Technical FAQs

# What values should be set for the potential transformer (PT) primary and secondary values in a CM4000 when connecting the voltage inputs of meters directly to the bus (with fuses)?

Issue
A user may want to know the PT primary and secondary values for a CM4000 upon initial setup

Product Line
CM4000

Environment
Setup

Cause
The meter may be set to "No PT" or the PT ratio may be used.

Resolution
When setting the PT ratio, one may select "No PT" for the PT Primary Scale - there is no requirement to set the PT ratio if "NO PT" is chosen.

The PT Primary and PT Secondary inputs are still available, but they are not used when "No PT" is selected for the PT Pri Scale.
A user would set the PT ratio to "No PT" when the system voltage used by the load to be metered is the same as the system voltage connected directly to the meter (i.e. direct connect with no potential transformers used).

The PT ratio needs to be set only when the load is using a different voltage from the meter voltage inputs (i.e. potential transformers are being utilized). Potential transformers are used when the voltage of the electrical system is higher than the rated specifications of the meter voltage inputs. The PT primary and secondary ratio on the meter should match that of the potential transformers in the field.

Example 1: When metering a 1.2 kV system with a set of PT's that contain a ratio of 1200:120, the PT Primary of the meter should be set to 1200 and the PT Secondary of the meter should be set to 120.
(Potential transformers are used in this scenario because 1.2 kV exceeds the acceptable voltage input threshold the CM4000 can withstand (1–690 L-L, 400 L-N)

Example 2: When metering a a 480V L-L system, no PT's are being used. The system voltage is directly connected to the metering inputs. In this scenario, you would set the PT Pri Scale of the meter to "No PT".

Note: If the meter receives voltage that is the same as the voltage used by the load and "No PT" is NOT selected, then a PT ratio of 1:1 would need to be used and is still acceptable. (i.e.This result can be achieved by setting the PT Ratio to 120:120 or 480:480)