limit switches testing

Before you grab your ohmmeter to measure contact resistance, consider this:

To determine the condition of the limit switch contacts you need to consider the level of current that the switch is to carry (more on this in a little bit). The fact that it is a "limit" switch indicates that it is a safety device of some kind. That being said, there are two possibilities for operating condition.

The first is one that requires "Normally Closed" contacts to be making a connection or the device will not run (commonly used on E-Stop circuits). If this is what you are using, then you will know if you have bad contact health by the fact that you have trouble starting or maintaining a normal run (i.e. frequent interruptions) condition.

If the contacts used are "Normally Open" they indicate a trigger device which is more likely to fail for mechanical reasons than electrical. Some people may scoff at this but it is true. Broken linkages, freedom of movement and missing lubrication are frequent causes but those things are not electrical.

Contact resistance is a field of its own. A standard analog or digital ohmmeter will give you different results depending on what scale you use. Most people do not appreciate how difficult it is to measure a low resistance value of less than one ohm. Special meters and techniques give better results. But more than that, it is important to take several readings and calculate an average result. It is also important to make sure that the pressure placed upon the switch is approximately the same for each reading. You will have to compensate for leadwire loss, etc. but the contact resistance of most appropriately selected switches should be fairly low.

I would expect an average contact resistance of up to 0.2 ohms to be good. An average contact resistance of up to 0.5 ohms to be fair. An average contact resistance of up to 0.9 ohms is beginning to be questionable. Above that, I would be looking for a replacement switch. If, however, your numbers are all over the place (i.e. a standard deviation > 0.4 ohms) then you should find another meter or test setup.

For circuits that pass more than 50 mAmps through the contacts, an ohm meter may give misleading results. In those cases you will need to set up a power source, a stable load and then measure the voltage across and the current through the contacts. Most Ohm meters are simply not designed to test impedance at higher current levels.