PROBLEM #1
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
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 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 contacts are eroding
too rapidly, check all of the factors listed under contact
welding, since these 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 contacts are chattering
when closing, erosion can be significantly increased.
• 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 contacts should be opened rapidly to minimize arc duration.
For both AC and DC circuits, contacts should close rapidly
with minimum bounce.
• Any device that is added to reduce arc duration or
intensity between 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 contact to the other
Material transfer is generally associated with DC circuits
due to the polarity of the circuit being interrupted. If
the 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 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 contact materials
that resist material transfer is certainly important for
DC applications. High melting and boiling points, good resistance
to welding, high electrical and thermal conductivity and
high hardness are properties that help reduce transfer. |
Problem #
4
High contact resistance
Generally caused by either
foreign contamination or corrosion of the contact material,
high contact resistance can lead to over-heating, contact
welding, high erosion rates, or no contact at all.
• Corrosion can be controlled by using a contact material
that is not readily attacked by the environment in which
the contacts will operate.
• 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 contact
system to provide some wiping action can be vital in keeping
contact surfaces clean during operation. |
