Floating Oscilloscope Measurements ... And Operator Protection

Battery-Powered Oscilloscopes

The TDS3000 Series (see Figure 1), when operated from AC line power using its standard power cord, exhibits the same limitations as traditional oscilloscopes discussed previously.

Figure 1a. - TDS30xx Digital Phosphor Oscilloscope.

Figure 1b. - Optional TDS3BAT Battery Pack.

However, AC power is not always available where you want to make oscilloscope measurements. The TDS3000 Series optional battery pack (TDS3BAT) allows you to operate the oscilloscope without the need for AC power. Observing the following precautions will provide safe operation of the TDS3000 Series oscilloscopes when battery powered.

Figure 2 shows the warning that is displayed on the TDS3000 Series screen when in battery operation mode.

Figure 2. - Warning displayed by TDS3000 Series when operating in battery mode.

This warning applies to circuits that have voltages of greater than 30 V_RMS (>42 V_pk). If the circuit under test has voltages that exceed 30 V_RMS (>42 V_pk), the TDS3000 Series chassis needs to be connected to earth ground using the grounding wire provided with the instrument (Battery Pack) to prevent electrical shock to the operator.

If you do not attach the grounding wire, you are not protected against electrical shock if you connect the oscilloscope to a hazardous voltage.

WARNING

Such electrical shock could result in serious personal injury or loss of life.

While in battery operation and following environmental specification limits for the TDS3000 Series, it is safe to "float" the "signal common" for making measurements provided you do not connect a signal greater than 30 V_RMS (>42 V_pk) from earth ground to either the probe tip or common lead. For measurements where higher voltages {>30 V_RMS, >42 V_pk) are present, the instrument's chassis must be connected to earth ground using the supplied grounding wire to prevent electrical shock to the operator.

WARNING

If there is any doubt whether more than 30 V_RMS is present or not, the supplied grounding wire should always be used and floating measurements SHOULD NOT BE ATTEMPTED! Be aware that hazardous voltages may exist in unexpected places due to faulty circuitry in the device-under-test.

It's important to remember that the "signal commons" for all channels are at the same potential and are NOT independent. Ensure that all probe common leads are connected to the same voltage or common point.

Do not connect a grounded device, such as a printer or computer, to the oscilloscope unless the oscilloscope's grounding wire is connected to earth ground!

In order to perform differential measurements with independent "signal grounds," true differential probes such as the P5205 or P5210 must be used.