Proper use for meters


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Old 08-01-13, 07:41 AM
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Proper use for meters

I am still doing an upgrade on an older house, that being said I have to move two power cables in the rood crawl space to allow access for a vent in the bathroom. I have two inductive meters I used for years in the automotive field one measuring current at 1/10 of an amp and the other at 1/100 of an amp as I used to do a lot of electrical diagnosis and repair on computerized control systems in high line cars. My question is can an inductive meter be used to determine if an AC circuit is live. In a DC circuit it was a simple and quick check but I don't have a lot of experience working with AC so that's my question for the morning, inductive meter of do I need to penetrate the sheathing with a probe and voltmeter. Thanks again
 
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Old 08-01-13, 07:58 AM
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Inductive meters only work on one conductor....... not one cable. A cable has more then one conductor.

Inductive current meters used for DC work can not be used on AC work unless there is an AC amp position on the dial. An inductive meter for DC uses batteries and a Hall effect current detection system. With DC the current flows only in one direction.

An AC inductive amp meter is strictly magnetic and many require no power. AC flows in two directions. If you put the AC inductive meter on a cable then the current flow would be cancelled and you would read 0.

You will need an AC voltmeter or an inductive AC test probe to check for power.
 
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Old 08-01-13, 08:14 AM
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PJ, tanks for the answer, I read it and dug out the operators manual in the garage. It indicates that I have to measure each separate leg of the circuit individually as in a 110 circuit I guess the current goes in one wire and returns via the other canceling the induced emf on the cable. That being said using my meters would require separating the two wires in a cable, or three I guess depending on the cable. My terminology might be a bit rusty as it has been a while. Both my meters in fact all of my meters are Fluke and they all have ac as well as dc ranges including probes as well as inductive clamps. I am however more familiar with hunting a spurious draw of 300 ma instead, if my assumptions are incorrect then please share as it has been a while since I did this stuff. Thanks again
 
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Old 08-01-13, 08:16 AM
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110 circuit
No 110 volt circuits in the U.S. We have 120 and 240 volt circuits.
 
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Old 08-01-13, 08:30 AM
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Thanks for the correction, is that a RMS value. In the old days we used lab scopes for chasing little variations that effected computer systems. AC leakage in a DC system was a biggy at that time. I did walk the house a minute ago and found 118.4 to 120.5 variation in the measured voltage.
 
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Old 08-01-13, 08:57 AM
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Yes, you get the A for this week. That is the root mean square voltage not the peak inverse. And I get a C for not writing ~120/240.
 
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Old 08-01-13, 09:13 AM
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One note...a current meter of any type will only tell you if a circuit is in use and has current flowing. It won't tell you if a circuit is live (has voltage applied).

Voltmeter is what you want. Whether it be a digital readout or analog or Wiggins type. The non-contact type are problematic. I don't think they will give false negatives...but lots of false positives. Never had one...just basing it on comments here.
 
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Old 08-01-13, 09:58 AM
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Vic, that was a good answer and brought a number of thoughts to mind. I was an instructor for an apprenticeship program at the end of my career. In a DC circuit voltage can be read up to the consumer and if the circuit has only one consumer there will be a 0 reading for voltage but the exact same current will be read on either side of the consumer be it a light bulb or whatever. In this case voltage isn't the killer but instead current is the dangerous element. An early ignition system might run at 15000v but no current and it will make you jump but it wont cook you, modern ignition systems have both a tremendous voltage potential but current as well and can send a person to the other side. How does this rule apply to an AC circuit, I don't believe there is a safe place to explore anywhere in a AC circuit, is that correct? I know you guys have better things to do than entertain old farts have flashbacks but perhaps others as well as myself can learn something about basic principals.
 
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Old 08-01-13, 10:49 AM
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Well...I'm no expert...but it's always safe to check a circuit using proper procedures. In the Navy that was supposed to be one hand in your back pocket and a rubber glove on the hand holding the probe. I did say "supposed to be"...almost impossible with electronics I worked on, though the electricians could maybe do it. I've never used such techniques in residential...but then, I've worked on very few 240V circuits.

