Measureing voltage across A phase line ,load

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khixxx

Senior Member
Location
BF PA
Is this a good practice? My old supervisor told me to use my fluke 87 3 meter and get a reading for A phase. measureing AC volts with my meter leads from the load side of A phase and the other lead on the line side of the A phase and if i read more than 1 volt there is a problem. he said this is common for motorstarters, relays, breakers. Is this a common practice for trouble shooting?
 

bphgravity

Senior Member
Location
Florida
Re: Measureing voltage across A phase line ,load

In order to obtain a voltage reading, the leads of the meter must be connected between two points of different potential. What you are suggesting is taking a readng of two points on the same conductor. This does not make much sense to me.

Any reading that you may get is most likely some sort of ghost voltage or a false reading from neighboring conductors.
 

Ed MacLaren

Senior Member
Re: Measureing voltage across A phase line ,load

This is one way to check for higher than normal resistance in a component or connection, providing it is done with the load in operation.

The voltage drop across a switch contact or connection is proportional to it's resistance.

I would suggest that, when using a digital voltmeter, you compare the no-load reading with the full-load reading.

Why just phase "A"? This test could be useful on any phase.

Ed

[ May 18, 2005, 06:42 AM: Message edited by: Ed MacLaren ]
 

ctrider

Member
Re: Measureing voltage across A phase line ,load

It depends what you're looking for. I've used this practice to determine if a breaker has high resistance internally without shutting things down. A voltage drop, (more than 1 volt) through that leg would indicate a possible problem. Contacts on starters or relays will build up resistance over time due to arcing.
 

BruceH

Senior Member
Re: Measureing voltage across A phase line ,load

Voltage drop readings are very useful in troubleshooting both ac and dc circuits. It basically measures resistance under load ( if performed correctly) and is more conclusive than just measuring ohms in a inactive condition.
 

brian john

Senior Member
Location
Leesburg, VA
Re: Measureing voltage across A phase line ,load

This is a very common practice often referred to as FOP or Fall of Pontenial Test Met hood,, of course there must be a load, and balanced loads make it easier to determine if the results are a problem. I usually lok for problems with balanced loads with a 20%-30% difference between the poles. I think at 1 volt you are into problems.

Of course this depends on size of equipment and type of equipment.

BP due to voltage drop there is always a difference of potential, along the length of any conducting body carryinig current.
 

Nick

Senior Member
Re: Measureing voltage across A phase line ,load

Where did you guys come up with 1 volt as the magic number? When I have seen this done before it was in the millivolt range. :confused:
 

khixxx

Senior Member
Location
BF PA
Re: Measureing voltage across A phase line ,load

hey ed A phase was used just to keep my post short. any phase will do. i used 1 volt because thats what my old supervisor told me to use as a rule of thumb. i'm not sure if there is a fomula or a better number to use. yeah i mostly see mv to. thanks guys for the input. another question would be is there any range limit as far as amps that you can do this test safely. ex. less than 60 amps or can you do this test up to 600 amps ect. thanks again.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Measureing voltage across A phase line ,load

Most likely the level of PPE required to do this test will increase with the amp rating of the circuit being tested.
Don
 

bphgravity

Senior Member
Location
Florida
Re: Measureing voltage across A phase line ,load

I am reading a relay spec sheet as I write that states, "Voltage drop of 1V - 1.6V should be expected through this device." There goes the 1 volt theory. I would think IR readings would be more reliable, unless you know the manufacturered resdistance of the device in question.
 
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