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bandit
17-11-2002, 04:26 PM
ok heres another question

i have been told by a very experienced engineer that most engineers tend to change oil switchs when there is an oil fault when infact they are not really required. he claimed this as an oil switch is probably used once in its life and the chances of it being faulty are low and it more then likely to be a problem with the oil etc

would you agree with this statement or what would you say on this matter

Dan
17-11-2002, 06:54 PM
i have been told by a very experienced engineer that most engineers tend to change oil switchs when there is an oil fault when infact they are not really required. he claimed this as an oil switch is probably used once in its life and the chances of it being faulty are low and it more then likely to be a problem with the oil etc

Your engineer friend has a valid point. I think one could say in general that a safety device is too often replaced for accomplishing its proper function. Even cycling controls such as fan-cycling klixons are jumped out when in fact they are working properly.

There are some sensible tests to determine whether an oil failure control is working or not. Apply voltage to the control with the compressor off, for example. The heater element should be receiving voltage and time out, opening the control circuit.

Or, more directly, just observe the net oil pressure during the run cycle, and the behavior of the oil failure control during the observation. Depending on the type of equipment you are working on and the type of oil failure control you have you should be able to manually insert fault conditions. On parallel racks, for example, you could valve off the oil supply and make your observations.

The oil pressure control has the simple responsibilty of opening the control circuit when the compressor's oil pump is not delivering a minimum net oil pressure. It has to be wired in correctly, of course. Compressor time delays can sometimes cause a useless trip because the oil control thinks the compressor is running when it really isn't, etc.

Current sensing relays on internally protected line break protection (Copeland 3D-body compressors) will cause an oil failure for the same reason: the oil failure control thinks the compressor is running when it isn't because of an overload situation. So you get an oil failure for the wrong reason.

There are two failure modes of safety devices: They trip out when they shouldn't, or they don't trip out when they should.

During a PM, it is important for the technician to make sure the safety devices trip out when they should.

When they appear to be tripping out when they shouldn't, a technician should be slow to condemn the safety device and look for the real problem.