With residential 120VAC normally you go to some sort of connection, be it light fixture, outlet, switch or junction box and verify OV hot to neutral and hot to ground. Sometimes you can get the probe in the wirenut and get a reading, sometimes you need to remove it. Digital meters can often show odd phantom voltages, so many pro's prefer analog meters. Analog loads the circuit enough to eliminate such readings. I don't think anyone has been seriously hurt from 120V that I know of, unless they fell off the ladder when they jumped from the bite. The breaker won't protect you so you could get full current (based on body resistance of course).

240V OTOH can be very dangerous since the circuits are normally higher amperage and the bite is much worse.

One thing you can't do with residential wiring which used to be pretty common in electronics is to use a needle tipped probe to go right through the insulation at the point you needed to check.

I got bit once by the output of a thyratron of a radar, which fed the magnetron. 15Kv through the sides of 2 fingers. No idea of the current, but it was up there. Tiny entry and exit burns, no damage where the fingers were touching each other, but I couldn't feel or move my arm up to the elbow for about 15 min. Probably lucky it didn't go in through one hand and out the other.
 
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Old 08-01-13, 11:47 AM
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Thanks for the dialogue, as for digital meters as it was explained to me a voltage Potential can be read without the ability to actually flow current say like in the event of a poor connection via a damaged internal board in a processor etc so in many cases a technician is fooled because he or she used a digital meter and saw what was considered voltage but with no current ability. The digital meter would show voltage but when even a small load is presented the value drops to 0v. I just made a circuit diagram for the attic and will go shopping first to secure a few needed parts and it's off to the races. The circuits have been cobbled together in the past to add functions, ceiling fans etc and the guy must have had a mile of green wire so I am going to restore the circuits and wire id back to something a stranger could look at and know what to expect. It's a cool day so the roof isn't a hundred degrees.
 
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Old 08-01-13, 06:13 PM
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A digital meter presents such a very low load to a tested circuit it actually doesn't make a very good device to use for routine electrical troubleshooting work. An analog meter presents a slightly higher load and can be a more effective electrical tester.

As an electrician I carry a wiggy...... a vibrating volt meter that applies a load to the circuit.
 
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Old 08-02-13, 08:16 AM
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I obviously didn't know a device like the wiggy existed. I read about it this morning and I will see about getting one or something like it. The wiggy itself is out of stock but it seems there are a lot of devices that serve the same purpose. Thanks again
 
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Old 08-02-13, 10:13 AM
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I carry three voltage testing tools with me. One is a non-contact tester that lives in my shirt pocket at work. I call it my "safety stick." The second is a relatively inexpensive fold-up digital multimeter that I've modified by adding magnetic sheeting to the outside so I can stick it on a panel door or inside an enclosure. That rides in my primary carry-in bag. The third is a rather large and heavy analog meter that stays in a bag of second-level tools in the truck unless I need to make some really accurate measurements and/or there appears to be a lot of noise in the current.
 
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Old 08-02-13, 04:59 PM
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Old 08-02-13, 07:18 PM
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If you really want to treat yourself. My tool of choice.

Fully digital with vibrating solenoid and a built in light. Fluke T+PRO

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Old 08-05-13, 09:21 PM
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Well we got it done. Both baths have been rewired. The extra GFI is in, the ceiling fan- heater is installed and the switches have been upgraded. That was an adventure. As for the meters, I did buy one of the Klein non contact tell tales which works real well and for the wiggy I tend to like the Fluke devices as I already own a small collection of them from my mechanic days, Thanks again for all the feedback and ideas. Next is the tub- shower replacement
 
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Old 08-05-13, 09:29 PM
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Next is the tub- shower replacement
The scuba training will come in handy here

Good job
 
 

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