Low impedance meters and ghost voltages

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JHZR2

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Power Systems Engineer
Hello,

im interested in getting a low impedance meter to avoid seeing ghost voltages. As I have a fluke 87v and 179, I'm interested in buying an analog meter.

is 20kOhm/V too much impedance? That's like 2MOhm at 100V, which is a lot lower than a dmm, but perhaps too high to dissipate.

Any recommendations for for a good low-Z analog meter?

thanks!
 
I think the newer Fluke T-5 has a low z setting.

As far as analogue Simpson is the classic model and you can find them on e-bay.

I still use my Knopp K-60 solenoid tester once in a while to eliminate ghost voltage readings. Mostly I use my old Fluke 12, it has a low z feature.
 
Rather than a model, is there a good value to target the meter resistance/impedance to dissipate the capacitance? It seems based on a bit more research that any of the analog meters you buy Will have an invoice impedance somewhere between 100kOhn and 1Mohm. But that's quite a wide range.
 
JHZR2 said:
Rather than a model, is there a good value to target the meter resistance/impedance to dissipate the capacitance? It seems based on a bit more research that any of the analog meters you buy Will have an invoice impedance somewhere between 100kOhn and 1Mohm. But that's quite a wide range.
Click to expand...

I may be wrong, but I am pretty sure that, on analog meters, the impedance changes when the settings are changed.

Having owned and operated many analog meters (I still have a couple) I would not suggest them for our work when there are low Z digitals and great solenoid testers available.

I have an Ideal solenoid tester that shows AC volts, DC + - and beeps when there is no voltage and shows continuity. Once a person gets used to reading the lights and hearing the beeps, ghost voltages are busted. That's why we should call such testers 'ghost busters'.

There are no settings to bother with and they are tough as rocks.

Some things to consider about analog meters:

They are fragile. Dropping them from just a few feet can and will destroy them.

The needle is subject to vibration, gravity and parallax.

They are very imprecise. Try seeing a change from 119.5 volts to 118.0 volts on an analog meter, which is very easy to see on a digital.

What is the CAT rating of the analog meter? You aren't supposed to use a CAT II meter in a CAT III or IV environment.

To me, a small 120 volt light bulb across the leads of a person's favorite DVOM is a better choice than an analog meter. The light thing, BTW, is something I have actually done and it works very well for busting ghosts.
 
You don't need a new meter. Fluke has a test lead kit with a low impedance adapter that you can use with your existing meter.
 
K8MHZ said:
To me, a small 120 volt light bulb across the leads of a person's favorite DVOM is a better choice than an analog meter. The light thing, BTW, is something I have actually done and it works very well for busting ghosts.
Click to expand...


Thanks for for your insight.

My my issue is that I'm still seeing the ghost voltagd when I put my outlet tester in (which dimly illuminates the bulb). There is a bulb installed in one fixture and I still see the ghost voltage. I guess technically it isn't "across" the leads, but it's still in the circuit and should be dissipating energy. Of course I guess it should also glow dimly too, but it's a much larger bulb so I guess that has some effect.

What at about just wiring a resistor to the probes via some alligator clips or something?

Actually i I may just buy the fluke 225.

But it end of the day I'm not sure that satisfies my desire for knowledge. For 120vac circuits, what impedance is the right amount to dissipate ghost voltages, yet not be an issue otherwise?

it seems to me that "good" true analog meters (not the dial on a digitized signal or vacuum tube types) can still be over a MegOhm.
 
JHZR2 said:
:)

That doesn't feed my quest for knowing the "right" amount of impedance, or the desire to have a decent meter!

:)
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I don't think there is a "right" amount. If you are looking for an impedance value that would be just enough to cancel out the voltages seen from capacitive coupling, aka 'ghost' or 'phantom' voltages, but not any more than that you have to realize that those voltages are not going to be the same from one circuit to the next.

Also, the effect of a certain value of reactance won't have the same effect on shunting ghost voltages as the same value (in ohms) of pure resistance. When an impedance value is given with no additional information we don't know how much is resistive, how much is inductive and/or how much is capacitive.
 
JHZR2 said:
:)

That doesn't feed my quest for knowing the "right" amount of impedance, or the desire to have a decent meter!

