What's the Voltage Rating of an RG8-U Coaxial Cable?

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Output cable and connectors are not trivial items for power supplies where output voltages can be 100,000 volts or higher. The cables and connectors used must function together as a system to safely and reliably access and provide the power supplies output for customer usage.

In many high voltage power supply applications, a shielded polyethylene coaxial cable is used. Polyethylene cables provide excellent high voltage dielectric isolation characteristics in a small but robust form factor. The shield conductor provided in a coaxial cable functions as a "Faraday Shield" for the center conductor of the cable that is referenced to the high voltage potential. If any breakdown in the main insulator occurs, the high voltage current will be bypassed to the grounded shield conductor that surrounds the main insulator. This inherent safety feature is one benefit of using a coaxial high voltage output cable.

RG8-U has long been used as a high voltage output cable in the high voltage industry. There is a variation of RG8-U that utilizes a solid polyethylene core. Specifications for this cable do not specify actual "high voltage" ratings, since this cable was not designed and fabricated with high voltage usage in mind. So the reality is, there are no high voltage ratings for RG8-U. Over the years others in the HV industry have used this cable at 20kV, 30kV and even higher voltages. Spellman does use RG8-U cable, but limits it usage to applications where the maximum voltage that will be applied to the cable is 8kV or less.

For voltages above 8kV where a coaxial polyethylene cable is desired, Spellman uses cables specifically designed and manufactured for high voltage usage.

These cables are of the same general design; as described above but the insulating core material diameter has been increased appropriately to obtain the desired dielectric insulating capability required. Frequently higher voltage versions of these cables utilize a thin semiconductor "corona shield". This corona shield is located between the metallic center conductor and the main polyethylene insulating core. This corona shield helps equalize the geometric voltage gradients of the conductor and by doing so reduces the generation of corona.

A high voltage cable and connector system can only be as good as the materials used to make it. Using cables that are designed, specified and tested specifically for high voltage usage assures that these materials are used within their design guidelines.

What Is the Difference Between Voltage Mode and Current Mode?

Power control, (a.k.a. power mode or power loop) is a third control mode that can be added to a variety of Spellman power supplies to provide another means to control and regulate the output of the supply. Voltage mode and current mode are the primary controlling modes of most units. Taking the voltage and current monitor signal and inputting them into an analog multiplier circuit, creates a power feedback signal (voltage x current = power). Using this feedback signal with an additional programmable reference signal in conjunction with error amplifier circuitry, a programmable power mode can be created.

Power control is typically used in two types of applications. The less common application is where the power into a load is the needed regulating parameter. A critical heating requirement may have very specific regulated thermal need. Using power mode, voltage and current limit levels can be established, and power mode will provide constant power to the load, immune from any impedance variations from the load itself.

The more popular usage of a power mode is in the area where a power source or load might be rated or capable of more current at reduced voltage levels, but limited to a particular power level. X-ray tubes frequently have this type of capability. If the maximum voltage were multiplied by this "increased current" capability, a power level above the rated power level would result. Power mode can address this problem by limiting the power to the maximum rated (or present) level.

Where Can I obtain Information On High Voltage Safety Practices?

One of the most comprehensive publications regarding high voltage safety practices is an excerpt from IEEE Standard 510-1983 known as "The IEEE Recommended Practices for Safety in High Voltage and High Power Testing." This information is available from Spellman in the form of a printed document included in our "Standard Test Procedures and Safety Practices for High Voltage Power Supplies" handout. Please contact our Sales Department for a copy. Safety Procedures

What is Solid Encapsulation?

Solid encapsulation, also referred to as "potting," is an insulation media used in a variety of Spellman's supplies. The "output section" of a high voltage power supply can operate at extremely high voltages. The design and packaging of the high voltage output section is critical to the functionality and reliability of the product.

Solid encapsulation allows Spellman designers to miniaturize the packaging of supplies in ways that are unobtainable when utilizing air as the primary insulating media alone. Improved power densities result, providing the customer with a smaller, more compact supply.

Additionally, solid encapsulation provides the feature of sealing off a potted output section from environmental factors. Dust, contamination, humidity and vibration typically will not degrade or affect the performance of an encapsulated high voltage output section. This is especially important where a supply will operate in a harsh environment, or where a unit must operate maintenance free.

What is Corona?

Corona is a luminous, audible discharge that occurs when there is an excessive localized electric field gradient upon an object that causes the ionization and possible electrical breakdown of the air adjacent to this point. Corona is characterized by a colored glow frequently visible in a darkened environment. The audible discharge, usually a subtle hissing sound, increases in intensity with increasing output voltage. Ozone, an odorous, unstable form of oxygen is frequently generated during this process. Rubber is destroyed by ozone, and nitric acid can be created if sufficient moisture is present. These items have detrimental affects on materials, inclusive of electrical insulators.

A good high voltage design takes corona generation into account and provides design countermeasures to limit the possibility of problems developing. Spellman engineers use sophisticated e-field modeling software and a Biddle Partial Discharge Detector to ensure that each high voltage design does not have excessive field gradients, preventing partial discharge and corona generation.

What is the difference between a modular supply and a rack supply?

Modular supplies and rack supplies are the two generic categories into which Spellman's standard products typically fall. These product categories were created and used to help classify hardware. Additionally, Spellman provides a variety of custom and OEM supplies that would not adequately fit into either category.

Typically, rack mounted supplies are higher in power than their modular counterparts; but this is a generalization, not a rule. Rack mounted units usually operate off-line, requiring AC input. Rack mounted units usually provide full feature front panels, allowing quick and easy operator use. Spellman's rack mounted supplies comply with the EIA RS-310C rack-mounted standards.

Modular supplies tend to be lower power units (tens to hundreds of watts) housed in a simple sheet metal enclosure. Modular units that can operate off AC or DC inputs, can be provided. OEM manufacturers frequently specify modular supplies, knowing the elaborate local controls and monitors are usually not included, thus providing a cost savings. Customer provided signals, done via the remote interface connector, usually accomplishes operation, programming and control of these units.

When ease of use and flexibility is required, like in a laboratory environment, rack mounted supplies are usually preferred. Modular supplies tend to be specified by OEM users, where a single specific usage needs to be addressed in the most compact and cost effective manner possible. These are guidelines, not rules.

High Voltage Power Supply Mission

The mission of this blog is to, from time to time, update this space with real world information as it relates to the daily and common use of high voltage power supply and supplies in our daily lives. For example, they are used in airport security scanners, xray machines, food and produce scanning devices and even the Genome Project!

When dealing with high voltage power and high voltage power supplies, it is most important that safety be given supreme and detailed attention. With this in mind, the first post will be on safety as it relates to high voltage power and will reference written material supplied by Spellman High Voltage Electronics Corporation:

What is a safe level of high voltage?

Safety is absolutely paramount in every aspect of Spellman's high voltage endeavors. To provide the maximum margin of safety to Spellman's employees and customers alike, we take the stand that there is no "safe" level of high voltage. Using this guideline, we treat every situation that may have any possible high voltage potential associated with it as a hazardous, life threatening condition. We strongly recommend the use of interlocked high voltage Faraday Cages or enclosures, the interlocking of all high voltage access panels, the use of ground sticks to discharge any source of high voltage, the use of external interlock circuitry, and the prudent avoidance of any point that could have the slightest chance of being energized to a high voltage potential. The rigorous enforcement of comprehensive and consistent safety practices is the best method of ensuring user safety.