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What is the difference between 2-wire and 3-wire KYZ pulse outputs

This article pertains to Powerlogic Meters that have the capability of Solid-State KYZ Pulse Output, specifically CM4000s, CM3000s, and CM2000s (with IOM modules). The KYZ output is a Form-C contact that can be wired either as a 2-wire pulse initiator or a 3-wire pulse initiator.
The two-wire pulse train looks like the following:

The majority of two-wire applications use a Form-C contact, but tie into only one side of the Form-C contact where the pulse is the transition from OFF to ON of that side of the Form-C relay. In the two-wire application, the transition from KZ to KY is the beginning of a pulse and the transition from KY to KZ is the falling edge or the completion of the pulse. The next pulse does not begin until there is another transition from KZ to KY.

In the above two-wire pulse train example, the numbers (1 - 6) represent the accumulation of kWh as monitored by the meter. So, the number 1 represents 1 kWh, while the number 2 represents the meter reading 2 kWh. With the pulse constant set to 1 kWh, each time 1 kWh is accumulated there will be a transition from KZ to KY or KY to KZ. However, since only the KY is monitored in the two-wire application, a pulse only begins when there is a KZ to KY transition. The gray areas in the pulse train represent the pulses.

The three-wire pulse train looks like the following:

The three-wire pulses are the transitions between KY and KZ. Unlike the two-wire application where a pulse only begins when there is a transition from KZ to KY, in a three-application pulses begin when there is a transition from KY to KZ or from KZ to KY. Consequently, the end of one pulse is the beginning of the next pulse.

In the above three-wire pulse train example, the numbers (1 - 6) represent the accumulation of kWh as monitored by the meter. Each time the pulse constant is reached there is a transition from KZ to KY or from KY to KZ. Since the three-wire application monitors both KY and KZ, pulses begin at each transition. Consequently, there are twice as many pulses in a three-wire application as there are in two-wire application for a given period of time (i.e. when the meter reads 3 kWh, the two-wire application has a pulse count of one and is beginning its second pulse, while the three-wire application has a pulse count of two and is beginning its third pulse). Each gray area in the pulse train represents a pulse.


If you are attempting to setup pulsing from a meter to achieve a certain number of pulses within a given time, the pulse constant must be divided by 2 for the two-wire application. With a pulse constant which is half of the pulse constant of a three-wire application, a two-wire application will then generate the same number of pulses as the three wire application over a given period of time. 


General Overview:
-In the two-wire application, only the KY or the KZ is monitored, hence "two-wire".
-If wired to KY, pulses begin when transition to KY and end when transition to KZ.
Over a Given Period of Time:
KZ to KY = start of pulse 1.
KY to KZ = end of pulse 1.
KZ to KY = start of pulse 2.

-In the three-wire application, both the KY and KZ are monitored.
-A pulse begins with the transition to KY and then ends with the transition to KZ. The transition to KZ also starts the second pulse, which ends with the transition back to KY.
Over a Given Period of Time:
KZ to KY = start of pulse 1.
KY to KZ = end of pulse 1 and start of pulse 2.
KZ to KY = end of pulse 2 and start of pulse 3.


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