# Thread: measure voltage + current travelling through negative wire

1. Can anyone tell me how to measure the voltage and current travelling through the negative wire after it has been through an appliance?

Is it as simple as attaching the positive of the voltmeter to the negative wire (after the appliance) and the negative of the voltmeter to an earth? If so could I use the earth found on a regular UK socket or would this be bad practice?

2.

3. Note that this is AC and so "positive" and "negative" don't really apply. If you are measuring voltage, make sure you have your meter on AC. And BE CAREFUL.

As to measuring voltage, it depends what you are measuring the voltage relative to! If that is not too obvious.

Normally, in a mains electricity supply, it is measured relative to the neutral ("negative"). But as it is the neutral you want to measure, then I suppose the obvious thing is to measure it relative to earth. And, yes, you can use the earth pin in a socket but be very careful how you access it; all exposed metal work on the appliance should be earthed (but as you may be looking for a fault...)

Neutral should be at or near zero volts. I don't know how much it is allowed to vary from this. A significant voltage suggests a fault somewhere.

As for measuring current, the easiest way is one of those clamps that goes round the cable. Otherwise you need to disconnect or cut the wire to put the meter in series (and, if you are asking a question like this, I'm not sure you should be doing that. ).

4. A clamp-on ammeter would be the way to measure the current.

The grounding conductor (usually a bare copper wire in the US) does not normally carry current, and is connected to ground at the service entrance. (Using American terminology here). Therefore since this conductor does not carry current, it should be at the same potential as the ground bus at the service entrance. However, if you have a poor connection to ground, then the voltage at the service entrance could vary quite a bit and might not be exactly at earth ground potential. If there is a concern about that, you would have to find some other reference to earth ground, like a metal stake in the ground, a metal water pipe, or something like that.

Likewise, if there is a short circuit in your appliance, the neutral could be shorted to the chassis of the appliance, and would therefore be at the same potential as the ground pin. This condition should trip your circuit breaker if you have a ground fault circuit interrupter.

5. thanks all! Can I not use the amps selector on my voltage meter to read the current? I presume a clamp-on is used to measure insulated wires and very high current...is this correct?

6. Originally Posted by fatman57
thanks all! Can I not use the amps selector on my voltage meter to read the current? I presume a clamp-on is used to measure insulated wires and very high current...is this correct?
As Strange said, you have to connect the meter in series with the load to measure the current. Generally, the voltmeters I've seen are not designed to measure a-c current, and then only very small d-c currents. The skinny leads on your meter would never be able to take it.

7. Originally Posted by fatman57
thanks all! Can I not use the amps selector on my voltage meter to read the current? I presume a clamp-on is used to measure insulated wires and very high current...is this correct?
The thing is, you can measure voltage between wires but you have to measure current along the wire. In other words, to measure current you need to break the circuit (e.g. cut or disconnect the wire) and then insert the meter in the circuit so the current flows through it.

If you put your meter on the current setting and connect it across the mains you will probably destroy it (or blow a fuse).

Harold mentioned a ground fault circuit interrupter - this is a residual current breaker (RCB) or earth-leakage current breaker (ELCB) in the UK. If you don't have one in your consumer unit ("fuse box") - they have only been mandatory in the UK relatively recently - then it might be worth buying one to put in the socket where the appliance is plugged in. This will tell you immediately if there are certain types of fault (the breaker will blow) and will give you a little extra protection if you do something silly.

8. Just be sure if you use a clamp on meter to only measure one of the conductors. If you put both the hot and neutral in the clamp, you won't get a reading.

It's far more difficult to use a multimeter, as Strange and Harold pointed out, you have to open up the circuit somewhere and place the meter in series:

9. thanks all and thanks for the safety tips!

10. Originally Posted by fatman57
Can anyone tell me how to measure the voltage and current travelling through the negative wire after it has been through an appliance?

Is it as simple as attaching the positive of the voltmeter to the negative wire (after the appliance) and the negative of the voltmeter to an earth? If so could I use the earth found on a regular UK socket or would this be bad practice?
To be entirely successful, and at ease, with the wonders of electricity, it is IMPERATIVE that one recognize that VOLTAGE does not travel around, through conductors, etc.

Voltage can exist only between two places, or points, such as points A and B, or point A and Earth Ground. Voltage may be present for an eternity, without any further effect taking place, until the opportunity presents itself for the voltage to cause current flow. Current flow is the INTENSITY or Amount of electricity flowing. Voltage does not "flow".

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