Electrical Reliability ensuring circuits will operate safely. This lowers Personnel, Asset and Insurance Risks.
The guidelines for periodic Electrical Maintenance for Industrial Sector is Three years with a visual annually.
Earth Loop Impedance is a test to ensure basic safety in an electrical circuit, the supply impedance is added to the R1 (Supply Resistance to the Motor) & R2 (Earth Resistance). The total must be below limits specified by Current rating. Earth Loop Impedance Results: The regulatory requirement is to take the highest measurement of the three phases and compare this absolute reading with the limits.
In the event of a Short to earth the current must pass unimpeded to ground, if that resistance is elevated the risk is that fault current follows a difference unplanned path. This is extremely dangerous.
The results of these tests must be recorded and repeated each three years.
This means every Motor circuit must be tested!!!!!
Two more considerations are to mentioned:
1. Circuit Breakers are rated to a maximum fault current, but on the other hand a very low fault current due to high Earth Loop Impedance both Phase to ground & Phase to Neutral means the circuit breaker may operate in the Thermal region of the trip curve.
A slower tripping time means circuits heat up under fault conditions and this is a major factor in starting fires.
2. Often overlooked is the Insurance factor, conducting testing and being able to show an insurance assessor your Preventative Maintenance program can mean a risk re rating lower. That has a financial incentive for Preventative Maintenance.
3Phi Reliability recommend in addition looking at the percentage resistance imbalance. A reading >3% indicates a problem eg Contactor contact, Circuit Breaker contact, Isolator corrosion, or Termination/crimp. A imbalance >5% is a serious problem that requires fault finding.
A Motor Circuit test is recommended at the Motor for Winding condition.
This is an efficient method of finding high resistance defects that are hidden from view of Thermography.
While the test schedule is three years, for a facility that has had no testing program it is recommended to conduct a program each year until the defect rate is below 0.8% of tested assets. At this level of defects, your facility is regarded as Best Practice.
The regulatory stick is a motivating factor but the biggest reward is the failure rates and the affect on Downtime. Just one failure eliminated can return financially more than the whole implementation costs.
So the return on investment is often Months.
Blog Free Advice on Electric Motor Reliability and Energy Savings
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