Basic Analog AC Voltmeters

PT Rated and "Self Contained"

Another common meter we build in the lab is the AC Voltmeter.  The two most common are the "PT rated" and the self contained.  "PT rated" meaning these meters must be used with a PT (potential transformer).  Just like with the AC Ammeters, PT transformers are expressed with a ratio.  For example 2:1, 240:120 or 300:150, the ratio is the same.  Figure 1 is a PT rated meter that requires a PT with a ratio of 2:1.  This meter must be used with a PT in order to measure 300 ACV.  The input to the meter is 150 ACV.  A drawing of a basic PT is in Figure 2.  Unlike the doughnut CT we discussed in AC Ammeters the PT in Figure 2 has two wires for the input (primary connections) and two wires for the output (secondary connections).

           
Below  in Figure 3 is a simple hook-up combining the two.  Lets say we're trying to measure the 300 ACV .  The 300 ACV comes in on the "primary" side and we'll say the PT "transforms" or "steps down" the 300 ACV to 150 ACV on the "secondary" side.  The 150 ACV is hooked up directly to the back of the meter (input) and the pointer will indicate 300 ACV on the scale.  This ratio is 2:1.  Note the red pointer on the meter that indicates 300 ACV on the scale.

Now with the same application see Figure 4.  Using the same meter and same PT, let's pretend that the voltage drops from 300 ACV to 240 ACV.  If the primary voltage drops to 240 ACV ,  what happens to the "secondary"?  It drops to 120 ACV, a 2:1 ratio.  With an input to the meter of 120 ACV the pointer indicates 240 ACV on the scale.

 Remember panel meters and PTs come in many sizes and shapes.  Some are more accurate than others, some are for specific purposes there are many variations, these Lab bulletins are only to give you a generalization of how they're utilized in the field.

            Technical Note: There are suppose to be fuses in the circuits.  For figures 3 & 4 these fuses are in line between the PT  and line.  For the meters without PTs these fuses are between the line and meter.  In both cases there is one on each leg.  I left them out to make the drawings as simple as possible.

"Self Contained" AC Voltmeters.

 In Figure 5 we have a simple application for a "self contained" ACV meter.  In the diagram below if we're trying to measure 300 ACV with a meter that is self contained.  This means we must be able to hook up 300 ACV directly to the meter, as shown in Figure 5.  A rule of thumb is that most manufacturers offer meters that can read up to 600 ACV.  Higher than 600 ACV  they may require a PT.  Yes we can have a meter 0-300 ACV that requires a PT or we can have a meter that is self contained.  If a customer is replacing a meter in a existing application, they may or may not have a PT.  This is why we have to know, is the meter "self contained" or is it a "PT rated" meter.

           
If they make a meter to read 300 ACV which is self contained,  why have a PT?  It could be a requirement of the customer not to have more than 150 ACV inside their panel which includes the panel meters. It could be like the example above where the customer is replacing a meter in an existing circuit.  There are many reasons but it's up to us to find out exactly what their requirements are specifically when no "part number" is conveyed to us.

So when can an ACV meter only be "self contained and will not require a PT?  Another thumb rule would be anything less than 150 ACV.  Again there are many exceptions to the rule and manufacturers are different in the way they "measure" certain signals so don't be surprised.  A good habit is to always ask for a "scale" and "input".