Polarity question

In a -48 volt DC power plant system how do you determine the polarity of 0v? trying to resolve an issue and not sure how to correctly identify

• how do you determine the polarity of 0v 0 V has no polarity, probably you mean "What is the ground reference?" "Ground" or "chassis" is the term EEs use to refer to the point that we define as being 0 Volt. Very often the (metal) case of a device is connected to ground, test this using a multimeter, connect - to the case and measure the voltage at the battery connections. In a -48 DC system the - contact of the battery will be -48 V, the + will be 0 V. – Bimpelrekkie Aug 10 '18 at 15:02
• Jose - You said that you are "trying to resolve an issue". Please explain the issue you are trying to resolve; in other words, please give us more context to your question. That will help readers to better understand your question (or to ask better clarification questions to you) since, as Bimpelrekkie explained, your question doesn't make much sense as it is currently written without any context to help explain it. – SamGibson Aug 10 '18 at 15:13

simulate this circuit – Schematic created using CircuitLab

Figure 1. (a) Negative ground. (b) Positive ground.

• If the power-supply negative is connected to earth / ground (Fig. 1a) then it will be the zero-volt reference and the other terminal will be +48 V with respect to your ground.
• If the power-supply positive is connected to earth / ground (Fig. 1b) then it will be the zero-volt reference and the other terminal will be -48 V with respect to your ground.
• If the power-supply is not connected to earth / ground (Fig. 1a) then it will be floating with respect to your ground. There will still be 48 V between its terminals but no defined voltage between either terminal and ground.

It seems that you have a system as shown in Figure 1b.

In a -48 volt DC power plant system how do you determine the polarity of 0v?

• It has no polarity relative to ground.
• It is +48 V relative to the power-supply negative.

That's a question of point of view:

• the voltage is 48 volts more negative than the ground, so it's -48 V relative to ground ;
• but, the ground is 48 volts more positive than your voltage, so it's +48 V relative to your supply voltage.
• you could as well take any other reference if you want.
• -1 Your answer doesn't say anything about the "polarity of 0V", which is the question. – Elliot Alderson Aug 10 '18 at 15:15