I'm measuring 17 Vac across my bathroom light fitting when the light switch is off. I'm also measuring 34 Vac across my landing light fitting when the light switch is off. If I isolate the upstairs lighting at the consumer unit then it drops to around 1.3 Vac.
What's going on?
What are you using to measure the voltage? If you are using a modern high impedance meter such as a Fluke, then what you are measuring is the capacitive coupling between a live wire and an isolated one, and it is quite normal. If on the other hand you are using an older meter based on a moving coil movement then it is time to worry!
AWEM
I'm using a Fluke 77. So why am I seeing 17 Vac on one fitting and 34 Vac on another fitting?
Beacause more or less that's the actual voltage of the wire, the wire is neither earthed nor is it connected to a supply. Frankly a probe with a
230Vac pigmy bulb in it will tell you much more _relevant_ info about mains wiring than a DVM.
modern
OK, you have a conducting circuit comprising your 'unconnected' wire, the tiny capacitances to adjacent conducting wires, and the very high impedance of the measuring instrument. Changes in the amount of capactive coupling will vary the perceived voltage at the Fluke. So it is probable that your 17v reading is on a wire with (about) half the capacitive coupling than your wire associated with the 34v reading. Neither are anything to worry about and are quite normal.
I don't wish to sound condescending or rude, but it is an example of a little knowledge being a dangerous thing. You need to understand the big picture and the limitations and scope of your tools.
AWEM
A little knowledge would be a dangerous thing if I ignored perceived anomolies rather than find out what they are. But anyway thanks for your input.
I'm amazed you bought such an expensive tool without knowing what it does.
But it's basic electrical theory. If you use a very high input impedance device you'll always get strange readings on an open circuit. Just touching the leads will give some sort of reading. It is high impedance to give an accurate reading under working conditions.
In message , LaserMark
You shouldn't be :-). I recently watched an air conditioning installer (clad in a boiler suit declaring under the company name that he was an "Installation Engineer") prodding one probe of a very expensive Fluke DMM at various contacts and saying that the reason the device he had just fitted was not working was because the mains was too weak (at 75VAC). When it was suggested that the second probe belonged somewhere other than his pocket he declared he was "Part P qualified" and knew what he was doing.
Pointing out that the destruction book said that the red wire should go to connector A and he had connected it to connector D led to much angst and a declaration that the instructions were not Part P and had to be modified accordingly.
Further mentioning that Part P didn't apply to industrial installations led to him stomping off saying he couldn't continue due to elfandsafety. In this he was probably right as I suspect my blood pressure was exceeding safe limits by this stage.
=A3203 I've just priced that meter at. When you can get a perfectly adequate meter out of Maplin for a fiver or less, that does somewhat beggar belief that that amount of money was spent without seemingly understanding the uses and characteristics of the instrument.
I do quite a bit of home electronics and would love to have spare money to buy quality instruments like that. Actually on the basis that 'quality' in my book also includes 'value for money' I'm not sure I would buy one.
Rob
Might explain why in every electronics lab I have worked every new batch of fluke meters bought seems to have such an alarming rate of disappearance! If you don't keep them in a locked drawer they grow legs!
Part of the attraction of the fluke meters is that they are nearly idiot proof - you have to work quite hard to kill one, so they are well suited to lab use.
Really? You must have tried very hard to find one that expensive. They are less than half that in the U.S.
It depends on what you want to do.
I have a Fluke 87 that I bought in the U.S. over 10 years ago (IIRC). This was one of the earlier version 4 true RMS ones and I think I paid around $200 for it at the time including a bundle of accessories such as thermocouple probe and adaptor and a current clamp.
During that time it has behaved perfectly - leads are still good, calibration reports fine, switches are solid and so on. It's rugged enough that if it's accidentally dropped it survives unscathed - not that I've dropped it more than twice. Even so, it's normally kept carefully and used as required on projects.
I've also had a succession of the £10-20 jobs that are typically used in the workshop for rough work that doesn't need a better device. They are OK for what they are, but accuracy is not great and there is quite a bit of variation with temperature for some reason which I've never investigated. It's the flimsy little leads that let them down the most and I've found that the switch usually becomes intermittent or causes errors. Fortunately, the errors seem to be gross enough to be obvious.
You can get quite a long way with these meters as long as you realise the limitations and are prepared to throw them out when they become troublesome.
The two are not really related that much. Quality is all about whether the product does what it is claimed it will do and continues to do so for a period of time represented by the price paid. It's therefore fairly well defined.
Value for money is about the degree to which the product does what you are looking for it to do and that varies by person and application.
Thus if you want to do a set of things that can be achieved by one of the Maplin superspecials and you are happy to buy replacements as they fail or become unreliable, then they are good value for money for you. Equally, if you want to do some things for which a higher spec. meter like the Fluke is needed or can be relied upon to do for longer without buying a new one, then the Fluke is good value for money and the Superspecial is not.
Indeed. A cheap DVM will be more than adequate for most who will only use it for voltage and continuity.
I did graduate to a Fluke some years ago. Deliciously tactile to hold and use. ;-) After you pay extra for some decent flexible leads - the originals were terrible.
Still easy to blow a fuse if you forget to change the leads back after doing current measurement, though. And the fuses ain't half expensive.
Common to a lot of meters, I can't understand why the maker of an expensive bit of kit cheapens the entire product by fitting leads that probably save all of 25p off the total cost.
My antique Solartron was the same.
At one time the Maplin ones were bogof and consequently cheaper than the batteries inside them. I've got a stack of batteryless meters somewhere ...
Ian
You are using a high impedance meter . And measuring capacitative pickup.
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