Troubleshooting Common Electrical Contact Problems
Problems with electrical contact welding, contact erosion, material transfer or electrical contact resistance can often be attributed to some common factors. PEP Brainin engineers recommend reviewing these variables as the first step in identifying the potential root cause of an electrical contact failure.
For more information about Brainin electrical contact and assemblies, go to our electrical contacts page.
Problem #1: Electrical contact sticking (welding)
This is one of the most common failure modes for electrical contacts. The following is a checklist of some possible contributing factors.
- Contamination or corrosion of the contact surface can increase contact resistance, thus raising the temperature at the points of contact and increasing the tendency to weld.
- A loose rivet joint, a poor weld or brazed joint can also contribute to contact heating which may lead to welding.
- Poor contact alignment reduces the effective contact area and can contribute to contact welding.
- A mechanical problem in the device which reduces contact force or reduces the opening force of the contacts may cause a contact welding problem.
- Currents that are higher than normal can cause contact welding. For new applications, in addition to the above.
- Make sure the contacts are a suitable size and material for the application.
- Support members for electrical contacts should be as highly conductive as possible to help keep the contacts cool. This factor should not be overlooked for it can make the difference between success and failure for a given contact set.
Problem #2: High contact erosion
If electrical contacts are eroding too rapidly, review all of the factors listed in the contact welding section (above), because the same factors can cause increased erosion rates even if the conditions are not severe enough to cause welding. In addition:
- Check for contact bounce. If electrical contacts are chattering when closing, erosion can be significantly increased as a result.
- For AC circuits, contacts should not open too rapidly. The contacts should be opened fast enough to minimize arc re-ignition, but slow enough to minimize the arc length (arc energy is proportional to arc length). The arc will tend to extinguish at the first current zero after the contacts have opened beyond a critical distance. For DC circuits, the electrical contacts should be opened rapidly to minimize arc duration. For both AC circuits and DC circuits, contacts should close rapidly with minimum bounce.
- Any device that is added to reduce arc duration or intensity between electrical contacts will reduce erosion.
- Mechanical erosion can be reduced by lowering the contact force within a reasonable limit. However, if the contact force is too low, the electrical erosion will far exceed any reduction in mechanical wear.
Problem #3: Material transfer from one electrical contact to the other
Material transfer is generally associated with DC circuits due to the polarity of the circuit being interrupted.
- If the electrical contacts operate under non-arcing conditions, a phenomenon known as “metal bridge transfer” causes material to migrate from the positive to the negative contact.
- If the electrical contacts are operating under arcing conditions, in addition to metal transfer, another phenomenon occurs that causes material transfer from the negative to the positive contact. This is a result of arc emission and is referred to as “arc transfer.” As a result, the arc duration and intensity will determine whether the net transfer will be to the negative contact or the positive contact.
- Selection of electrical contact materials that resist material transfer is important for DC applications. High melting and boiling points, good resistance to contact welding, high electrical and thermal conductivity and high hardness are properties that help reduce material transfer.
Problem #4: High electrical contact resistance
This problem is generally caused by either foreign contamination or corrosion of the electrical contact material. High contact resistance can lead to overheating, contact welding, high erosion rates, or no contact at all.
- Contract corrosion can be controlled by using a contact material that is not readily attacked by the environment in which the contacts will operate.
- Electrical contacts can be operated in a sealed controlled atmosphere to eliminate or reduce corrosion and foreign contamination.
- Contacts should be as clean as possible when the device is fully assembled.
- Designing the electrical contact system to provide some wiping action can be vital in keeping contact surfaces clean during operation.