By David Herres
When using a bench oscilloscope, it is critical that the scope and the work be reliably grounded, assuming they both connect to an external power source. (Of course, this doesn’t apply to hand-held, battery-operated instruments such as the Fluke Scopemeter, which has isolated inputs.) If the scope or the device-under-test floats with respect to the other, you may be in for trouble in the form of sparks and smoke. The result could be damage to the oscilloscope, equipment being investigated, or the investigator.
Unless differential probes are used, a bench oscilloscope does not do well with point-to-point measurements where neither side is solidly grounded. To emphasize, the probe’s ground line must connect with a wire or terminal that is at ground potential. And the oscilloscope must plug into a modern three-prong receptacle where the equipment ground has intact, low-impedance continuity back to the entrance panel or load center.
It is possible to verify the veracity of ac-plug ground connections using a digital multimeter on a low-range ac setting. Just make sure there is no significant voltage potential (usually tens of millivolts) between the two grounds or between the equipment ground (bare or green) and the neutral conductor (white). Also, inexpensive circuit analyzers (an example is visible below, courtesy of Ideal Industries) and neon test lights are good for making initial assessments.
Of course,the only way an instrument can be correctly grounded is if the premises is properly grounded where ac power enters the building. A visual inspection and instrument readings are advisable when setting up an electrical shop or laboratory.
The National Electrical Code, which has jurisdiction in such matters, mandates that the ground resistance not exceed 25 Ω. This measurement cannot be taken with a conventional ohmmeter because doing so would require a known “perfect” ground for reference. But if a perfect ground was available, the procedure would be unnecessary.
It takes special equipment and procedures to measure ground resistance. Accordingly, an exception in the Code permits a second ground rod to be placed at least six feet away from the first one, in lieu of the resistance measurement. This is how most residential and small commercial occupancies are grounded. Larger facilities, such as industrial buildings, and sensitive locations, like telephone central offices and laboratories, make use of ground plates, ground rings, and Ufers. Ufers are named after their inventor who described premises grounding based on connection to the rebar network inside the concrete footing. This is the lowest impedance ground you can have, but it requires preparation prior to pouring the cement for the footing.
In most cases, a well-constructed building with a good electrical system will have adequate grounding provisions at least to the entrance panel. Typically, the grounding electrode conductor, for a small service, is a number six American wire gauge (AWG) bare copper wire coming into the entrance panel and terminating at a lug made specifically for the purpose. A main bonding jumper connects the neutral bar to the entrance panel so it picks up the grounding. This is the one and only connection between the equipment grounding conductor and the neutral (white) conductor. They leave the panel in the same conduit or cable but they are insulated from one another, never to rejoin downstream even in tools and appliances. Such improper double bonding would make for dangerous circulating currents.
The equipment-grounding conductor (green or bare) must be intact with no missing, improperly torqued or corroded joints, all the way to the oscilloscope and to the work being measured. It is the responsibility of the technical personnel running the measurement, in consultation with a building electrician, to make sure the correct grounding infrastructure is in place before connecting the oscilloscope.