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secutanudu 02-17-2010 07:56 AM

Using a multimeter inside a panel
 
There was a short in a J-Box in my parents house that left everything in the box dead. No breaker tripped (tried turning them all off and on).

I finally got around to looking at it...I figured I'd check each breaker to make sure it has voltage. I used my Fluke T5 in voltage mode, and touched one probe to the breaker screw, the other to the neutral bus bar. They were all working fine, which didn't help me solve my problem, but I did think of the possibilty of arcing the hot & neutral bars inside the panel.

Is this something I should be worried about? Is there any function in a multimeter that would lead to an arc when doing tests like I was doing? Is what I was doing safe?

jbfan 02-17-2010 08:30 AM

Yes, it does happen.

I was installing a neutral wire in a live panel, slide my screw driver into the screw, and found a skinned hot wire.
Took a nice chunk off the tip of my screw driver and tripped the breaker.
House 15 years old, and I was the first person into the panel since it was built.
I'm just glad my hand did not find that wire.

HooKooDooKu 02-17-2010 08:37 AM

You can not cause an arc through a (properly functioning) multimeter used in voltage mode because in this mode, the meter is a high resistance device. Virtually no current is flowing through the meter, so an arc can not occur.

Now if a meter were setup to measure current, they you've got the equivilent of a a bare wire between the connections and arcing could possibly occur (at least until a breaker trips, a fuse breaks, etc).

On the topic of poking metal parts around inside a live panel being "safe", I'll leave that to others to discuss.

secutanudu 02-17-2010 09:28 AM

I didn't think meters could measure current, at least not the kind with probes. My understanding was that you need a clamp- or jaw-type meter for that.

Basically, what I'm asking, if you simply touch one probe of a multimeter to a hot and one to the neutral bar, is there a mode on the multimeter that would cause an arc? I get that if your screwdriver or other conductive device accidentally touches both hot and neutral you would...I am specifically asking about a multimeter.

This is the one I have:

http://us.fluke.com/usen/Products/Fluke++T5.htm

HooKooDooKu 02-17-2010 09:42 AM

Quote:

Originally Posted by secutanudu (Post 401478)
I didn't think meters could measure current, at least not the kind with probes. My understanding was that you need a clamp- or jaw-type meter for that.

Basically, what I'm asking, if you simply touch one probe of a multimeter to a hot and one to the neutral bar, is there a mode on the multimeter that would cause an arc? I get that if your screwdriver or other conductive device accidentally touches both hot and neutral you would...I am specifically asking about a multimeter.

This is the one I have:

http://us.fluke.com/usen/Products/Fluke++T5.htm

The type of multi-meter I had back in college did indeed measure current. However, from what I recall, you had to move the probe to a different input slot (i.e. the meter had three slots for probes, Common, Volt/Ohm, and Current). It also had a 10 amp fuse inside for over-current protection.

When I look at the details of specifically the T5, it looks like it too can measure current, but it looks like it does so indirectly by placing a wire between the jaws and detecting the electric field. By contrast, the "old school" meters measured current by placing the device in the current flow path.

AllanJ 02-17-2010 10:26 AM

You will blow out the meter (and possibly see a small arc) if you try to measure continuity (or ohms) with power turned on.

Do not poke the meter probes (test leads) into a box or panel with power on unless the meter is set to AC volts, 250 or higher. (You can use lower voltage ranges when you are sure the maximum voltage you will encounter is less.)

The most common arc scare when using a meter comes when your hand slips and the probe or screwdriver touches two things and shorts them together.

When using an old school meter to measure current (amperes) you disconnect wires in the circuit and connect the meter into the circuit. For example remove one wire from a screw, attach that wire to one probe, and touch the other probe to the screw. You don't just stick probes in. The majority of consumer multimeters, if they measure current the connect-into-circuit way, have a maximum measurement of five amperes or less.

Billy_Bob 02-17-2010 11:48 AM

I think I just figured out why the multimeters in retail stores they sell can only measure up to 10 amps DC!

I get upset when I see these. What is 10 amps? Everything is higher than that! What good is this for? Etc.

And it is quite easy to install a "shunt resistor" to give the multimeter a higher amperage capacity...

But NOW I see why they do this! In the wrong hands and on the wrong setting, a higher capacity amperage multimeter could cause some serious arcing and damage!

Anyway here is a somewhat reasonably priced 600 amp AC/DC clamp meter safe in all hands...
(Many clamp amp meters are AC only and cost a fortune.)
http://www.amazon.com/gp/product/B00...&redirect=true

About amperage shunt resistor...
http://www.allaboutcircuits.com/vol_1/chpt_8/4.html

secutanudu 02-17-2010 11:54 AM

Quote:

Originally Posted by AllanJ (Post 401499)
You will blow out the meter (and possibly see a small arc) if you try to measure continuity (or ohms) with power turned on.

I'm pretty sure I did that...with no arc or blowout. I guess I got lucky...

HooKooDooKu 02-17-2010 12:23 PM

Quote:

Originally Posted by secutanudu (Post 401556)
I'm pretty sure I did that...with no arc or blowout. I guess I got lucky...

I could see where some meter manufacturers might have their meters designed to avoid this type of situation since it can be a relatively simple mistake (i.e. setting knob turned to the wrong setting).

Given that at typical multimeter tries to measure ohms by applying a small voltage across the terminals and measure the resulting current, trying to test continuity (or ohms) on a live circuit at best will give a false reading, at worst, what AllenJ says.


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