Testing at 20KV: Selecting High Voltage Reed Relays for Medical and Industrial Applications

Testing at 20KV: Selecting High Voltage Reed Relays for Medical and Industrial Applications

When your test system needs to switch or measure at 10KV, 15KV, or even 20KV, the switching component becomes the system’s weakest link. The relay isn’t just passing a signal — it’s standing between lethal voltages and the test fixture, the DUT, and potentially the operator. At these voltage levels, the physics of insulation, arcing, and breakdown govern everything, and not all relay technologies can handle the challenge.

High voltage reed relays have earned a dominant position in extreme-voltage switching applications for a reason: the hermetically sealed glass envelope that encloses the reed contacts provides inherent insulation advantages that electromechanical relays and solid-state alternatives simply cannot match. This post examines the engineering principles behind high voltage reed relay selection, explores MiRelay’s HVR, HRX, and HVFR series, and provides practical guidance for medical and industrial applications where 20KV relay ratings aren’t a luxury — they’re a requirement.

Understanding High Voltage Relay Specifications

Before selecting a relay, engineers need to understand what the datasheet parameters actually mean in the context of high-voltage switching. The terminology is precise, and misinterpreting it leads to either over-engineering (wasted cost and space) or under-engineering (arc-through, safety failures, or catastrophic DUT damage).

Breakdown Voltage vs. Working Voltage

Breakdown voltage (also called dielectric strength or standoff voltage) is the maximum voltage the relay can withstand between open contacts or between contact and coil before insulation fails and an arc forms. This is a peak or instantaneous rating — it describes the absolute limit, not the safe operating envelope.

Working voltage (rated voltage) is the maximum continuous voltage the relay can safely switch or carry during normal operation. Working voltage is always lower than breakdown voltage because it accounts for long-term insulation degradation, temperature effects, and the need for a safety margin.

A relay rated for 20KV breakdown typically has a working voltage in the 10–15KV range. Engineers who use breakdown voltage as the working specification are building a time bomb.

Insulation Resistance

Insulation resistance measures how effectively the relay’s body (between open contacts, or between contacts and coil) resists current leakage at high voltage. Measured in gigaohms (GΩ) or teraohms (TΩ), insulation resistance is typically specified at a specific test voltage (e.g., “10 GΩ at 1000VDC”).

At high voltages, even GΩ-level insulation resistance produces measurable leakage current. For precision measurement applications — such as hipot testing or insulation resistance measurement — the relay’s own insulation resistance must be orders of magnitude higher than the expected DUT insulation to avoid measurement error. A relay with 1 GΩ insulation resistance at 20KV produces 20 µA of leakage — significant when your pass/fail criterion is 1 µA.

Air Gap and Creepage Distance

The air gap is the physical distance between conductive surfaces through air. The creepage distance is the shortest path along an insulating surface between conductors. Both must be adequate for the rated voltage — air breakdown occurs at approximately 3 KV/mm at sea level, but surface contamination, humidity, and altitude reduce this significantly.

Reed relays have an inherent advantage here: the sealed glass envelope provides a controlled, contamination-free environment for the air gap. The reed spacing is fixed by the glass envelope geometry and cannot change due to mechanical wear, vibration, or contamination. Compare this to an electromechanical relay where contact arm position, spring tension, and housing contamination all affect the effective air gap.

Why Reed Relays Win at Extreme Voltages

At voltages above 5KV, the choice narrows. Solid-state relays (SSRs) are limited by semiconductor junction breakdown and thermal runaway at high voltages. Electromechanical relays (EMRs) can achieve high voltage ratings but suffer from contact wear, bounce, and mechanical wear that degrades insulation performance over time. Reed relays offer a unique combination of advantages that make them the preferred technology for extreme-voltage switching.

Hermetic Sealing

The sealed glass envelope eliminates atmospheric contamination of the contact gap. No humidity, no dust, no oxidation — the insulation environment inside the envelope remains constant for the relay’s entire lifecycle. This is critical because atmospheric breakdown voltage drops dramatically with humidity and altitude, and open-relay architectures are vulnerable to both.

Fixed Contact Geometry

The reed contacts are rigidly fixed at their base and cantilever toward each other. The air gap doesn’t change with use — no mechanical wear alters the standoff distance. An EMR, by contrast, depends on spring tension and pivot wear to maintain contact gap, and those degrade with cycling.

