In an electronics lab, a recently-built advanced oscilloscope is one of the more costly test instruments. To protect this investment from instant destruction or long-range degradation, engineers and technicians must be aware of potential hazards.
In a bench-type oscilloscope powered by a grounded utility supply, the most immediate danger is inadvertent connection of a probe ground return lead to a voltage that is referenced to and floats above ground potential. This action completes a high-current fault circuit. If the user is fortunate, the relatively small wire that is the ground return lead will act as a fuse and interrupt the current flow before there’s serious damage to the oscilloscope, equipment under investigation, or user.
When using a grounded bench-type oscilloscope, the stage is set for this high fault current because the point of connection at the analog channel input is solidly grounded. This is due to the equipment-grounding conductor that is a part of every compliant premises branch circuit. Notice at the oscilloscope input the metallic outer part of the connector. This piece of hardware is solidly grounded, and you don’t want to hook a hot wire to it. That is true even if the instrument is powered off, whenever it is plugged into a branch-circuit receptacle.
Some scope operators, to prevent accidental grounding of a hot ground return lead, cut off the power cord plug’s ground prong. This expedient does protect against fault current. Unfortunately, it also disables the life-saving equipment-grounding conductor, which prevents conductive bodies within the test setup from becoming energized. Besides being a Code violation and creating a shock hazard, cutting off the ground prong introduces a new set of hazards that result from elimination of the critically important ground plane within the instrument.
The obvious remedy is to simply remain vigilant each time you connect the ground reference lead. Remember: It is the ground reference lead, not the probe tip, that is the issue. The probe tip may be, and often is, connected to voltages that are referenced to and float above ground potential. Likewise, the ground return lead can safely be connected to higher voltages provided they are not referenced to ground. As an example, an oscilloscope probe can be connected to a 9-V battery without regard to polarity, because there is no ground potential involved.
One must be careful when measuring telephone and other communication or data circuits, because there may be a ground plane situation that is not immediately evident. A neon test light is a convenient high-impedance test device that is helpful for checking out floating voltages prior to doing an oscilloscope hookup.
A bench-type mains-grounded oscilloscope is capable of dealing with floating voltage measurements if it is equipped with a differential probe. For example, in measuring internal voltages within a variable-frequency drive (VFD) for an ac motor, it is necessary to look at circuits where both sides are referenced to and float above ground. These voltages may be substantial and with heavy industrial wiring, the available fault current is immense. In a 440-V unit, due to the full-wave rectification, the voltage on the dc bus is 1.414 times the RMS supply voltage, which comes out to 678 V. So these measurements should be made carefully, using a differential probe.
The problem is that since the differential probe is expensive and not often used, many oscilloscopes don’t have one. For this reason, many users, in making floating measurements, prefer a hand-held, battery-powered oscilloscope such as the Tektronix THS3024. This type of instrument is fully insulated from ground and the inputs are insulated from one another. (Some portable scopes, although they are insulated from ground, do not have inputs that are fully isolated from one another, so they may be hazardous for certain types of measurements.)
Generally, the hand-held, battery-powered oscilloscope is a viable solution for making measurements where both sides of the circuit float above the ground plane.
Besides the voltage hazard in using a grounded bench-type oscilloscope, there are other situations that can damage the instrument and/or harm the user. Additionally, there is the potential for gradual, long-range degradation if certain precautions are not observed.
First, we should mention CAT ratings and how they are applicable to oscilloscopes as well as to other electronic test equipment. Test instruments are assigned unique designations, ranging from CAT I to CAT IV. Each of these categories pertains to a specific electrical environment, CAT I being the least hazardous and, moving upstream to the ultimate power source where there is invariably more available fault current, CAT IV being the most hazardous.
Cat I pertains to voltage measurements of specially protected secondary circuits. Such voltage measurements include signal levels, special equipment, limited-energy parts of equipment, circuits powered by regulated low-voltage sources and electronics.
