Ambiguous Voltage Units

It is extremely irritating when people aren’t specific enough, or don’t elaborate on details.  See? Let me explain!  The topic specifically on my mind relates to the different types of unit quantities, and the failure to acknowledge which is intended in a given situation (because we’re all mind readers).  Let’s use voltage as an example, of which there are different types; for example, RMS voltage (Vrms), line-to-line voltage (Vll), line-to-ground voltage (Vlg), peak voltage (Vp), peak-to-peak voltage (Vpp), AC voltage (Vac), average and of course, regular old voltage (V).

The regular old voltage, or V for short refers to an instantaneous quantity in its purest sense at a moment in time.  So let’s say we are measuring something with an oscilloscope, and you ask “what is the voltage?”  Well, in a way, that is an incomplete question, and you should expect an incomplete answer in return.  If the quantity is sinusoidal in nature as an example, did you mean to say “what is the RMS voltage?” or “what is peak voltage?” Probably you did.  When we use a different measurement type of voltage, it gives us insight into the nature of the waveform.  Perhaps it is sinusoidal, perhaps it is DC.  Knowing an instantaneous value of a DC voltage could be more relevant of knowing an instantaneous voltage for a sinusoidal waveform.

An example is when discussing power system voltages.  Someone might say or you may see written somewhere, 480V, aka four hundred and eighty volts.  What does this mean and imply about the system in question? For those of you not familiar with the convention, this refers to an RMS, AC (sinusoidal), and line-to-line quantity all in one.  A line-to-line quantity implies that it is a 3-phase system.  But if you are indeed unfamiliar with the convention, how could you possibly know that?  Would it be so difficult to write “480Vrms,ll,ac,3-phase” so that an unfamiliar person could gain more insight without having to wonder?  Writing or saying “480V” also implies what type of 3-phase system it is, and that it does not have a neutral connection.  So the layers of the onion keep peeling away as you can see.

My point is that it is perfectly OK to give more detail than is necessary.  Don’t be afraid to break the convention.  I do it all the time.  No one ever complains, and it also reinforces in my own mind fundamentally what I am talking about.

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