Much has been written on the subject of preventive maintenance, and surely by now PM is well established as part the raison d'être for biomedical engineering departments. In actual fact, however, PM is rarely properly conducted, and many hospitals place undue emphasis on electrical safety testing only.

The essential point is, perhaps, that the technician comes into contact with each item of equipment on a periodic basis. The second point is what the technician then actually does!

The following guidelines may serve as a useful checklist and "default" procedure:-

1)  Check and Clean it
2)  Replace and/or Repair it
3)  Tighten and Lubricate it
4)  Adjust and/or Calibrate it


Check and Clean it

Examine the complete equipment and all accessories to verify total serviceability. Check mains cable and plug. Check fuse(s) for correct rating. Clean the equipment exterior, and accessories, to preserve appearance and prevent operational problems. Where appropriate, inspect the interior of the unit and look for accumulations of dirt, dust, spilled fluids, foreign objects, excessive lubrication and signs of mechanical wear.

Replace and/or Repair it

Replace filters (liquid, gas and air); deteriorating, cracked or dry-rotted tubing; motor brushes; spent batteries; missing spare fuses; o-rings; rubber or silicone parts; and other defective components found at inspection, or at intervals recommended by the manufacturer. Carry out any "make good" small repairs found to be necessary. But if anything other than minor repairs are needed, defer the PM and open a repair work order in the usual way.

Tighten and Lubricate it

Ensure that all mechanical fixtures and fittings are safe, sound and tight. Replace any damaged or missing screws, nuts, washers etc. Lubricate mechanical components such as motors, bearings, chains, wheels, hinges, latches etc. that have friction points. Remember, however, that excessive or inappropriate lubrication can cause danger.

 
Adjust and/or Calibrate it

Carry out electrical safety tests as applicable. Perform calibration and adjustments on electrical components as recommended by the manufacturer or indicated by inspection results. Verify the integrity and proper operation of all mechanical components and hardware. Ensure that the equipment is left clean and fully fit for use. Leave controls in normal (safe) position. Fix a signed and dated sticker to reassure user staff.
 

... so, it's as easy as 1-2-3-4!


Now a word about electrical safety testing. A piece of equipment is electrically safe if it passes the less than 0.2 ohm protective earth test, does not fail an insulation test, and has an enclosure leakage current with the earth open circuit (sometimes known as "risk current") of less than 500 µA. All other tests are esoteric and beyond the scope of routine inspections. Much more important is the physical condition of the equipment, and whether it works correctly (is within calibration limits, etc.).

In a strictly practical sense, only two basic electrical safety tests are actually necessary to ensure that a unit is safe:-

1)  Grounding resistance
2)  Risk current

Grounding resistance Check that the resistance from grounding prong at the mains plug to the equipment chassis is less than 0.2 ohm. The protective earth conductor must be strong enough to blow the equipment fuse or breaker if an electrical fault occurs (this point should be emphasized)!

Risk current Measure with ground open (known as a "single-fault condition", or SFC). This is properly known as "enclosure leakage current" (or "touch current") under SFC. Current through a 1 K load between chassis and earth must be less than 500 µA (that's < 500 mV across 1 K). This is the maximum current that could flow through the patient if the ground becomes open and chassis becomes live (ie, if a double-fault condition arises), and can be called "risk current" to distinguish it from "touch current" under SFC.

Note that the "double-fault condition" mentioned above is very unlikely to occur in well-maintained machines, and as long as earth continuity is preserved, the equipment fuse or breaker will blow if the chassis becomes live in the event of a fault (due, for example, to liquid ingress and [or] component failure). Note also that the maximum allowable "risk current" of 0.5 mA is below the level of perception (typically 1 mA), and well below harmful levels of current. It can be seen, therefore, that the integrity of the grounding conductor is the essential element of electrical safety.

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