Eddy Current
Senior Member
We were discussing in class how an open coil circuit can have an infinity reading. What would this look like on a meter, would the resistance just keep getting larger? Wouldn't it blow the resistor in the meter?
Infinity readings
- Thread starter Eddy Current
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I think that your meter would give you an error message to save itself.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Along with some less common methods, an ohmmeter can apply a fixed voltage, and measure the current. The display must read the inverse of the current, making for an odd looking scale on an analog meter. Or it can try to force a fixed current (varying with range) and measure the voltage across it.
With either method no higher voltage can exist than that provided by the battery or power supply in the meter.
Nothing threatening can happen as the resistance gets too large. Instead it either loses accuracy (the first method) or displays an error message ( the second method).
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
With an ohm meter the scale is reversed from a voltage meter with 0 being the highest reading at the end of the needle movement, A ? reading is the same reading you will get when the leads to the ohm meter are not connected to anything, this in no way can harm the meter as you can't always have the meter leads connected to something, when you get a ? reading it just means that the resistance is to high for the meter to detect any current path, this in most cases would be called an open circuit or that the resistance is beyond what the meter can read, you might have the option to select another range that is higher or use a more sensitive meter, and as we know when we have an open circuit we know there is no current flowing, some very sensitive meters known as Meggers we can read into the gig-ohm range or billions of ohms, we use these to check insulation from decaying and to make sure there are no high resistance fault that can turn into a low resistance fault by arcing over.
It all just depends upon the ability of the ohm meter you use.
Never connect a meter set to the ohms range to a voltage source, many meters can be destroyed doing this, some will have protection that will prevent a meter from failing catastrophically in most cases it is a special very fast acting fuse, but this does not always protect the meter as since the ohms range is very sensitive applying any voltage to it can cause components to burn out even if the fuse blows, the fuse is not there to protect the meter it is there to protect you from getting burns and the components can fail catastrophically which was a very bad safety problem until we started getting Cat rated meters, most meters are now cat 3 or cat 4 rated which basically means it is protected by OCPD's that will limit the current that the meter can create when used in a wrong manner, I had a coworker who went to check for voltage in a 480 volt disconnect with a $10.00 Radio Shack analog multimeter, it was the one that didn't have the range knob on it, you changed the function by inserting the leads into different holes in the face, and what he did was he had it in the holes for 10 amp DC current which has a shunt between the leads and is a dead short, for current the meter uses this shunt which is of a known resistance so when you place it in series with the load the meter will measure the millivolts and read in current on the scale, but when he touched the leads to the bottom of the fuses which was capable of supplying thousands of amps instantaneously the meter exploded in his hands which then cause an arc flash in the disconnect causing it to blow off the wall, he spent many days in the burn unit up in Chicago and was never the same again, so knowing how to use a meter and what meter to use for the job is a very important lesson to learn, today in a cat 3 rated meter all that would have happened would be the very special fast acting fuse in the meter to blow, also never replace the fuses in a meter with just any fuse or you will loose the cat rating, OSHA rules require that they only be replaced with a factory replacement fuse that are calibrated and tested to make a meter safe
It all just depends upon the ability of the ohm meter you use.
Never connect a meter set to the ohms range to a voltage source, many meters can be destroyed doing this, some will have protection that will prevent a meter from failing catastrophically in most cases it is a special very fast acting fuse, but this does not always protect the meter as since the ohms range is very sensitive applying any voltage to it can cause components to burn out even if the fuse blows, the fuse is not there to protect the meter it is there to protect you from getting burns and the components can fail catastrophically which was a very bad safety problem until we started getting Cat rated meters, most meters are now cat 3 or cat 4 rated which basically means it is protected by OCPD's that will limit the current that the meter can create when used in a wrong manner, I had a coworker who went to check for voltage in a 480 volt disconnect with a $10.00 Radio Shack analog multimeter, it was the one that didn't have the range knob on it, you changed the function by inserting the leads into different holes in the face, and what he did was he had it in the holes for 10 amp DC current which has a shunt between the leads and is a dead short, for current the meter uses this shunt which is of a known resistance so when you place it in series with the load the meter will measure the millivolts and read in current on the scale, but when he touched the leads to the bottom of the fuses which was capable of supplying thousands of amps instantaneously the meter exploded in his hands which then cause an arc flash in the disconnect causing it to blow off the wall, he spent many days in the burn unit up in Chicago and was never the same again, so knowing how to use a meter and what meter to use for the job is a very important lesson to learn, today in a cat 3 rated meter all that would have happened would be the very special fast acting fuse in the meter to blow, also never replace the fuses in a meter with just any fuse or you will loose the cat rating, OSHA rules require that they only be replaced with a factory replacement fuse that are calibrated and tested to make a meter safe
kwired
Electron manager
- Location
- NE Nebraska
resistance to high for an analog meter to display simply results in no movement of the pointer.
Digital meters measuring something outside their range usually display some error type of code, which does vary from one unit to another.
Digital meters measuring something outside their range usually display some error type of code, which does vary from one unit to another.
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