Recently, we were working with a Public Safety customer that had new diesel generators installed at several two way radio and microwave system sites. We happened to meet the generator maintenance technician at one of the sites while doing other work. An interesting discussion ensued. It turns out that the technician, in his instructions had been told to set the maintenance routine up so that the generator ran for 15 minutes once a week, under no load.
Generators are no different than an automobile when it comes to engine maintenance. It’s common knowledge that the worst thing you can do to an automobile engine is to only run it for 15 or 20 minutes and then shut it off. This has to do with several items that affect an internal combustion engine.
First and foremost is the oil. Oil oxidizes while sitting in the crankcase. This causes acids to form and chemistry to change. What removes these acids and normalizes the oil is heat and pressure over a minimum period of time. 15 minutes is not long enough to heat the oil and maintain that heat and pressure to remove the acids that form from the oil oxidation.
Oil and heat has a secondary affect on an engine. Oil attaches and migrates into the pores of the metal. This limits rust and other contaminates from bonding to the metals of the engine. The classic example of this is in the form of a cast iron skillet. Anyone who cooks knows that a well seasoned cast iron skillet will not have food stick to it. The vegetable oil permeates the pores in the iron, thus not allowing things to stick. To maintain this, you must use the skillet regularly, get it hot and allow the cooking oils to penetrate into the iron. A skillet like this also will not rust, and will last for years.
Secondly, moisture affects an engine – particularly the exhaust system. The engine needs to heat the exhaust system enough to boil all the water condensation out. Have you ever noticed that during the winter your car exhaust has “smoke” coming out for some time after you start it, but it disappears once the engine gets hot? That’s the water vapor escaping as the system heats up. A generator is no different, and the exhaust system is critical to proper operation of any gen-set.
It’s important to remember that the engine is only half of the generator. The other half, the actual generator has its issues too. It is also affected by moisture and heat. That is why it’s very important to exercise the system under a load. This allows the electrical portion of the generator also to come up to operating temperature.
Lastly, the NFPA has specific standards for testing. The critical section is in NFPA 2010 Section 110, Chapter 8. The maintenance routine is in Section 8.4. Specifically, Section 8.4.2:
“Diesel Generator sets in services shall be exercised at least once monthly for a minimum of 30 minutes, using one of the following methods:
(1) Loading that maintains the minimum exhaust gas temperatures as recommended by the manufacturer.
(2) Under operating temperature conditions and at not less than 30 percent of the EPS nameplate kW rating”
I will take issue here: I don’t believe that monthly testing is adequate. In our industry, and for critical sites, weekly is required, with all the above criteria met.
There are several other standards dealing with standby generators, their installation and maintenance. Specifically, NFPA 70/NEC 701, which deals directly with “Critical Operations Power Systems” and the Joint Commission on Accreditation of Healthcare Organizations. I would recommend that if you have not reviewed these standards, that you do so.