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oT, alert on multimeter

surplusjohn

Diamond
Joined
Apr 11, 2002
Location
Syracuse, NY USA
I was fixing something the other day and checked its voltage with my sperry multimeter, which is 10 or 12 years old. I got a reading of 190 volts on a 110 circuit! yikes, this is in a commercial building so I thought maybe they had their transformer tapped wrong. Anyways i went and got the land lord and he checked the voltage with his meter and it was ok. So i put a new battery in mine and got the right reading. Point is, wouldnt you think that when a meter was failing it would have a fail safe mode where it would simply turn off or some thing other than giving a misleading and possibly dangerous reading? something to be aware of.
 
Does your meter not give a low battery warning? Or did you just ignore it and keep using the meter?

The low batt warning is the way designers get around this problem. Even the high end Fluke units give funky readings if you keep pushing a dead battery for a long time.

I would worry a lot more if it read zero, or substantially lower than reality. The worst it did was make you look a bit silly running to your landlord. If it read zero, you might have been seriously hurt or worse when you touched a "dead" circuit....

FWIW, OSHA and NFPA 70E standards require you to verify the meter on a known voltage source before and after checking your circuit for voltage.
 
One issue about using small hand-held DVMs for checking power voltages is, the meters
are not typically fused correctly for the application.

If you open up the better meters you'll find two fuses in series, a small fuse and one
large fuse. The large fuse is there to shut down any heavy current flow if the meter happens
to be on an amps or an ohms range when it's put across the line.

Without the large fuse with an appropriate current interrupt rating you can have a runaway
situation where the small fuse develops an arc inside and the current contiues to flow. The
meter melts in your hand.

The older simpson meters are prone to this, but any inexpensive DVM is a poor choice for checking
line voltage.

The correct test instrument is an electricans 'wigger' which is designed just for that purpose. It's
a handy tool to have.
 
One strange day..... I had one video monitor die suddenly.....
An hour later... Another video monitor suddenly dies for no possible reason..

I get out the Fluke 73 meter, and look at the line voltage.
HOLY SMOKE! It is High.

I call the power company, and they have a truck dispatched in a matter of minutes.
I was very impressed.

They check the voltage, and everything is fine..
I had an old battery in the meter, and it was reading funky.
It did not give a low battery alarm.


Now, WHY did two monitors die with minutes of each other?
One on my desk, one on a CNC machine? That mystery is more treacherous.
 
According to pg 6 of manual here:

www.sperryinstruments.com/documents/products/dm-4100a.pdf‎

, you should get a low battery warning on the display.

I agree with Jim that I wouldn't trust that meter for working on any kind of high energy systems. It was designed well before any of the CAT rating standards were developed, and explosive failures of these kind of meters can and have happened in the past when used on high energy circuits. You really want at least a CAT III rated meter and leads if you are troubleshooting machines in an industrial environment:

http://support.fluke.com/find-sales/download/asset/1263690_6116_eng_h_w.pdf

Does Your Meter Safety Measure Up? | content content from Electrical Construction & Maintenance (EC&M) Magazine

In addition to obvious differences like shrouded test leads, etc., properly designed DMMs incorporate high rupture current rated fuses, gas tubes, MOVs, etc, as well as rugged physical construction to contain the blast should something fail catastrophically.
 
I had an old Lutron multimeter from 1998 and literally smoked it ten years later trying to measure 900VDC on the 1000V range: combination of a cheap meter with a well used range switch allowed tracking to occur and set the board on fire.
 
When I got a three phase line in the shop, the first thing I did was measure it. The 240 V line read over 500 V. I called the power co and suggested that they had hooked me to 480 V instead. I was told, not very politely, that they would never make such a mistake. I went home for dinner and came back to the shop in the evening. The line read right on 240 V. To shorten some serious head scratching, the culprit was an AM radio station about 1/2 mile away. My little pocket analog meter was rated 60 to 400 cycles. Because there was a small fall off at 400, they added a capacitor across the resistors that slightly increased the current to the meter at higher frequencies. That was fine at 400 cycles but at 1,000,000 cycles it was a very low impedance, way overdriving the meter. The radio station was a daylight only one and was off the air in the evening. When I checked with my Fluke digital meter, the readings were consistent day and night.

