Floating Oscilloscope Measurements ... And Operator Protection

• Limitations of Traditional Oscilloscopes
• Management And Safety In The Workplace
• Floating An Oscilloscope: A Definition

Limitations of Traditional Oscilloscopes

Traditional oscilloscope designs cannot effectively handle these problems alone. Let's examine why:

Most oscilloscopes have their "signal common" terminal connected to the protective grounding system, commonly referred to as "earth" ground or just "ground." This is done so that all signals applied to or supplied from the oscilloscope have a common connection point. This is usually the oscilloscope chassis and is held at (or very near to) zero volts by virtue of the third-wire ground in the power cord for AC-powered equipment. It also means that, with few exceptions, all measurements must be made with respect to "earth" ground. This constrains the typical oscilloscope (at least in a single measurement) from being used to measure potential differences between two points where neither point is at earth ground.

Tektronix provides several solutions for "floating" measurements which are not only safe but also much more accurate than the sometimes used potentially dangerous procedures (see Table 1).

These solutions meet the safety engineering principles. They fall into four general categories:

• Battery-powered oscilloscopes
• Monolithic isolation amplifiers
• Differential measurement systems
• Isolated input oscilloscopes

Management And Safety In The Workplace

WARNING

While the subject of this technical note is about Floating Measurements, some definitions of terms and general precautions must be understood before proceeding. Historically, floating measurements have been made by knowingly defeating the built-in safety ground features of oscilloscopes or measurement instruments in various manners. THIS IS AN UNSAFE AND DANGEROUS PRACTICE AND SHOULD NEVER BE DONE! Instead, this technical note describes instruments, accessories, and practices which can make these measurements safely as long as standard safety practices and precautions are observed.

WARNING

Whenever making measurements on instruments or circuits which are capable of delivering dangerously high-voltage, high-current power, measurement technicians should always treat exposed circuits, bus-bars, etc., as being potentially "live," even when circuits have been shut off or disconnected. This is particularly true when connecting or disconnecting probes or test leads.

Floating An Oscilloscope: A Definition

"Floating" a ground referenced oscilloscope is the technique of defeating the oscilloscope's protective grounding system - disconnecting "signal common" from earth, either by defeating the grounding system or using an isolation transformer. This allows accessible parts of the instrument such as chassis, cabinet, and connectors to assume the potential of the probe ground lead connection point. This is dangerous, not only from the standpoint of elevated voltages present on the oscilloscope (a shock hazard to the operator), but also due to cumulative stresses on the oscilloscope's power transformer insulation. This stress may not cause immediate failure, but may lead to future dangerous failures (a shock and fire hazard), even after returning the oscilloscope to properly grounded operation!

Not only is floating a ground-referenced oscilloscope dangerous, but the measurements are often inaccurate. This results from the total capacitance of the oscilloscope chassis being directly connected to the circuit under test at the point where the common lead is connected.

TEKTRONIX RECOMMENDS ONLY THOSE MEASUREMENT TECHNIQUES THAT COMPLY WITH SAFETY ENGINEERING PRINCIPLES AND ENSURE ACCURATE MEASUREMENTS.