No External Arc Paths

High-voltage arcs follow the path of least resistance. In a relay with exposed terminals, that path can include the PCB surface, housing contaminants, or adjacent traces. The sealed reed envelope forces any breakdown to occur within the controlled environment of the glass tube, where the gap is optimized and the medium is pure.

High Voltage Relay Technologies: A Comparison

Parameter	High Voltage Reed Relay	HV Electromechanical Relay	Solid-State Relay
Max Breakdown Voltage	Up to 20KV+	10–15KV typical	5–10KV typical
Contact Bounce	Low (0–50 µs)	High (1–5 ms)	None
Insulation Stability	Constant (hermetic seal)	Degrades with wear	Depends on junction temp
Lifecycle	10⁸+ operations	10⁵–10⁷ operations	Unlimited (no wear)
Leakage Current	Very low (pA–nA)	Low–moderate	Higher at high temp
Size	Very compact	Large	Medium (needs heatsink)
Cost	Moderate	Low–moderate	High at HV ratings
Suited For	Test, measurement, precision	Power switching	High-cycle AC loads

MiRelay’s High Voltage Reed Relay Series

MiRelay manufactures three high-voltage reed relay series, each engineered for specific voltage and application profiles.

HVR Series

The HVR series is MiRelay’s flagship high-voltage reed relay, designed for applications requiring the highest breakdown voltages — up to 20KV. The HVR features extended glass envelope length, optimized reed geometry for maximum air gap, and isolation resistance exceeding 10 GΩ. Common applications include hipot test equipment, capacitor discharge testing, and high-voltage power supply validation.

The HVR’s extended creepage distance along the glass surface — achieved through the elongated envelope design — makes it particularly robust in humid environments where surface breakdown is the dominant failure mode rather than air breakdown.

HRX Series

The HRX series targets medium-to-high voltage applications in the 5KV–15KV range with emphasis on compact form factor and fast switching. The HRX is frequently used in medical device test fixtures where board space is limited but voltage requirements exceed what standard reed relays can provide. Its lower coil power consumption makes it suitable for battery-powered or portable test systems.

HVFR Series

The HVFR series combines high voltage capability with high-frequency signal handling. Designed for RF test systems and communication equipment validation, the HVFR minimizes parasitic capacitance while maintaining isolation ratings above 10KV. When your test system needs to switch between a 15KV hipot source and a 50Ω RF measurement path through the same relay matrix, the HVFR is the right choice.

Medical Applications: Where 20KV Relays Are Essential

Medical device testing imposes some of the most demanding voltage requirements in any industry. The stakes are uniquely high — the device being tested will ultimately connect to a patient, and the test system must verify that the device is safe under fault conditions.

Defibrillator Testing

Defibrillators deliver energy pulses at 3KV–5KV to patient electrodes. The test system must verify waveform shape, energy delivery, and — critically — patient leakage current under fault conditions. The test fixture’s relay matrix must withstand the defibrillator’s output voltage while maintaining insulation resistance sufficient to measure leakage currents in the microamp range. This is exactly the application profile where a 20KV reed relay excels: high standoff voltage, low leakage, and hermetic reliability.

MRI System Verification

MRI gradient coil drivers and RF amplifiers operate at high voltages with fast switching transients. Test systems for MRI subsystems need relays that can switch high-voltage signals with minimal bounce (to avoid corrupting fast transient measurements) and high insulation (to avoid coupling noise into sensitive receiver chains). Mercury wetted or low-bounce high voltage reed relays are standard choices in MRI test fixtures.

IEC 60601 Compliance Testing

The IEC 60601 family of standards governs the safety of medical electrical equipment. IEC 60601-1 specifies dielectric strength test requirements that depend on the equipment’s applied part classification (Type BF, Type CF, etc.). For Type CF applied parts (cardiac floating), the dielectric strength test requires 4KV–5KV AC or 5.7KV–7KV DC between applied part and mains, with leakage current limits as low as 10 µA under single-fault conditions.