CAT II pertains to local-level electrical distribution, such as that provided by a standard wall outlet or plug-in loads. Examples of CAT II are measurements performed on appliances and cord and plug-connected power tools.
CAT III pertains to measurements on hard-wired equipment in fixed installations, distribution boards, circuit breakers, wiring, cables, bus bars, junction boxes, switches, socket outlets and hard-wired motors.
CAT IV pertains to origin of installation or utility level measurements on primary over-current protection devices and on ripple control units.
Besides CAT ratings, there are other factors which must be ascertained prior to using the oscilloscope. One is the maximum voltage that can be applied at the analog channel inputs without subjecting the internal circuitry to overload. The voltage level in specific instances may be either higher or lower than the CAT rating. The max voltage rating may be found in the product documentation and/or printed on the oscilloscope enclosure, usually adjacent to the relevant input. Another point to note is that allowed voltage at the inputs varies also according to the frequency.
User manuals invariably have lengthy sections that dispense safety warnings and cautionary information. Some of it sounds more like it was written by lawyers concerned with product liability than by actual applications engineers. That being said, because the oscilloscope is a valuable and sensitive instrument, we will do well to go through this material just to be certain that we do not make a costly mistake.
As an example, going through the Tektronix MDO3000 Series user manual, the operator is warned not to use the oscilloscope in a wet or damp environment. Moreover, it is noted that condensation may arise if a unit is moved from a cold to a warm environment. The bench oscilloscope is more vulnerable to moisture than a hand-held model, in part because of the metal case with open venting, and in part because of the higher supply voltage. Moisture causes degradation of electrical insulation, particularly in the power supply transformer. Also there is the matter of corrosion at terminations, not to mention line-to-line and line-to-ground short circuits and attendant arcing and heat.
The manual further warns against using the scope in an explosive atmosphere. Most of the time this is obvious, but there are gray areas, so it is essential that the user be aware of hazard gradients that may exist. The National Electrical Code provides guidance in this area and for those interested in such matters, it makes interesting reading. Hazardous areas with respect to electrical safety are divided into three classes.
• Class I locations are those in which flammable gases, flammable liquid-produced vapors or combustible liquid-produced vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures.
• Class II locations are those that are hazardous because of the presence of combustible dust.
• Class III locations are those that are hazardous because of the presence of easily ignitable fibers or where materials producing combustible flyings (sawdust is considered a flying) are handled, manufactured or used.
As may be seen in these definitions, Class I is the most hazardous and Class III is less hazardous. Each of these three classes is further subdivided into two divisions. In Division 1, the hazard is more immediate, while in Division 2 the hazard is less likely to be present at any given point in time, but under certain circumstances, it may arise.
Equipment that is permitted in these hazardous areas must be designed to be safe within the environment, and that may involve heavy cast aluminum housing, gasketed construction, and UL or other certification. That is the rationale for the statement in the Tektronix MDO3000 Series user manual that the oscilloscope is not to be used in an explosive atmosphere.
Besides cautionary warnings in the manual, several other protective measures make good sense. Many oscilloscopes record hours of usage. Whether or not this is the case, it is prudent to power down the instrument when not in use. A related matter involves the choice of channels. Since Channel One is turned on by default, many users tend to use this channel exclusively. It seems like a better idea to rotate channels so that wear is distributed more evenly.(On modern digital scopes, the main component subject to wear is the encoder attached to the front-panel knob.)
Most areas are prone to lightning from time to time. Transient voltage spikes can damage connected electrical equipment, and the oscilloscope is vulnerable to this damage. Surge protection at the premises service is effective, and multi-outlet plug-in strips work well. Robust lightning protection results when there is a cascading configuration consisting of variously rated protectors placed in descending order at intervals between the service and the oscilloscope. When a severe thunderstorm is predicted, the instrument should be unplugged from the power source and any LAN connection should be disabled.
All in all, modern oscilloscopes, like other high-end test equipment, are built to last, and careful usage will extend the life expectancy many times over.