How to know wire polarity in an open circuit without any electrical power?

You measure *continuity* using the continuity/ohms setting on your multimeter to determine which of your two same coloured wires go to positive or negative.

Alternatively, apply power to one side of the wire while the load is disconnected, using the multimeter in volts to determine polarity.

The wire may be the same colour but also may have a subtle stripe down one side. This can indicate negative, but this is not universal.

Otherwise, ask the manufacturer, or disassemble the device and test for continuity of the wire to a polarised component (e.g. the negative of an electrolytic capacitor on the input).

- Gough

Thank u so much 

Dearest Gough,

I have followed ur advice n did a partial power on , without closing the circuit. I was able to detect polarity using a digital multimeter. Tq very much.

How do u see the ohms which is positive or negative by testing continuity? Does the numbers of ohms determine POS or neg ? Tq very much

z234 said:

How do u see the ohms which is positive or negative by testing continuity? Does the numbers of ohms determine POS or neg ?

Ohms can only be positive value. The range is from 0 to infinity.

Gough suggested to measure, with a continuity meter, which wire goes to the + point of the circuit. And which wire to the - point. You would put one probe on a known + or - point. Then touch one wire with the other probe. The one that shows continuity is connected to that point. The other wire is then the oposite one.

When the meter is used to test continuity, it applies a small positive voltage to the red lead of the meter. Hopefully the small voltage avoids causing damage in case the actual polarity is reversed. 

Thank u very much for ur kind help.

To recap, The multimeter does produce a very small positive voltage so as to check for the polarity but it is so small a voltage it may not damage the Component in case it really is of reverse polarity ya?

I have followed these post and chimed in to confirm my understanding without breaking out the meter.

Using the ohm scale on the VOM, is it not reasonable to determine polarity by measuring each dash camera lead to ground in order to determine polarity. I assuming the -ve connection is tied to gnd. Using the ground plane of the dash camera as the -ve polarity not acceptable?

It is an assumption. The designers may have taken an other decision.

Just like saying that when measuring resistance or continuity, the red lead will send positive voltage. The only mode where a meter will send out positive voltage, guaranteed, is diode mode. For resistance and continuity, there are meters that send the positive via the common connector.

Not all advices in this post's comments are true at all times.

I agree it is an assumption but what do you do when you don't know what to do an assumption is the start.

If it's a cheap device that's easy to replace, I'd go with the assumption.

If it puts a dent in your budget, or is hard / impossible to replace, I would not.