ELECTRICAL SWITCHING CONTACT WEAR & EROSION
Electrical Contacts experience severe erosion from the action of the arc during switching
Electric Arcing occurs during most switching operations and is effectively a high current plasma. Effects of individual switching events may be subtle but cumulative effects will still determine switching device reliability and lifetime
Contact wear is fundamentally linked to lifetime performance of the switching device, and therefore linked to system reliability – this provides a prime motivation for understanding the individual and cumulative effects of switching operations on the contact surfaces.
Application areas for devices – Automotive, LV Power systems.
XYris 4000 Surface Profiling system is the ideal solution to measure arc erosion on these surfaces.
Most samples are of the order of a few mm’s and can easily be mounted and measured.
Detection and quantification of erosion from nm’s to mm’s scales with the same device
Both type of confocal sensor can be used – CL and WL.
In this case study we have used the WL system.
When switching electrical power the requirements for safety, isolation and very low “in-line” power loss specify performance that can often only be met by an electro-mechanical (EM) switching device. EM devices provide very low “on” resistance, while offering the capability to physically isolate the circuit – this cannot be achieved by the solid-state (SS) alternative.
In applications, such as Automotive, increased emphasis on the use of electrical power has led to a drastic increase in the number of switching devices employed. Physical isolation of the circuit under typical conditions results in the generation of an arc (spark) or a high current plasma, which acts to dissipate the stored energy in the circuit. The consequence of the arcing is an interaction between arc and electrical contact surface resulting in damage that eventually degrades performance and limits the lifespan of the EM switching device. While the wear effects may be extremely subtle for an individual, or even many switching events, it is the cumulative effect of this surface wear that degrades and causes the device to eventually fail.
In the case of low voltage (LV) power systems devices (shown below) EM devices are used in all systems where there is a safety requirement for circuit and human protection. These devices are often AC, but with increasing demand for DC systems, e.g. (Automotive, PV and other energy sources).
A switching device has investigated and its switching performance evaluated through quantification of the microscopic volume contact material transferred being quantified. It is the cumulative failure of these microscopic effects that lead to eventual failure of a device.
To test the performance of relay contacts used in an LV DC device for 12 V automotive applications, Ag/Ni contacts are operated for 3000 cycles, resistive load, “make and break” at circuit conditions of 14.8 V DC and 10.2 A. The switching surfaces are then investigated using a XYris 4000CL surface measurement system. The results are shown below.
The fixed cathode contact, showing build-up of material, not visible in the top down micro-scope image, but apparent in the 3D data view, measured over 1 mm x 1 mm. To determine the mass of the material transferred form the anode (see next section), we are able to remove the curved surface datum, using established methods in BODDIES. [1-4]. The resulting surface is shown in the next image. It clearly shows the transferred material in yellow.
Anode Surface. Again the top down micro-scope image is unable to show the details of the arc induced wear region. The wear region shown below is measured using the XYris 4000 CL over a region of 1 mm x 1mm. The data shows curvature on the surface which is removed in BODDIES, shown below. This highlights the complex nature of the event, where the anode shows a small region where the material has been removed, but a wider region over which there is an apparent increased roughening of the surface. The lower image shows a cross section of the data, highlighting this.
The removal of the datum curved surfaces allows BODDIES to calculate the volume of the material relative to the datum. With information on the density of the material we are then able to determine the mass transfer.
1. McBride J.W, Cross K.J, Sharkh S.M, “The Evaluation of Arc Erosion on Electrical Contacts using 3D Surface Profiles”, IEEE Transactions on Components Packaging and Manufacturing Technology, Vol. 19, No 1, March 1996, pp 87-97.
2. McBride J.W, “The Volumetric Erosion of Electrical Contacts”, IEEE Transactions on Components and Packaging Technology, Vol 23, No 2, June 2000, pp 211-221.
3. Zhang, D., McBride, J.W. and Hill, M. (2004) A feature extraction method for the assessment of the form parameters of surfaces with localised erosion. Wear, 256, (3-4), 243-251.
4. Zhang, D., Hill, M. and McBride, J.W. (2006) Evaluation of the volumetric erosion of spherical electrical contacts using the defect removal method. IEEE Transactions on Components and Packaging Technologies, 29, (4), 711-717