In electrical and electronic systems where reliability cannot be compromised, the relay is often a deceptively critical component. A relay may perform a simple function—opening or closing an electrical circuit—but its operating environment can make that task considerably more difficult. Moisture, corrosive gases, dust, pressure changes, temperature extremes and mechanical vibration can all affect switching performance over time.
This is where the question “what is a hermetically sealed relay?” becomes important.
A hermetically sealed relay is an electromechanical switching device whose critical internal components are enclosed within a tightly sealed environment designed to prevent the exchange of gases, moisture and contaminants with the surrounding atmosphere. Unlike ordinary plastic-enclosed or environmentally sealed relays, hermetic designs are intended for applications where the stability of the internal atmosphere is essential to long-term electrical performance.
They are widely associated with aerospace, defense, high-reliability industrial systems, instrumentation and other applications where component failure can carry a high technical or financial cost.
What makes a relay hermetically sealed?
The defining feature is not simply the presence of a cover around the relay. Almost every modern relay has some type of enclosure.
Hermetic sealing is about controlling the boundary between the internal switching mechanism and the external environment.
Depending on the relay architecture, manufacturers may use:
- welded metal enclosures;
- glass-to-metal electrical feedthroughs;
- ceramic-to-metal seals;
- sealed reed-switch capsules;
- controlled internal atmospheres;
- inert-gas-filled or evacuated switching chambers.
Glass-to-metal technology, for example, is widely used in components intended to separate a controlled internal atmosphere or vacuum from demanding external environments.
The objective is straightforward: protect the contacts, armature and other sensitive elements from environmental influences that could alter their electrical or mechanical properties.
How does a hermetically sealed relay work?
At its core, a hermetic electromechanical relay operates according to the same basic principle as another electromagnetic relay.
An electrical control signal energizes a coil. The resulting magnetic field moves an armature or activates a magnetic switching element, causing one or more contacts to change state.
Depending on the configuration, the relay may be SPST, SPDT, DPDT, 4PDT or another contact arrangement.
The relay therefore allows one electrical circuit to control another while maintaining galvanic isolation between the coil circuit and the switched circuit.
The hermetic enclosure does not fundamentally change this switching principle. Instead, it creates a controlled environment around the switching mechanism.
That distinction is important.
Hermetic sealing is primarily an environmental and reliability feature rather than a separate relay operating technology.
Why protect the contacts from the atmosphere?
Electrical contacts are sensitive interfaces. Their performance depends on surface condition, contact pressure, material, switching load and environmental exposure.
Moisture or contamination can contribute to:
- oxidation;
- corrosion;
- unstable contact resistance;
- insulation degradation;
- leakage current;
- surface tracking;
- reduced switching reliability.
In very low-level signal applications, even small changes in contact resistance can affect measurement accuracy or signal integrity.
The same principle becomes important at higher voltages, although other factors such as contact spacing, arc management, dielectric withstand and insulation geometry become increasingly significant.
Maintaining a stable internal environment helps designers control these variables more effectively.
Hermetic relay versus ordinary sealed relay
One of the most common specification mistakes is assuming that every “sealed relay” is hermetic.
A plastic relay described as sealed, washable or flux-resistant may be designed primarily to prevent contaminants from entering during PCB manufacturing or normal service. Its sealing method and leakage performance may differ substantially from those of a true hermetic component.
Some sealed relays may even need to be vented after soldering depending on the application and manufacturer recommendations. TE Connectivity notes that contaminants generated by switching arcs can accumulate inside certain sealed relay constructions, which is one reason venting strategy can matter.
Hermetic relays generally take a different design approach, commonly using welded metal packages or dedicated hermetic switching capsules.
Engineers should therefore examine the actual construction and test requirements instead of relying on the word “sealed” alone.
Where are hermetically sealed relays used?
The technology becomes particularly valuable when environmental exposure and reliability requirements overlap.
Typical applications include aerospace power distribution, satellite electronics, military systems, high-reliability communications equipment, test and measurement platforms, industrial controls, marine equipment and selected medical or scientific instruments.
Military specifications provide a useful illustration. MIL-PRF-39016 covers established-reliability electromagnetic hermetically sealed relays for electronic and communication equipment, including devices intended for switching from low-level signals through higher-current loads.
Commercial high-performance devices can also be designed to tolerate substantial temperature, vibration and mechanical shock. TE, for instance, specifies hermetically sealed high-frequency relay families for harsh environments including aerospace and satellite applications.
What are the main advantages?
The value of a hermetic relay goes beyond simply keeping dust away from the contacts.
More stable contact performance
A controlled internal atmosphere reduces exposure to moisture and corrosive contaminants, helping preserve contact surfaces.
Better performance in harsh environments
Hermetic construction is well suited to equipment subjected to humidity, altitude changes, chemical contamination or demanding storage conditions.
High reliability for low-level switching
Signal-level contacts can be particularly sensitive to films and contamination. Hermetic construction can help maintain predictable contact behavior.
Compatibility with aerospace and defense requirements
Many high-reliability relay families are designed around military performance specifications covering shock, vibration, temperature and electrical endurance.
Long-term environmental isolation
For long-life equipment, controlling the atmosphere surrounding the switching mechanism can reduce one source of performance variability.
Does hermetic sealing make a relay indestructible?
No.
A hermetically sealed relay still has electrical and mechanical limits.
Contacts can wear or weld if switching current, voltage or inrush exceeds their rating. The coil can overheat if operated outside specification. Mechanical shock may exceed the qualified level. Insulation can break down if voltage exceeds dielectric capabilities.
Even the hermetic seal itself must be considered as part of the reliability model.
This means designers must still evaluate switching voltage, current, load type, coil voltage, contact arrangement, temperature range, shock, vibration, insulation resistance and expected operating life.
Hermetic sealing solves an environmental problem. It does not eliminate the need for correct electrical engineering.
How should engineers specify one?
A practical selection process starts with the electrical load before moving to environmental requirements.
Determine the actual switching voltage and current, including startup or inrush conditions. Identify whether the load is resistive, inductive, capacitive, lamp-based or low-level. Then examine coil supply, switching frequency and required contact arrangement.
Next, define environmental requirements such as temperature, vibration, shock, altitude, humidity and contamination.
Finally, determine whether compliance with a specific military, aerospace or internal qualification standard is required.
This approach avoids over-specifying an expensive component simply because the word “hermetic” sounds more robust.
Conclusion: what is a hermetically sealed relay in practical terms?
A hermetically sealed relay is best understood as a relay whose critical switching mechanism operates inside a controlled, tightly sealed environment that minimizes exposure to external gases, moisture and contaminants.
Its value becomes clear in systems where stable contact performance, environmental resistance and long service life matter more than minimum component cost.
For engineers designing aerospace, defense, instrumentation or industrial systems, the right question is therefore not merely whether a relay is sealed. The real question is whether its sealing technology, electrical ratings and environmental qualification match the mission profile.
Need to select a relay for a demanding application? Start by defining your switching load, environmental conditions and required qualification level before comparing hermetically sealed relay specifications.