When the continuous auto-cycling mode is initially selected, the unit

will briefly display ‘Auto On’ before continuous cycling begins. Mo-

mentarily touching the ‘SEL’ button again will cause the unit to briefly

display ‘Auto OFF’ before it returns to the fixed VOLTS reading mode.

3. Calibration: Due their digital design, ACM20 power meters cannot be

calibrated in the field. ACM20 power meters are factory-calibrated to

meet their specified accuracies with the supplied L1 current trans-

former. Use of any other current transformer will produce significant

errors for the Amps, Watts, and Power Factor measurements.

Contact Murata Power Solutions if additional information is required

regarding calibration, setup, or any other technical issue pertaining to

ACM20 power meters.

4. Wiring: All power supply wiring must be rated for the voltages and

currents they will carry and must comply with any code or applica-

tion-mandated requirements pertaining to the user’s specific installa-

tion.

5. Supply Fusing, and Grounding: Wires specified in the Functional

Specifications section must be used for making connections to

ACM20 series power meters. No connection is required for earth/

chassis ground.

ACM20 series power meters are not internally fused. Terminal block

TB1 is to be used only for powering the power meter's internal circuitry;

it must not be used to supply power to external loads. The supply wires

feeding these power meters must be fused with a 0.5A/250V time

delay/time lag fuse, in accordance with applicable regulatory codes.

TYPICAL WIRING DIAGRAMS

H

N

B

B

A

A

A

TB1

120 Vac (50/60 Hz)

120 V

LOAD

Epoxy encapsulation

Polarity dot on bottom

Fuse*

Figure 1. Wiring diagram for 110/120V single phase systems

Wire insulation must be stripped to within ±10% of the stated dimen-

sions, and wires should be inserted into TB1 such that their insulation

is not pinched by the screw terminal.

6. Current Transformer Polarity: In order to perform accurate Watts

and Power Factor measurements, connections to the two rear power

supply inputs, TB1-A and TB1-B, and built-in current transformer L1

must have the proper polarity. That is, the load current flowing in the

wire passing through L1's center hole must have the same polarity as

the line voltage connected to TB1-A and TB1-B.

The wiring diagrams in Figures 1-4 ensure that, for a purely resistive

load, current flowing in current transformer L1’s primary circuit will

have the same polarity as the applied voltage at TB1. If proper polarity

is not followed, the Watts and Power Factor readings will be zero. To

correct a zero Watts reading, simply reverse the direction of the load

wire passing through the hole in L1.

7. Connector Torque Ratings: It is important to tighten TB1’s, screw-

terminals to their rated torque specification of 3.6 pound-inches

(0.4Nm). Proper tightening will minimize connector losses and ensure

safe, reliable operation.

8. Isolation: Except for the 2-56 thread metallic mounting studs, all of

the ACM20 power meter’s internal components (printed circuit board,

resistor, capacitors, current transformer L1’s secondary leads, etc.)

are at the ac-mains potential connected to TB1. ACM20 power meters

are designed to measure and be powered from one ac power-source

only. Any other connection schemes will introduce significant mea-

surement errors.

Note: If the ACM20 displays zero Watts with

power applied to the load, reverse the direction

of the wire passing through the CT's center hole.

See Technical Note 6 for more information.

*See technical note 5.

ACM20 Series

Four-Function AC Power Meters

www.murata-ps.com/dpm

email: sales@murata-ps.com

03 Mar 2011

MPM_ACM20 Series.A09 Page 3 of 6