:)
Click to expand...
Any amount that gets rid of the "ghost" voltage and does not cause excessive voltage drop or cause the OCPD to trip will be fine.
 
If you're dead set on an analog voltmeter, literally the only one I know of that is suitable for commercial/industrial use would be the Simpson 260-9 series that is CAT III 600V rated. All the other analog meters I've seen are really only suitable for home use.

Simpson-260-9SP.jpg

We have a couple that come in handy sometimes for HV substations.

Otherwise, just buy the SV225 adapter for your multimeter. Cheap, safe solution:

SV2251.jpg
 
teufelhounden91 said:
Newb question here, but what are ghost voltages?


Sent from my iPhone using Tapatalk
Click to expand...

Most Digital meters are very high impedance (high resistance). Ghost voltages are voltages read by high-impedance meters when there is no metallic connection between the voltage source and one or more of the conductors connected to by the leads.

Consider a black, white, blue and green wire on a conduit. The black wire is connect to 125VAC hot; the white to the 125VAC neutral; the blue is unterminated, and the green connected to ground.
Now the black and blue wire act as a capacitor (two conductors separated by insulating materials). The capacitor has a high impedance at 60HZ, but the meter has a much higher impedance (10--100 say). Now the circuit the meter is seeing (blue -- white) is a voltage divider with 90% or more of the voltage across the meter which then reads somewhere in the vicinity of 120--125V.

Using a low impedance meter (1kOhm -- 30kOhm per volt) the impedance of the meters to so much lower than the capacitive coupling that the reading is essentially zero -- as it should be.

The SV225 has a resistance of roughly 2kOhm. Low impedance digital meters have such low impedance when set for "low-Z voltage".

Sent from my hand build tower system using UBUNTU.
 
151016-1148 EDT

A typical analog voltmeter consists of a microammeter with a series resistor to translate voltage to current. For AC a bridge rectifier is added to convert AC to DC.

Since the meter movement is of a fixed range different full scale voltage ranges are obtained by changing the series resistor.

Meters are rated as so many ohms-per-volt. Use ohms law to figure out why.

A Simpson 260/270 has a DC sensitivity of 20,000 ohms/volt ( 50 microampere full scale), and for modern meters the AC sensitivity is 5,000 volts/volt. In early 260s (1940s and 1950s) the AC value was 1000 ohms/volt ( 1mA full scale).

Romex cable between black and white has a capacitance of about 20 pfd/foot. A 100 ft length of Romex would be a capacitance of about 2000 pfd or 0.002 mfd. This has a capacitive reactance of about 1.2 megohms at 60 Hz. A Simpson 260 on AC and the 250 V range is about 1.25 megohms. I have to leave, but you can see the voltage reading will be quite large, near 0.7 times line voltage.

.
 
big john said:
If you're dead set on an analog voltmeter, literally the only one I know of that is suitable for commercial/industrial use would be the Simpson 260-9 series that is CAT III 600V rated. All the other analog meters I've seen are really only suitable for home use.

View attachment 13588

We have a couple that come in handy sometimes for HV substations.

Otherwise, just buy the SV225 adapter for your multimeter. Cheap, safe solution:

View attachment 13589
Click to expand...


How much for the Fluke unit in the US?

£50 over here!
 
151016-1436 EDT

To continue from my post numbered 14.

The maximum current thru a 0.002 mfd capacitor when connected across a 120 V 60 Hz voltage is about 120/1.2 = 100 microamps. A Simpson 260 at 5000 ohms/volt on AC is a 200 microamp movement. The 2.5 V scale is 12,500 ohms, and this is small compared to 1.2 megohms for the capacitor. Thus, a Simpson 260 on its 2.5 V AC range would read about 1.25 V when in series with a 0.002 mfd capacitor and 120 V 60 Hz.

Experimentally my Simpson readings with a series 0.002 mfd capacitor are:
Code: 
Range   Reading
 250      120      direct no capacitor
 250       85      0.002 mfd
  50       23      0.002 mfd
  10        4.6    0.002 mfd
   2.5      1.2    0.002 mfd

Do not ever do this on an unknown source.

.
 
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