Introduction

Relay contacts can become welded—or stuck together—in various ways, usually due to excessive heat or electrical arcing. Here’s an overview of some common causes:

1. Overcurrent or Overload

   Cause: When a relay’s contacts are exposed to a current above their rated capacity, they may heat up and soften, which can lead to the contacts fusing or welding together.
   Example: Suppose a relay rated for 10 amps is exposed to a 15-amp current for a prolonged period. The excessive current causes the contacts to overheat, deform, and eventually weld.

2. Electrical Arcing

   Cause: Arcing happens when the relay opens or closes, especially if there’s an inductive load (like a motor or solenoid). When contacts separate, the circuit’s energy can create an arc. This arc can heat the contacts to the point of melting or welding.
   Example: In relays switching high-power AC or DC inductive loads, the inductance opposes the sudden change in current, leading to arcing when the relay contacts separate.

3. Voltage Spikes and Transients

   Cause: High-voltage transients or surges in the circuit can produce intense arcs when the relay switches, particularly in DC circuits. These arcs can weld the contacts together even if they are designed for the regular voltage.
   Example: When relays are used in circuits with motors or other inductive loads, the sudden current changes can lead to high-voltage spikes, which may cause arcing and eventual welding of the contacts.

4. Contamination or Foreign Materials

   Cause: Dust, oil, or other contaminants can get onto the contact surfaces, which can result in increased resistance, leading to localized heating and welding.
   Example: In a dusty or oily environment, contaminants may coat the relay contacts, causing hot spots as current flows through the added resistance.

Preventing Relay Contact Welding

To prevent contact welding, it’s essential to choose relays rated for the load’s current and voltage. For inductive loads, use relays with arc suppression or consider additional snubber circuits. Also, ensure the relay is regularly inspected and kept free of contaminants.