Test for 240VAC 3 Phase circuit power?

[biff44]

Q1: How does one go about one-handed testing using a standard voltmeter to see if a 240VAC 3 phase circuit is de-energized?

In single phase, I could just hook an aligator clip to neutral or earth ground, and use a voltage probe to see if any of the wires are hot.

But in 3 phase, can I do the same thing? The 3 phase voltages are not really referenced to earth ground, so how do you make the measurement?

Question 2: can a "non contacting voltage sensor", like an Extech 623, accurately predict if a 3 phase circuit is not energized?

Thanks.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[mcgyvr]

measure L1-L2, L2-L3, L1-L3

 

[ScottyUK]

Is the three phase supply from an earthed source? If the source is earthed then you can measure the line-neutral voltages to determine whether it is de-energised or not. If the source is ungrounded then measurement of line-neutral voltage will not prove that the supply is definitely de-energised, in which case mcgyvr's solution is a suitable method of testing.


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If we learn from our mistakes I'm getting a great education!

 

[biff44]

Yeah, that would do it, but violates the one-hand test rule! I would have to hook up a lead to one of the wires first. If I am not sure if a line is de-engerized, how can I safely hook up an alligator clip probe to L1 without frying myself. I obviously can not hold one lead to L1 and the other lead to L2 using both hands, as that is really dangerous.

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[dpc]

If I am not sure if a line is de-engerized, how can I safely hook up an alligator clip probe to L1 without frying myself.

You can't - so you should hire someone who knows what they are doing and has the proper test equipment and saftey gear. Seriously.



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[ScottyUK]

"I obviously can not hold one lead to L1 and the other lead to L2 using both hands, as that is really dangerous."

Obviously. But in reality that is what the rest of us do when necessary. The denger is reduced if you use suitable test leads with minimal exposed metal and HBC fuses within the lead, and if you are working with very high energy sources you should consider arc flash PPE even if you are not bound to use it by legislation.


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If we learn from our mistakes I'm getting a great education!

 

[biff44]

What is your opinion on something like this?

http://us.fluke.com/usen/products/Fluke+1ACII+VoltAlert.htm?catalog_name=FlukeUnitedStates

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Maguffin Microwave wireless design consulting

 

[oldfieldguy]

biff--

good for troubleshooting, but a bad thing for safety.

Our company requires the use of gloves rated for the proper voltage when doing live testing. It's inconvenient, and I know that for most of my career I never used such protection, but that's the way it is now. Inspect the gloves before using them.

Use a contact voltage meter for safety. Test it first on a known live source, then test what you're going to be working on, then retest the meter on the known live source. That way you KNOW the meter was good when you checked voltage on something you're going to be touching.

Seriously, if you've not done this sort of thing before, get a qualified and knowledgeable person to guide you through it. Electricity can kill you for a very long time.

old field guy

 

[ScottyUK]

They're ok as a quick go / no-go indicator. Much as I like Fluke's products I would not use one as a proving device immediately prior to grabbing hold of the copperwork though: something like this instrument would be my preference for proving dead at LV. These are the accepted standard proving device throughout UK industry. Prove the instrument works, then test the circuit, then prove it still works afterwards. That way there are no nasty surprises.

I am not sure how any of these devices behaves on an ungrounded source when the instrument is connected line-earth, or at least I'm not sure how reproducible whatever behaviour it did exhibit would be, hence the earlier question about understanding the nature of the system you are trying to test.


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If we learn from our mistakes I'm getting a great education!

 

[biff44]

Good suggestion.

I am only thinking about using a non contact voltage sensor to tell me if the mains switch worked and de-energized the circuit enough for me to want to get near it. If the switch is off, the non contact sensor says it is de-energized, and the electrical hum goes away, then I would feel safer using two hands and a standard voltmeter--as suggested above, L1 to L2....

This is on the supply to a glass oven, and it is a little hard to get at the terminals because of the extreme heat.

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[ScottyUK]

OFG,

Same thoughts at the same time!


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If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

There is also no way of knowing that the terminal, which assumed to be a ground is not energized to begin with. So whether one hand or two hands are used, proper safety gear/gloves and instruments are necessary.

Rafiq Bulsara

http://www.srengineersct.com

 

[biff44]

I was looking at gloves too. That would solve the problem of not being sure that the disconnect worked. But this is an oven running at 2300 deg F. All I could see for gloves rated to 600VAC were rubber gloves, or leather covered rubber gloves, which may melt. Does anyone make synthetic rubber gloves that have a very high operating temeperature?

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Maguffin Microwave wireless design consulting

 

[rbulsara]

Oh, this is new information!.

Then according to your original post, which alligator clips you were trying to hook up to a bus at that temp? Oven temp may be 2300F, are the electrical connections too?? I am not aware of any 600V buses rated for that temperature or the standard measuring instruments that can work at that temp.

I think your expertize is microwaves/wireless and not power. I would hire some professional/licensed electrician to take the voltage measurements.

Rafiq Bulsara

http://www.srengineersct.com

 

[biff44]

No kidding!

The terminals are, obviously, on the outside of the oven, but still are pretty hot. Apparently they breakdown every year or so and cause some interesting fireworks. I have not seen the setup yet, but was asked some questions about the situation by a bunch of artists, and am trying to figure out what to tell them and how to keep them alive.

I am interested professionally, though, too becasue I am starting to deal with high power microwaves on a different project, and they might be fed by 3 phase ac.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[rbulsara]

If safety is the issue, probably a visible disconnect will take all the guess work out, regarding the presence of the voltage. This does help the measurement.

Rafiq Bulsara

http://www.srengineersct.com