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TECHNICAL NOTES

IMPORTANT! To ensure safe and reliable operation, DMR20-1-FM ac 

frequency monitors must be installed and serviced by qualified 
technical personnel. Contact Murata Power Solutions if there is 
any doubt regarding their installation or operation.

1. Measurement Type:

 DMR20-1-FM meters are designed to be powered 

from sine wave ac supplies, over their rated input of 85-264Vac (45-
65Hz). Operation from triangle, square, and quasi-sine waveforms is 
limited to a maximum of 85 to 120rms (45-65Hz).

2. Calibration: 

DMR20-1-FM frequency monitors are factory calibrated; 

there are no provisions for calibrating these meters in the field.

3. Operating Range: 

As the sine wave ac input voltage is lowered below 

100Vac (50 or 60Hz), the DMR20-1-FM’s display may begin to dim 
slightly, but the readings will remain accurate down to 85Vac. Input 
frequencies below 45Hz and above 65Hz will display as four dashes, 
indicating an out-of-range input condition.

4. Panel Installation: 

All electrical connections to DMR20-1-FM must 

be made after the meter is securely attached to the panel, and with 
the input ac supply de-energized (off). In high-vibration environments, 
adequate strain reliefs must be used on all supply wiring. See Figure 1 
for typical panel mounting details.

 Tightening 

Torque:

 

The recommended tightening torque for the M30 x 1.5 plastic hex nut 
is 10 to 17 in-lbs (1.3 to 1.9 N-m). If a torque wrench is not available, 
this recommended range can be approximated by hand tightening the 
hex nut SLOWLY until it just bottoms out against the panel’s rear sur-
face. From this bottomed-out reference position, using a suitable tool, 
tightening the hex nut ¼ turn clockwise will produce approximately 10 
in-lbs (1.3 N-m) of torque; tightening it 3/8 turn clockwise will produce 
approximately 17 in-lbs (1.9 N-m).

 

Check to make sure the housing’s anti-rotation key is aligned with the 
notches on both the panel and the gasket before tightening the hex 
nut. Over tightening the hex nut will distort the rubber gasket and may 
damage the threads on both the nut and the housing, thereby compro-
mising the installation’s mechanical integrity and its ability to protect 
against environmental effects.

 Panel 

Thickness:

 

When using both the factory supplied EPDM gasket and hex nut (the 
recommended standard installation method), the DMR20-1-FM can be 
mounted in panels ranging from 0.032” to 0.250” (0.8 to 6.4mm) thick. 
When the gasket is not used, the panel thickness range is 0.075” to 
0.325” (1.9 to 8.3mm). However, using the factory supplied hardware 
and tightening torque recommendations provides optimal resistance to 
vibration, dust, and moisture ingress.

!

 Panel 

Materials:

 

Acceptable panel materials include aluminum, mild steels, plastics, 
FR-4 pc-board (fiberglass), and many other materials with a flat matte 
surface on both sides of the cutout. The DMR-20-x-KP tools can be 
used with most panel materials except stainless steel or other hard-
ened metals. 

 

DMR20-1-FM has passed vibration testing combined with temperature 
cycling while mounted to the materials noted above, using the specified 
tightening torques. If the meter will be mounted to extremely smooth, 
slippery surfaces, the user is advised to test the completed assembly 
under the environmental conditions encountered in the end application.

5.  AC Input Fusing and Wiring: 

DMR20-1-FM frequency monitors 

contain an internal, self-resettable fuse. However, the supply wires 
connected to input terminals TB1-A and TB1-B must be fused with an 
external, user supplied, 0.25A/300V time delay/time lag fuse, in accor-
dance with applicable regulatory codes.

 

All supply wiring must be rated for the voltages and currents they will 
conduct and comply with any code or application-mandated require-
ments pertaining to the user’s specific installation. 300V, UL-rated 
hook-up wire suitable for the intended application is required. TB1 is to 
be used only for powering the meter’s internal circuitry; it must not be 
used to supply power to external loads.

 

The recommended TB1 supply wire size is 18AWG to 22AWG (0.83mm

2

 

to 0.33mm

2

) solid or stranded copper wire. The supply wires must be 

properly stripped and attached to TB1 such that their insulation is not 
pinched by the screw terminal. Recommended insulation strip length is 
0.25” (6.4mm). When using stranded wire, verify that there are no loose 
or stray strands that could potentially cause a short circuit across the 
ac supply input.

6.  AC Input Polarity and Grounding: 

The ac supply inputs TB1-A and 

TB1-B terminals are not polarity sensitive, that is, neither terminal is 
designated as “AC LO” or “AC HI.” DMR20-1-FM frequency monitors do 
not include, nor require, a connection to earth/chassis ground.

Figure 1. Panel Installation

www.murata-ps.com/support

DMR20-1-FM

Self-Powered, 

LED Display, AC Line Frequency Monitor

MPM_DMR20-1-FM.A02 

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