High-Voltage Connector Design: Connector Types & Design Issues

Connectors designed for such applications must accommodate particular safety requirements, and present special design challenges.

What is Considered High Voltage?

The numerical definition of high voltage depends on context. Two factors considered in classifying a voltage as “high voltage” are the possibility of causing a spark in air, and the danger of electric shock by contact or proximity. The definitions may refer to the voltage between two conductors of a system, or between any conductor and ground.

At the low end, voltages greater than 50 V applied across dry unbroken human skin can cause heart fibrillation if they produce electric currents in body tissues that happen to pass through the chest area. On the other hand, in electric power transmission engineering, high voltage is usually considered any voltage over ~35,000 V.

Table 1 shows the IEC definitions for low and high voltage and their defining risks.

IEC voltage range	AC	DC	Defining risk
High Voltage	> 1000 Vrms	>1500 V	Electrical arcing
Low Voltage	50 – 1000 Vrms	120 – 1000 V	Electrical shock
Extra-Low Voltage	50 Vrms	<120 V	Low risk

Table 1: IEC voltage definitions (Source: Wikipedia)

High-Voltage Circuit Design: Issues & Considerations

High voltages impose severe stress on all connection system components. Insulation that is perfectly adequate at 12 V, say, may rapidly degrade or fail altogether at 12 kV.  In general, as voltage increases, a corona forms around an HV conductor followed by dielectric breakdown, leading to arcing or catastrophic failure.

Corona Discharge

A corona discharge is an electrical discharge brought on by the ionization of air surrounding a conductor that is electrically charged. The corona will occur when the strength (potential gradient) of the electric field around a conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects. This can occur at voltages as low as 300 V. Corona can also occur due to the ionization of air within a void in a dielectric or interface inside of a connector.

While corona is a low-energy process, over long periods of time, it can substantially degrade insulators, causing a system to fail due to dielectric breakdown.

To minimize corona effects in connector design, it’s important to maximize the distance between conductors that have large voltage differentials, use conductors with large radii, avoid designs that have sharp points or sharp edges, and use dielectrics without voids.

Arcing in Electrical Circuits & Dielectric Breakdown

An electric arc, or arc discharge, is an electrical breakdown of a gas that produces an ongoing plasma discharge, resulting from a current through normally nonconductive media such as air.  At STP, air breaks down at approximately 3 kV/mm.  

In the case of a solid medium such as a dielectric, a dielectric breakdown occurs when the voltage stress is significant enough to cause an arc through the dielectric between the conductor and ground. This failure is catastrophic because the current flow through the dielectric leaves voids filled with carbon and the dielectric will no longer be able to withstand the required voltage.

High Voltage Safety Standards

These effects can have serious, possibly fatal, consequences in high-voltage equipment, including connectors, so there are numerous safety standards that have been developed, depending on the application.

 

Safety Standard	Application
IEC/EN-61558-2-17	Power transformers, power supply units,  switch mode power supplies
IEC/EN-60950	IT Equipment
IEC/EN-60601-1	Medical electrical equipment
IEC/EN-60079	Electrical Apparatus for Explosive Gas Atmospheres
IEC/EN-60335	Household and similar electrical appliances
IEC/EN-60065	Audio, video and similar electronic apparatus

Table 2: Some common safety standards (Source: Feryster)

Other fields, such as automotive, have their own set of standards established by organizations such as the SAE.

High Voltage Clearance & High Voltage Creepage

In order to minimize the risk of failure in high-voltage equipment and give an adequate safety margin, conductors carrying high voltages must maintain a certain minimum distance apart (separation).  These distances, called clearance and creepage, vary by application and are specified in the appropriate safety standard.

 

Figure 1: Clearance and creepage. (Source: Feryster)

The clearance distance is the shortest distance between two conductive parts such as connector pins measured through air. An adequate clearance distance helps prevent dielectric breakdown between pins caused by air ionization. The dielectric breakdown level is also influenced by relative humidity, temperature, and degree of pollution in the environment.

The creepage distance is the shortest path between two conductive parts (or between a conductive part and the binding surface of the equipment) measured along the surface of the insulation. A proper and adequate creepage distance protects against tracking, a process that produces a partially conducting path of localized deterioration on the surface of an insulating material as a result of the electric discharges on or close to an insulation surface. The creepage distance is equal to or larger than the clearance distance.

The Comparative Tracking Index (CTI) is used to measure the electrical breakdown (tracking) properties of an insulating material. For a given application, the minimum creepage distance required by safety agencies such as UL is dependent on the insulator’s CTI value.

Pollution Degree

For a given voltage and insulating material, the clearance and creepage distances are also affected by dry pollution and condensation present in the environment, also known as pollution degree. Pollution degree classification ranges from degree 1 (extremely low, equivalent to a clean room environment) to degree 4 (persistent conductivity caused by conductive dust, rain, or snow). Pollution degree can have a major impact on connectivity because clearance and especially creepage distances increase dramatically with pollution degree.

For example D-subminiature (Dsub) connectors are commonly used in many applications. They are available from many vendors and are inexpensive. The distance between the connector pin and the ground shield is about 1.6 mm. This distance meets UL creepage safety standards for both pollution degree 1 (0.3 mm) and pollution degree 2 (1.6 mm) environments at 150 V.

For 300-V operation, though, the creepage distance for pollution degree 2 increases to 3.0 mm, so the connector meets safety standards only if used in a pollution degree 1 environment. Therefore, within a typical test environment—which is classified as degree 2—using a Dsub connector above 150 V does not meet applicable safety standards and is considered unsafe.

High-Voltage Connector Design

How do designers of HV connectors minimize the effects of HV and maximize safety?

Figures 2 and 3 show a typical high-voltage circular connector and receptacle, rated for up to 27 kV operation. Typically a deep-well female receptacle, shown at right, is located on the HV side. The male connector pins are mounted in individually insulated channels.

 

Figure 2: LGH high-voltage circular connector. (Source: TE Connectivity)  

Figure 3 shows the internal construction. Notice how as the two halves are brought together, the male insulation ring completely surrounds the female before the two pins make electrical contact. To avoid arc-over between adjacent pins on the same housing, potting compound must be added to fill the rear cavities after the pins are inserted into the housing shell.

 

Figure 3: LGH connector cross-section. (Source: TE Connectivity)  

To protect against dielectric breakdown, HV connectors such as this one use high dielectric strength materials such as PTFE, also marketed under the brand name Teflon®. The dielectric strength of PTFE resin is very high (23.6 kV/mm for 1.5 mm thickness as measured by the ASTM short-time test), and does not vary with temperature and thermal aging.

System-Level Safety

In cases where it is possible that the connector may be disconnected while the power is still on, there are other strategies that can be used to ensure safety. Incorporating a High Voltage Interlock (HVIL) circuit is a system-level strategy to enhance connector safety. An HVIL circuit is a separate closed circuit built into the connector design that is a mate-last/break-first type of connection.  

As an HV connector starts to disconnect, the HVIL circuit detects that movement and signals the power electronics to discharge high voltages present at the terminal below a predetermined level before the final disconnection of the terminal. This typically must happen within a half second of the HVIL detecting the beginning of the connection break within the power electronics unit. Ideally, that results in no high voltages being present at the terminals when the connector is fully separated.

This strategy is used in HV connectors designed for automotive electric vehicle use, which carry potentially lethal voltages.

High-Voltage Connectors from Arrow

Specifying connectors for high-voltage applications is a specialized area: Arrow and TE Connectivity offer a range of high voltage connectors and the technical assistance to help you make the right choice.