Fluke 2 Pole Tester always shows a reading

[Jupiter01]

Please excuse the naivety of this question.
When using a contactless tester, it was always binary: there either was a beep if electricity was present or no beep if thee was no electricity. I also appreciate the shortcomings of this tester! This is why I invested in a 2 Pole Fluke Tester with a Voltage reading.

Q. Unlike the contactless tester this usually shows a positive reading (and I think it beeps too) when there is no electricity present. I expect that technically, there is some electricity...
As an example, I was testing the switched live and when the switch was active, it beeped and gave me a reading of around 240. When the switch was off, it beeped and have a reading of around 18v. In this instance the reading was so much lower than 240, that I knew it was safe to work on. Could this “phantom” reading fluctuate around 80 in some instance and would I then be left in doubt?

I’d appreciate your guidance on this please.

 

[CBW]

Could be ghost readings, do a search in the forum, should bring up some results. What is the switch live being tested?

 

[flameport]

By default these devices have a high impedance input, which can give incorrect readings if connected to cables that are in close proximity to others.
If it's the Fluke with black button on each half, pressing those buttons will remove that problem.

For voltage indication, it's the yellow/red coloured lights on it that matter - NOT the voltage reading.

 

[Jupiter01]

I’ve got the Fluke T130. I’ve never used the black buttons on the probes. Would this give me a 0 volt reading and eliminate the beeping in the above situation?

I recall a positive reading and beeps but don’t remember any lights on the device. It sounds from your response, that it’s the lights that ultimately determine if the circuit is safe to work on?

 

[Jupiter01]

Hi Everyone,

Can someone please clear this up for me please? It's a niggling doubt whenever I am using my tester.
Thanks in advance.

 

[DetlefSchmitz]

It's as Flameport says. All the space around you is full of a 3D field of varying electric potential. It comes essentially from the mains cables in your walls. If you wave the probes of a high input impedance around they will measure this voltage. This voltage is meaningless because it only remains as a significant number when you draw no current from it.

If you try to measure the voltage between two pieces of wire which are not connected to anything, they may well show a voltage, for the reasons above. However if you connect a suitable resistance between the probes, then because the stray voltages are unable to source any significant current through this resistance, the measured voltage will drop to close to zero.

I was unaware that voltage testers had this high impedance, but it seems to be so you can measure voltages without tripping RCDs. My tester has a resistance applied all the time so as not to cause false positives. Anyway, the instruction sheet with your tester, though extremely badly written, does mention holding the load button down when using to check for live, so that is what you must do.

 

[blup]

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