Nuts and Volts - March 2018

16 March 2018

cause a flashover from the wiring to anything touching the case, such as your hand.

Electrocution caused by using meters beyond their voltage rating happens regularly. In the photo, note that the probes are recessed for extra insulation — use the right probes for the full rating.

If you use a voltmeter to measure high voltages, power-down the equipment, discharge the high voltage points, then attach the meter to the point being measured. Place the meter where you can see it. Then, power-up the equipment and take a reading. That way, you’re not touching the meter when it’s energized.

Special high voltage probes (such as the Fluke 80K- 40 or B&K HV44-A) are available for the multi-kV voltages sometimes found in tube gear. Remember, it’s not a race. Take your time.

Switch to Safety

Finally, follow the “switch to safety” rule long promoted by the ARRL (American Radio Relay League) in its literature. One of the ARRL’s early technical pioneers, Ross Hull, died in 1938 from an electric shock when working on transmitting equipment. The “Switch to Safety” campaign began thereafter.

Don’t work on live equipment unless it’s absolutely necessary, and then take precautions. Follow basic safety procedures at all times.

Don’t work alone and know how to respond if someone else does get a shock or burn. CPR courses are often available through a public safety agency like police or fire at very low cost or free. Electrical safety classes may be available through your power utility or an electrician’s trade union.

Know how to remove power and show your friends and family how to remove power in your shop or station.

This all sounds like a lot of bother I’m sure, but you won’t regret having those skills if they are ever needed.

Static Voltages and ESD

ESD stands for electrostatic discharge. Anyone who has gotten a shock from walking across a carpet and

n FIGURE 3. The IEC 60001- 4-2 electrostatic discharge (ESD) waveform (A). The Human Body Model (HBM) circuit at B is used to perform ESD testing on equipment.

n FIGURE 2. Voltage ratings are shown next to a voltmeter’s probe jacks (1,000V DC and 750V AC).

Exceeding these voltages can result in a flashover, also presenting a severe electrocution hazard.

Avoid Crack Attacks

If you’re like most electronic-ers, you never throw anything away — especially test instruments and accessories. In the world of high voltage, this can be dangerous.

As probes, fixtures, and connectors age, their insulation can become brittle and crack. That crack compromises the insulation, and you can even come in direct contact with the internal wiring.

Before beginning a high voltage project, carefully inspect your test probes and other equipment. Make sure the insulation is clean, flexible, and not scuffed or cracked so it can protect you as you expect.