Bill
 
Heh.

In my attic you can get loudspeaker volume from the local AM radio station (WLNA)
via a crystal set. Granted it's an old Western Electric speaker horn, but still that's
an impressive number of volts/meter.....
 
I'm all for using a meter for a "real" measurement. It's what engineers do, right?

But for checking power, there's a lot to be said for a "Wiggy". (originally a "Wigginton Voltage Tester" I believe)

First, the Wiggy will take any common voltage and tell you what it is. So you can check an unknown outlet and it will identify it for you, AC or DC from 120V to 600V.

Second, the Wiggy takes some actual power to make an indication. So, it will NOT be fooled by capacitive leakage to a motor case, or to an un-energized wire. A meter is usually so high an impedance that it will read a pretty good voltage on a wire that is near energized wires, but not connected to them. Won't happen with the "Wiggy", it separates "leakage" from a real power connection..

Third, it is pretty hard to fool. Radio waves won't bother it, it's pretty much basically a solenoid with a spring-loaded plunger. Stone hammer simple, reliable, no batteries, and easy to check.

There are newer versions, Klein makes one, with lights that show the voltage. I don't know if they draw the kind of current the Wiggy does, so they may not have the same results.
 
"There are newer versions, Klein makes one, with lights that show the voltage. "

Fluke makes one of those. They work great. The continuity check has a light and
a beeper. Very handy.
 
I'm old, I have a Wiggy next to my Fluke - no batteries needed.
VT1CP_lg.jpg
 
Heh.

In my attic you can get loudspeaker volume from the local AM radio station (WLNA)
via a crystal set. Granted it's an old Western Electric speaker horn, but still that's
an impressive number of volts/meter.....

When I used to work nights at the local radio/t.v. station....you couldn't pick up anything else.

That hill (just south of Erie, holds 4 local t.v. stations) and we ran 3.5 mw IIRC and WJET ran 4mw.
Along with 35kw f.m. and some others up there ran 50kw f.m.
 
AM radio stations, voltage spikes, maybe Tesla was onto something ?

I have Tesla's own description of radio wave propagation. He thought most was carried through the ground, which means that if he was right, aircraft and satellite communication would not work. Besides, he was dispersing most of it to the universe where it would be wasted except for the small amount intercepted by customers. It was done, though. During the unregulated period of broadcasting, Powell Crosley had WWL up to 500 Kw. A couple of engineers put an electric motor in a small car with an antenna feeding a rectifier and drove it around the WWL antenna. They couldn't get very far away.

Bill
 
I have Tesla's own description of radio wave propagation. He thought most was carried through the ground, which means that if he was right, aircraft and satellite communication would not work.
Bill

I once saw a very complex derivation that proved (or purported to prove) that the electric current was carried in the insulation, and not the conductor. I decided not to slog through the 5 pages to see where the problem was....
 
During the unregulated period of broadcasting, Powell Crosley had WWL up to 500 Kw.

That wasn't exactly "unregulated". Crosley had to petition the FCC for special authorization to do so, claiming that the experience gained would advance the state of the radio art or whatever. He kept renewing this "experimental" authorization until other stations complained loudly enough about interference, lost listenership and unfair competition that the FCC had to withdraw the temporary permit, and cut WLW back to its former 50kW.

A couple of engineers put an electric motor in a small car with an antenna feeding a rectifier and drove it around the WWL antenna. They couldn't get very far away.

Cool. Have heard a lot of neat stories about WLW starting grassfires by causing arcing between fence wires, being received on dental fillings, zapping people in the shower, etc., but not that one. Where did you find that?

Lotsa cool WLW stuff here:

WLW's Big-Arse Transmitter
WLW's 500,000 Watt Transmitter - YouTube
JIM HAWKINS' WLW Transmitter Page
 








 
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