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: ACM20 power meters are not inter-

nally fused. If supply fusing is required it must be externally installed

according to the supply wire’s maximum amperage rating. No con-

nection is required to earth/chassis ground.

ACM20 power meters draw minimal steady-state supply currents

(110mA max.), and in most applications can be fused according to the

supply wire’s maximum amperage rating. However, these models can

draw significantly higher surge currents for brief periods when the ac

line voltage is initially applied to TB1.

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.

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

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

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.

ACM20 Series

Four-Function AC Power Meters

www.murata-ps.com/dpm

email: sales@murata-ps.com

08 Oct 2009

MPM_ACM20 Series.A03 Page 3 of 6