheme of the dial background, markings, and pointer shall be designated by a single letter as follows:

W - White dial background, black markings and black pointer.

The center scale value and nominal voltage rating shall be designated by two digit numbers as specified in 1.2.1.4.1 and 1.2.1.4.2.

The first digit shall designate the center-scale value as follows:

4 - 400 Hertz: (Hz) scale range 395-405 Hz

The second digit shall designate the nominal voltage rating as follows:

1 - 120 volts.

2 - 450 volts.

3 - 208 volts.

4 - 220 volts.

5 - 440 volts.

Two letters shall designate the kind of current for which the meter is designed as follows:

AF - Alternating current (a.c.) 400 Hz nominal frequency.

A single letter shall designate the special feature as follows:

H - High-impact-shock resistant.

The type designation MR38W41AFH signifies:

(a) MR - Frequency meter.

(b) 38 - 3-½-inch round flange sealed, flush mounted 90-degree nominal scale.

(c) W - White dial background with black markings and black pointer.

(d) 41 - Center-scale value 400 Hz, 120 volt nominal voltage rating.

(e) AF - Alternating current 400 Hz, nominal frequency.

(f) H - High-impact, shock resistant.

intended Use:

Frequency meters furnished under this specification are intended for use on type III, 400 Hz power systems of MIL-STD-761.