The test system’s relay matrix must introduce negligible leakage relative to these limits. High voltage reed relays with insulation resistance above 10 GΩ contribute less than 0.5 µA of leakage at 5KV — well within the measurement budget. Lower-quality switching introduces systematic errors that can cause compliant devices to fail certification testing.

Industrial Applications: Hipot, Power Supply, and Beyond

Industrial high-voltage testing demands are growing as power electronics move to higher bus voltages (800V–1000V in EV systems) and insulation requirements tighten.

Hipot (Dielectric Withstand) Testing

Hipot testing verifies that a product’s insulation can withstand overvoltage conditions without breakdown. The test applies a voltage significantly above rated voltage (typically 2× rated + 1000V) and measures leakage current. Hipot test equipment relies on high-voltage relays to connect the HV source to different test points on the DUT. The relay must hold off the test voltage with minimal leakage, switch cleanly without bounce (to avoid false current spikes), and cycle reliably through thousands of test sequences per day.

Power Supply Validation

Switch-mode power supplies for industrial and telecom applications operate at bus voltages up to 1500V with switching transients that can spike well above rated voltage. Test fixtures that validate these supplies need relay matrices rated for the transient voltage peaks, not just the steady-state bus voltage. A properly selected high voltage reed relay accounts for these transients in its voltage rating with appropriate derating.

Cable and Harness Testing

Aerospace and defense cable harnesses require high-voltage insulation verification — often at 10KV–15KV DC. Automated harness testers scan through dozens or hundreds of conductors, applying HV and measuring insulation resistance between each pair. The relay matrix in these testers must be both high-voltage rated and highly reliable, as a single relay failure stops the entire test sequence.

Derating Strategies for High Voltage Relays

No engineer should operate a high-voltage relay at its absolute maximum rating. Derating extends reliability, reduces arc probability, and provides margin for environmental variation. Here are practical derating guidelines:

Voltage Derating

Operate at no more than 60–70% of the relay’s rated breakdown voltage for DC applications, and 50% for AC applications (peak voltage, not RMS). For a relay rated at 20KV breakdown, this means a maximum working voltage of 12–14KV DC. If your application requires 15KV, select a relay rated for at least 21KV — don’t try to push a 15KV relay to its limit.

Altitude Derating

Air breakdown voltage decreases with altitude due to lower atmospheric density. At 5000 feet (1500m), breakdown is roughly 80% of sea-level value. At 10000 feet (3000m), it drops to 65%. If your test equipment will operate at altitude (aircraft testing, high-altitude facilities), apply altitude correction to your voltage derating calculation.

Humidity Derating

Surface creepage breakdown is highly sensitive to humidity. In environments above 80% relative humidity, reduce the effective creepage voltage rating by 20–30%. For outdoor test equipment or tropical installations, consider conformal coating on the PCB around the relay to improve surface insulation resistance.

Temperature Derating

Insulation resistance decreases with temperature. At the relay’s maximum rated ambient temperature, insulation resistance may drop to 50% of its room-temperature value. Factor this into your leakage current budget — especially for medical applications where leakage limits are stringent.

Selecting the Right High Voltage Reed Relay

Use this decision framework when specifying a high-voltage reed relay:

1. Determine your actual working voltage — peak voltage including transients, not just nominal
2. Calculate your insulation resistance requirement — divide working voltage by your maximum acceptable leakage current
3. Apply derating — voltage (60–70%), altitude, humidity, temperature
4. Check the resulting relay rating — the derated relay rating must exceed your derated application requirement
5. Verify PCB layout supports the voltage — relay rating means nothing if the PCB trace spacing can’t hold the voltage

For guidance on applying these principles to your specific application, see our detailed article on how to choose the right high voltage reed relay for your application.

Getting Started

High-voltage relay selection is too important to get wrong. Whether you’re designing a hipot test system, a medical device compliance fixture, or an industrial power supply tester, the consequences of an underspecified relay include equipment damage, safety violations, and invalid test results.

MiRelay’s high-voltage reed relay portfolio covers applications from 5KV to 20KV+ with options for low leakage, high frequency, and compact form factor. Our applications engineering team can help you select the right relay, design appropriate drive circuits, and verify that your PCB layout meets the voltage clearance requirements for your application.

Contact our engineering team to discuss your high-voltage switching requirements, request samples, or get application-specific design support.