FLOATING MEASUREMENTSPresenter or date goes here

March 01, 2014

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FLOATING MEASUREMENTS

A floating measurement reads the voltage between two points, neither of which is at ground potential.

“Signal common” may be elevated to hundreds of volts from earth

FACETS OF FLOATING MEASUREMENT

Operator safety

Form factor and cost of the measurement tools

Measurement quality

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Performance Form SafetyReal-time bandwidth Portability (size) InsulationSample Rate Weight Isolation from groundAccuracy Probing solutions Isolation between channelsChannel count Cost Voltage ratingTriggering capability Ease of use UL/IEC1010 certification

Table: 1 Attributes that make the facets

APPLICATIONS

Power control circuits (motor controllers, uninterruptible power supplies, and switching DC power supplies) and industrial equipment

To measure one machine with reference to another where the two machines are connected to different power distribution circuits with different ground reference levels

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FLOATING MEASUREMENT TECHNIQUES

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Battery Powered DMM

Well suited for floating measurement within their bandwidth limits

Insulated housings facilitates safe handling

Due to battery operation, provide good isolation from earth ground

Emphasize the form facet, trades off performance benefits

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Isolated Input Oscilloscopes Measurements

Provides true and complete channel to channel and channel to power line isolation

Channels are individually isolated

Requires special probes which has insulated BNC connectors and reference lead which can withstand rated float voltage

Safe and reliable measurement; trades off balanced floating measurement

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Differential Probe Measurements

These systems enable floating measurements with grounded oscilloscope

Safer way of operation

Neither of the probe contacts are at earth ground nor the probe system at chassis ground

May require independent power supply

Emphasize performance and safety facts; trades off form factor

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Voltage Isolator Measurements

Isolators do not have direct electrical connection between floating inputs and ground referenced outputs

Signal is coupled through optical or transformer

Provides safety; trades off cost and form facet

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A minus B Measurements

Uses conventional oscilloscope and passive voltage probe to make an indirect floating measurement

One channel measures ‘positive’ test point and other ‘negative’ test point and difference between the two results in floating voltage measurement

Probes must be matched to maximize common mode rejection ratio

Easily done; trades off measurement capability

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Floating A Conventional Grounded Oscilloscope

Uses isolation transformer which does not carry ground through to the secondary (or) uses an oscilloscope with disconnected AC mains power cord grounding connector

Puts all accessible metal parts at the same voltage as the test point where the probe reference lead is connected

Uses existing device, trades off safety and measurement capability

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