Important Precautions and Safety Considerations Discussion
When dealing with stray electrical shaft current phenomena, it is important to realize that there is a very real potential that hazardous conditions can develop, such as overheating, sparking, and electrical shock potential. Stray electrical shaft currents have caused catastrophic failures of critical, expensive equipment. Safety to personnel can be an important issue. This is not a situation to approach casually, and the approach will vary from industry to industry.
We at Sohre Turbomachinery are not Electrical Engineers, and although familiar with many industries, do not wish to be considered experts in these matters. Our expertise is in designing and producing the world’s finest shaft riding brushes for high speed rotating equipment. It is important that those investigating shaft current phenomena and applying our brushes carefully and thoroughly determine all possibly necessary precautions and follow these with great care. It would be our pleasure to suggest companies and individuals experienced in the matter of stray shaft currents who can be used as consultants.
It is the primary purpose of our brushes to allow measurement and monitoring of stray electrical shaft currents, not to ground them. However, even after corrective action has been taken (such as demagnetization), many machines will still develop some amount of current. Although this may not be sufficient for "self excitation" or other destructive phenomena, it may still be enough to cause deterioration and unscheduled shutdowns. The resulting amount of current, although modest, can usually be neutralized by proper grounding and monitoring with our brushes. This will minimize damage of bearings, seals, and other critical components.
It is not the purpose of our brushes to ground strong electrical currents, since this is neither possible nor desirable, and could be detrimental and dangerous. Every attempt has been made in the design of our brushes to compensate for detrimental operating conditions. However the responsibility for the application of a brush remains with the user, who must determine the suitability of the design and material for the specific application, operating conditions, and environment. If hazardous conditions such as overheating, sparking, and electrical shock potential develop, immediate corrective action should be taken, and the brush removed or taken out of service.
If high levels of magnetism or significant voltages and current exist in equipment not designed for that purpose, the unit should be expertly analyzed and corrective action taken before "self excitation" or other potentially catastrophic phenomena can develop. To our knowledge, there are no standards or hard-and-fast rules on what constitutes "significant" currents. However in general, stray electrical shaft currents should always be either non-existent, or minor in nature if the equipment is well designed and in good order. The services of an expert in these matters is suggested.
Stray electrical shaft current phenomena are not well understood and can vary considerably in behavior. The shaft current situation can be different for each installation, depending upon where and how these currents are generated. Analysis of shaft current problems requires the active participation of skilled engineers with considerable experience and specialized equipment.
There are so many possible sources of stray electrical shaft currents that it is impossible to predict the strength of such currents and the paths they will take. There are no brushes which will protect a machine against all possible combinations of such currents. If the electrical source is powerful and the currents are strong, any brush will deteriorate rapidly, resulting in frosting and spark damage to the shaft surface under the brush. Because of eddy currents, etc. heating of the brush and shaft may occur. This is one reason cooling oil is suggested for some applications. If sparking due to excessive current transfer has progressed beyond a certain point, the brush will lose electrical contact with the shaft and become ineffective. This can be detected by measuring current transfer through a brush. If shaft currents are strong enough to cause loss of contact within a relatively short time, immediate corrective action is necessary.
In summary, the function of a shaft brush when used for stray electrical shaft currents, is to permit measurement of shaft voltage and current, thereby allowing assessment of the shaft current situation inside the machine. In the case of modest currents, within the rating of each brush, damage to bearings, seals, couplings, gears, etc. can be prevented in most cases. However, it is possible that more current will be generated than the brush can handle. In this case, the rate of bristle depletion will be excessive, sparking and heating may take place, and the brush will cease to function properly. Also, unless electrical current is limited to reasonable values, "self excitation", a potentially catastrophically destructive phenomenon may occur.