How Switches and Relays Fail

Why this matters

Switches and relays are the cheapest parts in most machines and the ones that strand a customer the most. A relay is just a small electrically operated switch: a coil pulls a set of contacts together so a low-power signal can turn on a high-power load. When one fails it rarely dies clean. It chatters, sticks, or passes voltage that drops to nothing under load. Knowing the failure modes turns a frustrating intermittent into a five-minute call.

The two halves that fail differently

Every relay has a coil (the control side) and contacts (the power side). They fail for different reasons.

• The coil fails open from a burned winding, or it fails to pull in because it is not getting full control voltage. A coil that hums but never seats is usually starved for voltage or fighting a weak power supply.
• The contacts fail from arcing. Every time contacts open under load, a tiny arc burns metal off the surface. Over thousands of cycles the surface pits, oxidizes, and grows a resistive film. Eventually they weld shut or stop making clean contact.

A plain mechanical switch is the contact half with no coil: same arcing wear, same pitting, same eventual failure.

The failure modes you will actually see

• Stuck closed (welded). The load runs and will not shut off, or a fuse blows. High inrush current welded the contacts together. Common on motor, compressor, and heater circuits where startup current is several times the running draw.
• Stuck open (burned). Nothing turns on. The contacts pitted until they no longer touch, or the coil burned open.
• High-resistance contact. The sneakiest. The relay clicks, voltage reads fine with the meter, but under real load the voltage collapses and the device starves or runs hot. A resistive film or pitted surface drops voltage only when current flows.
• Chatter. Rapid clicking. The coil is not getting steady voltage, or a control signal is bouncing. Chatter destroys contacts fast because every bounce is another arc.
• Intermittent on vibration or heat. Works cold, drops out warm, or quits when something nearby vibrates. Internal connections are cracked or the contacts barely meet.

How to test one

Power off first when you can. The fastest checks:

1. Coil resistance. With power off, measure across the coil terminals. A reading of zero (dead short) or infinite (open) means a bad coil. A good coil reads a specific low-to-moderate resistance.
2. Contact continuity, energized and de-energized. A normally-open contact should read open with no power and near-zero with the coil energized. A normally-closed contact reads the opposite.
3. Voltage drop under load. This catches the high-resistance failure the other tests miss. With the circuit running, measure voltage across the closed contacts. A good contact drops almost nothing. More than a fraction of a volt across closed contacts means burned contacts even if continuity looked fine.
4. The swap test. Many control panels use several identical relays. Swap a suspect with a known-good neighbor. If the fault follows the relay, you found it.

Warning signs before it strands the customer

• A relay or contactor case that smells hot or shows discoloration around the terminals.
• Audible buzzing or chatter when the load should be running steady.
• Visible pitting or black film on accessible contacts.
• A device that works on a cold start but quits after running, then works again once cooled.
• Lights or motors that flicker or surge in time with another load switching on.

What to do about it

Replace, do not file. Filing contacts smooth removes the hardened surface and buys days, not months. Match the new relay or switch to the original by coil voltage, contact rating (amps), and contact configuration. Undersizing the contact rating is the most common repeat failure: a relay rated below the load's inrush current will weld within weeks. If a relay keeps failing, the relay is the symptom, not the cause. Look upstream for a stuck load, a short, or a coil voltage problem driving the early death.

References

• NFPA 70 (National Electrical Code), conductor and device sizing for continuous and inrush loads
• Manufacturer relay and contactor datasheets, contact rating and coil voltage specifications
• Trade-standard electrical troubleshooting practice (voltage-drop testing under load)
• See related: the baseline reading you should always take; how capacitors fail (generic)