Answer MET Question 43

Question : With reference to the condition monitoring of electrical machinery: A. State TWO important parameters that may be recorded;

B. Explain how the parameters are measured and what defects may be revealed.

Answer: Maintenance can be classified as:
1. Break down maintenance
2. Planned maintenance
3. Condition monitoring
  Breakdown maintenance (corrective maintenance) is when equipment is left untouched until a breakdown occurs. At this time the equipment is repaired or replaced and any other specified maintenance procedure carried out.
Planned maintenance (preventive maintenance) is when equipment is regularly inspected and maintained according to a fixed timetable and set of procedures specifying the actual work to be done to prevent equipment failure.
Condition monitoring (another form of preventive maintenance) is when equipment is regularly monitored and tested. When monitoring indicates that a breakdown is imminent, the equipment is repaired or replaced and any other specified maintenance procedures are carried out.

 A. Regular insulation testing and vibration testing are two forms of condition monitoring. Condition monitoring is carried out at fixed regular intervals. The aim is to forestall breakdown by predicting probable failure from the trend shown by the monitoring results. The advantage of this type of maintenance is that equipment is not subjected to unnecessary maintenance. Equipment is regularly condition monitored according to a monitoring schedule. Measurements are taken of insulation resistance, temperature and vibration (of motors). Contacts and other parts subject to deterioration are inspected.
All findings are recorded in an historical record file. No maintenance is carried out until the trend of test results indicate that it has become necessary.
The equipment is then either replaced, repaired or subjected to a major overhaul as specified on a job card. A maintenance records system is required.

B. The recorded measurements of insulation resistance may show a falling trend indicating a progressive degradation of insulation. The equipment should be inspected and repaired before the insulation resistance falls to a dangerously low value. Hot-spot temperatures emitted from live electrical equipment can be monitored from a safe distance using an infra-red detector or camera.

  The recorded measurements of the vibration of a motor may follow a rising trend indicating progressive bearing deterioration. Bearings should be replaced before failure occurs. Immediate repair or maintenance is probably not necessary but should be put in hand at the earliest convenient moment.