Low voltage Capacitance-to-Ground (CTG) limits are not found in industry standards. A single measurement of CTG has limited diagnostic value, but if trended correctly it is a very useful tool that could help with the determination of the actual root cause of a failure mode and assist in removing that cause. When trending, ask yourself what direction is it trending?
A popular formula for capacitance is:
- C=0.2249(KA/d)
Where:
- C = capacitance (pF)
- A = area of plate (in2)
- d = distance between plates (in)
- K = dielectric constant of insulating material
- .2249 = a constant when converting from Metric to British units
Some common dielectric constants:
- Vacuum = 1.0000
- Air ≈ 1.0006
- Paraffin paper ≈ 3.5
- Rubber ≈ 2.0-37
- Water ≈ 78
- Petroleum ≈ 2
Based off the formula, when the CTG values go down, this could be an indication of thermal degradation. As the insulation deteriorates, the layers start to delaminate which reduces the dielectric constant. Being directly proportional if the dielectric constant goes down then the CTG goes down as well.
On the other hand, when the CTG values go up, this is an indication of insulation contamination or water saturation. Similar to the above example, the contamination of the dielectric can increase the dielectric constant resulting in a higher CTG. Additionally, the presence of general leakage paths from surface contamination can effectively bypass the dielectric resulting in an increase in current flow during testing and a higher CTG. Due to the sensitivity of the CTG it will often be an early indicator of dielectric changes or surface contamination.
Trending the Resistance-to-Ground along with the Capacitance-to-Ground and the Polarization Index with the Insulation Resistance Profile will give an extremely accurate assessment of the motor’s insulation system.
Thank you to Chris Harvey of Ingredion for the CTG tip. Your PdMA hat or mug is on the way.
