⚡ When Is a Phase Failure Relay Not a Phase Failure Relay?
When it’s failed, of course — says Malcolm Greenhill. Phase Failure Relays occasionally fail, and the biggest enemy is heat: heat caused by incorrect installation, harmonics, or power-line interference such as voltage spikes and surges. To prevent this, it’s important to understand what you expect from a relay, how it’s designed, and how careful installation can prevent premature failure.
🔌 Understanding the Role of a Phase Failure Relay
Ask yourself: how many electronic devices in your control panel operate directly on a 400/415V AC supply? Probably only one — the Phase Failure Relay. While it’s powered by the incoming three-phase line, the relay itself internally operates at a much lower voltage. This voltage drop must be dissipated as heat, which is an unavoidable part of the design.
Manufacturers address this using capacitors to step down voltage. Higher-quality relays use capacitors with internal series connections to share the load evenly, but capacitor quality varies across manufacturers. Experience and testing often reveal which ones stand the test of time.
⚙️ Why Heat and Electrical Noise Cause Relay Failure
To accurately measure voltage across all three phases, a relay must have a low input resistance. Unfortunately, this makes it more susceptible to electrical noise. Other devices, such as sensors or logic controllers, typically have a high input resistance that helps filter out this interference. This fundamental design difference means relays are exposed to more electrical stress than many other components in your panel.
🔥 Common Heat Sources and How to Avoid Them
1️⃣ Incorrect Installation
Neat wiring layouts are always appealing, but functionality must come first. Too often, Phase Failure Relays are “crush-mounted” right next to motor contactors. A contactor’s surface temperature can exceed 60°C at normal operating voltage. Combined with the relay’s own heat generation, the internal temperature can easily exceed the tolerance limits of its components.
To avoid this, ensure there is at least a 10 mm air gap between the relay and any adjacent heat source. Anything more than 10 mm makes little additional difference.
2️⃣ Harmonics on the Supply
Harmonics increase current flow through the relay’s power supply. More current equals more heat, and this can shorten component life dramatically. In many plant installations, harmonic distortion has been identified as the leading cause of overheating and early relay failure.
3️⃣ Spikes and Surges
Voltage spikes and surges gradually degrade the foil capacitors used in the relay’s power supply. Flashovers between foil layers can occur, and even self-healing capacitors eventually lose their effectiveness. Combine that with ambient or nearby heat sources, and the capacitor electrolyte begins to evaporate — reducing lifespan further.
The first warning sign is often relay chatter — where the output relay flickers or fails to latch due to insufficient internal power. This indicates a weakening power supply inside the device.
💡 Should You Use a Transformer-Based Phase Failure Relay?
Transformer-based relays can reduce susceptibility to supply interference by providing galvanic isolation. However, they come with trade-offs — limited operating voltage ranges, additional heat issues, and up to three times the cost of a standard capacitor-based design. In most industrial applications, galvanic isolation isn’t necessary, and a well-designed relay with high-quality capacitors is the better investment.
🧩 Choosing Reliable Components for Longevity
To maximise reliability, choose a supplier that uses high-grade self-repairing capacitors rated for 105°C and 3,000 operating hours. These follow the “doubling 10°C rule”: for every 10°C reduction in temperature, lifespan doubles. For example:
- At 105°C – 3,000 hours
- At 65°C – 48,000 hours
- At 55°C – 96,000 hours
- At 45°C – 192,000 hours
- At 35°C – 384,000 hours (approx. 44 years!)
By combining careful installation practices with high-quality relays, you can achieve decades of reliable service — protecting your control panels, motors, and machinery.
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For technical guidance or help selecting the best Phase Failure Relay for your application, contact our experts or call 01424 850660. We’ll ensure your system runs efficiently and safely.