High voltage relays are electromechanical devices used to switch high voltage (> 1kV) signals. They operate using the same basic principles as electromechanical relays, but include features designed to allow use in high voltage applications. A high voltage relay's contacts are typically contained in a vacuum enclosed by glass or ceramic, which prevents the contacts from arcing. High voltage products are also constructed differently than normal relays, as their coil is located outside of the vacuum and further from the contacts.

High voltage relay construction. Image credit: TE Connectivity

Specifications

Performance Specifications

Buyers of high voltage relays should pay special attention to the product's performance specifications, including maximum voltage and dielectric strength.

A relay's maximum switching voltage simply refers to the amount of voltage the product can withstand without physical damage. Likewise, a product's maximum current refers to maximum continuous current allowable across the output terminals under specified environmental conditions. Maximum current is sometimes known as maximum switching current.

Dielectric strength is also known as isolation voltage and is defined as the maximum potential gradient an insulating material can withstand without physical damage. This spec is expressed in the volts (V) or kilovolts (kV) the material can handle before breakdown.

Switch Specifications

The Engineering360 SpecSearch database contains information about a high voltage relay's switch, including the number of poles and throws.

The term pole describes the number of separate circuits controlled by a switch. The number of circuits controlled by the relay determines the number of switch contacts, which in turn determines the poles needed to make or break the contacts. Switches typically have between one and four poles.

The image series below illustrates, from left to right, a single pole (SP), double pole (DP), and triple pole switch (3P). Note that, in the last image, the switch is connected to three separate circuits and has three contacts.

Image credit: Enasco | Skycraft Surplus | Frank Alapini

A throw defines the number of distinct positions a switch is capable of. It is important for a relay switch's throws to suit its application.

Single throw (ST) switches are open in one position and closed in another. For example, a single pole single throw (SPST) switch is a simple on-off switch, such as a light switch. A double pole single throw (DPST) switch is an on-off switch that opens and closes two contacts with a single motion.

Double throw (DT) switches are two-way devices. Double throw relays have three contacts and two positions: in the first position, Contacts 1 and 2 are in contact, but the third remains open. In the second position, this connection is reversed to Contacts 2 and 3.

Specifications about contacts, including contact orientation and maximum ratings, are important to consider when selecting electromechanical relays.

Contact orientation refers to the switch's position when a relay coil is not energized. As its name implies, a normally open (NO) switch is open in a resting, non-energized position; when current is passed through the relay, the switch then closes. A normally closed (NC) switch is therefore reversed: closed at rest, and open when energized. Changeover switches contain both NO and NC contact types.

Contacts are frequently rated to accept a maximum amount of current allowable under specified heat dissipation and ambient conditions. Maximum current is sometimes referred to as maximum switching voltage (expressed in volts) or maximum switching current.

The speed specifications of a relay include make time and break time. Make time refers to the amount of time a switch needs to operate and make contact, while break time is the amount of time required to release and break contact. Switch speed is typically measured and specified in milliseconds. Higher speed devices have several advantages over lower speed ones. In high voltage applications fast switching reduces arcing and the possibility of physical damage.

Mounting

High voltage relays may be mounted using a number of different methods.

• Bracket (or flange) mounted relays are equipped with a flange for mounting. The flange is typically installed by bolting the device to a matching flange which is then welded to a corresponding wall.
• DIN rail mounted devices are equipped with fasteners capable of mounting on DIN rails. DIN rails are mounting devices standardized by the Deutsches Institut fur Normung (DIN).
• Panel mount relays are manufactured for mounting to an electrical panel.
• PCB relays are mounted on printed circuit boards (PCB) using through hole contacts or surface mount technology (SMT).
• Socket relays are mounted to PCBs using pin sockets.

Isolation

High voltage relays may use one of several different methods to isolate the contacts. As mentioned above, the product may use a vacuum as an isolation method and also to prevent arcing. Other relays may use air or a gas mixture as the dielectric isolation material. Reed switches are also used for isolation and can switch devices such as solenoids, contactors, and motors.

References

National Instruments - How to Choose the Right Relay