Fluke Corp. has manufactured electrical test and measurement instruments for decades, and these instruments have been well-known for accuracy, durability and great user-friendly interfaces. Maintaining those qualities, Fluke is now emphasizing electrical safety.
This emphasis is apparent in Fluke’s new line of electricians’ hand tools, which are designed and built with the idea that after putting in a full day in the shop or field, electricians should return home safe and uninjured despite dealing with electrical hazards on a daily basis.
Traditional electronic instrumentation involves a close approach to live voltages, and there is the potential for high fault current. Technicians use meters such as the classic 287 true RMS multimeter and Fluke’s versatile ScopeMeter to measure electrical energy and display waveforms at far higher levels than encountered in a residential or office setting.
The reality in this type of work is that if you touch a cold and inert-appearing conductor or terminal that is energized, you could be injured or killed. Typically, electrons enter the body at a hand or fingertip and flow at close to the speed of light, seeking ground. You needn’t be ankle-deep in water. Standing on an apparently dry concrete slab will complete the circuit. The arteries and veins are pipes fill with conductive liquid and they terminate at the heart. Voltage peaks disrupt the heart’s rhythm and that is why ac is more hazardous than dc.
Arc flash is a still greater hazard than shock. In this grim scenario, current does not actually pass through the body. Instead, the shock wave and heat associated with fault current between two terminals or between a terminal and grounded surface cause catastrophic injuries to people even some distance away. Arc blast often happens when a metal tool is dropped or slips so it completes a circuit between two opposite poles that have high available current. “Available” implies that the full current may or may not flow when the circuit is completed.
Voltage, current and resistance are related by the Ohm’s law equation:
E = IR
Where E = voltage (electromotive force), I = intensity (the old word for current), and R = resistance (in ohms). Solving for current:
I = E/R
When R goes low, I goes high. It never reaches infinity because even in the dropped-wrench event, there is a small resistance and the duration is short before the tool explodes, opening the circuit. And more importantly, the current is limited by the supply impedance, which includes conductors, transformer windings, internal generator copper and utility-owned current-limiting devices including fuses and breakers. Even so, when the area of contact is small, the temperature at the fault can equal that at the sun’s surface.
There are some ways to avoid arc fault hazards. Insulated tools help, but also important is to observe CAT location voltage limits, prominently displayed adjacent to the inputs on electrical test and measurement tools such as multimeters and hand-held oscilloscopes. Beginning with the most hazardous of these locations:
• CAT IV pertains to the source at a building’s low-voltage mains installation. (High and low voltage are relative terms. In this context the transition point in a single-phase service is 240 V.)
• CAT III is applicable to the downstream distribution of the low-voltage mains installation.
• CAT II applies to utilization points such as receptacle outlets.
• CAT I pertains to electrical wiring and equipment not directly connect to the utility or alternate supply.
Each of these locations is associated with a maximum voltage that the marked test instrument can probe. This is a maximum voltage your meter can safely measure. It relates to the act of handling the instrument and is not to be confused with the maximum rated voltage at the meter’s input, which is often different.
Some test tools can be read remotely. For example, you could power down a variable-frequency drive, connect leads, then from a safe distance, power up the drive and record the measurement. The new Fluke 374, 375, 376 and 381 current probes are rated for 600 V in a CAT IV location and for 1 kV in a CAT III location. The Fluke 381 clamp meter permits the user to record measurements 33 ft from the device under test, even when it is in a steel enclosure.
The Fluke ScopeMeter 190 Series II hand-held oscilloscope, like smaller Fluke multimeters, can withstand harsh industrial settings and is dual-rated CAT III 1,000 V and CAT IV 600 V.
The other robust defense against hazardous arc flash appears in Standard Number 1926, promulgated by the Occupational Safety and Health Administration, part of the U.S. Dept. of Labor. Rules applicable to electrical work include:
• Only qualified employees may work on or with exposed energized lines or parts of equipment.
• Only qualified employees may work in areas containing unguarded, uninsulated energized lines or on parts of equipment operating at 50 V or more.
• Electric lines and equipment are to be considered and treated as energized unless they have been de-energized.
• At least two employees are to be present while any employee performs the following types of work:
1. Installation, removal or repair of lines energized at more than 600 V.
2. Installation, removal or repair of de-energized lines if an employee is exposed to contact with other parts energized at more than 600 V.
3. Installation, removal or repair of equipment such as transformers, capacitors and regulators if the employee is exposed to contact with parts energized at more than 600 V.
• Not included are the following:
1. Routine circuit switching.
2. Work performed with live-line tools.
3. Emergency repairs to the extent necessary to safeguard the general public.
IEEE has outlined accurate calculations that take into consideration the true operating conditions of a facility. The energy exposure level associated with a specific task determines the type of personal protective equipment electrical workers should wear. IEEE standard 1584 establishes the following analysis process:
• Collect system and installation data
• Determine modes of operation
• Determine bolted-fault current
• Determine arc-fault current
• Find protective device characteristics and arc duration
• Document system voltages and equipment class
• Select working distances
• Calculate incident energy
• Calculate the flash protection boundary.
Fluke’s new line of 1,000-Vac, 1,500-Vdc-rated insulated hand tools have actually been tested to 10,000 V. Many others, besides those illustrated here, are offered. They are available individually, in selected groups and as the complete set. These ergonomic hand tools are manufactured in Germany and may be purchased world-wide through authorized dealers such as Amazon. They carry a lifetime warranty.
The Fluke T6-1000 is rated to measure 1,000 V, ac or dc, and 200 A in the current mode, 1,000 V in a CAT III location and 600 V in CAT IV, where there is greater available current. This means that it is suitable for three-phase, 480-V work, including the 678-V dc bus in variable-frequency drives for high-horsepower motors. The open-fork design accommodates a 4/0 conductor as used in a 200-A electrical service.
The Fluke T6-1000 electrical tester resembles a clamp-on ammeter but actually operates quite differently. The clamp-on ammeter performs current readings by detecting the magnetic field surrounding a conductor through which current is flowing. The jaws close around the conductor, inducing current flow through the instrument’s secondary coil.
In contrast, the T6-1000 capacitively senses the electrical field surrounding an energized wire even when there is no load and no current flowing. Neither meter can read anything in a cable such as Romex or a flexible cord that contains a hot wire and neutral return conductor or two or more opposing phase wires, because the current and/or voltage and consequently magnetic or electrostatic fields cancel out and you get a zero reading. The solution is to slit the cable jacket and pull out a single conductor which is then inserted into the jaws so that the current can be read.