Digital multimeter and Meggar 1

[eejacky]

I encountered the following problem during our work. I hope the experts can help to solve this problem.

I work in 132kV transmission cables. At one time, I tried to use the digital multimeter to identify the correct cable from one end to the remote end. The cable is about 3km long. In one end, I used the digital mulitimeter such that the positive wire was connected to the core of the cable and the negative wire was connected to the earth. In the remote end, my colleague used a copper wire to connect the core of the cable to earth. The test is very simple. If the circuit is closed, the buzzer of the multimeter will alert whereas no buzzer is alerted in case of open circuit. Howver, during the test, I found that the buzzer alerted every time when I shorted the positive wire of the multimeter on the core of the cable and the negative wire to the earth, but without shorting the remote end to earth. After that, I used Meggar meter to test again. I switched to 500V and the testing procedure is the same as mentioned above. In this time, the test result is normal. My question is why multimeter showed closed circuit any time in this case. In fact, it is impossible for the core of the cable short circuit to the earth physically. Is this related to the limited function of the digital multimeter? But I want to highlight that successful story was achieved in past job reference (I mean use multimeter to identify the transmission cable to test a long cable (>3km))

I really hope the experts can explain the reasons behind this.

 

[Skogsgurra]

Hi,

A cable of 3000 metres length has a capacitance of less than one microfarad. So it cannot be the charging of the cable that the multimeter buzzes for. It must be someting else.

A cable running 3000 metres is very likely to pick up some inductive or capacitive voltage from nearby lines and that voltage will be present between cores and ground as an AC voltage.

Now, some multimeters will indicate an AC voltage in ohms/continuity mode by buzzing and that is probably what you have experienced. Try using an instrument that is less sensitive to voltage (like a real electro-mechanical buzzer) and you will probably not have the false indications.

The fact that a megger does the work further indicates that it is about inductive or capacitive voltage. A 500 V megger works with a much higher voltage than the buzzer's low voltage (about one volt) and is not sensitive to induced voltage.


Gunnar Englund

http://www.gke.org

 

[eejacky]

Hi skogsgurra,

Thank you for your reply. Do you mean if I use handheld anlogue multimeter, it will not have such false indication? Also, can you explain more why some digital multimeters are more sensitive to induced AC voltage?

Thanks again.

 

[Skogsgurra]

Hi jacky,

Yes, that's right. An analogue meter does not have the built-in non-linearities that a DMM has. But beware, the induced voltage can be high enough to hurt your d'Arsonval movement and the pointer. Better measure the induced voltage first (AC range!) and if it is substantially higer (two or three times higer)than the voltage of the battery used for the ohm meter circuitry then you should be careful.

The reasons for different sensitivity are many. The most significant is how high a battery voltage you use in your DMM. Those with nine volts usually work better than those with three volts. Another factor is how well the front end has been designed. If it has unsymmetric clipping (clipping negative voltages before positive - or vice versa) an AC voltage will be clipped unsymmetrically and the resulting signal - the signal that reaches the A/D converter - will have a positive (or negative) offset and indicate that on the display.

If the insttrument is in OHMS it will probably indicate that there is current in the cable, even if there isn't any.

You can probably get rid of that false indication by connecting a ten microfarad capacitor between cable and ground. It shall be a non-polarized type, not an electrolytic capacitor.

Gunnar Englund

http://www.gke.org