Measurement Computing WavePort 312P rev.1.0 - Manual (Page 36)

4-2 Device Configuration PowerVista/312 User’s Manual
Power system measurements always begin with a connection of external voltages to hardware inputs. The
connection points, voltage levels, and arrangements can vary depending upon equipment installation.
EasyPower Measure has been designed to allow easy and safe connection to three phase power systems as well
as other miscellaneous connections. Six of the most common power system connection arrangements are
described below.
WARNING !
ELECTRIC SHOCK HAZARD! The need for proper and safe connection procedures cannot
be stressed enough. All power system voltages from 120V and higher are lethal and can
severely damage instrumentation equipment when applied incorrectly. In all cases, external
connections should never be rushed or performed in a hurry. The connection procedure
described below should be followed to minimize accidents and equipment damage. Failure to
observe safe electrical practices could result in death or serious injury from electric shock.
It should be noted that the “PT Connection” item in the System Configuration is appropriately labeled when the
hardware is connected to potential transformer secondaries. For external connections where access to the
system being measured is direct, i.e. a 480V load center bus, “PT Connection” defines the electrical connection
arrangement of the system, even though no potential transformers are involved. The system arrangement can be
defined by equipment connections near the measurement location, or a long distance away via cable or busway.
4.1.1 Differential Inputs
The EasyPower Measure hardware utilizes differential inputs for all channels. Because of this, there are
tremendous advantages and flexibility in monitoring capabilities. Differential inputs offer the following
advantages:
• Ability to measure drive DC bus voltages.
• Ability to measure AC System and drive DC bus voltages simultaneously.
• Ability to perform a true measure of line-to-line voltages.
• Ability to measure four unrelated high voltage and four unrelated low voltage signals.
• Able to connect low voltage differential signals utilizing common mode rejection capability.
Along with these strengths however, there are some instrumentation compromises that must be considered. The
following compromises are a result of differential inputs:
• Unused channels will float, and under higher gain conditions, will drift to the rail (the minimum or
maximum level of the input).
• Drifting means that unused, unshorted channels will not deliver a near zero measurement.
• Drifting creates a need for shorting out unused input channels. Shorting plugs are supplied with the
hardware.
• Single ended signals (like clamp-on CTs) need to use an input adapter (supplied with hardware) that
includes an additional connection from the negative input to Safety Ground, which is also the analog
system ground. This aids in shielding and helps to eliminate noise pickup by clamp-ons that have
poor shielding.
Note: For all connection procedures discussed, if the extra inputs V4 and E1 are not being used, their inputs
should be shorted with the supplied shorting jumpers to supply a clean, near zero measurement.