OT: Tell me whats wrong with this situation...

TConnolly

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A 5A CT is connected to a panel mount amp meter. Engineer doesn't believe the amp meter, so he grabs his multimeter and a couple of alligator clips. He removes one CT lead from the lug on the amp meter and opens the alligator clip lead all the way up and puts it on the lug. Then he puts the other one on the CT lead and clips them to the ends of his multimeter probes and configures the multimeter to measure curretn.

Would you do this? Why or why not?

I sure as hell wouldn't do it.
 
Quite simply, once he 'open circuited' the CT, the terminal voltage will rise to dangerous levels, If the secondary circuit has a very high resistance (open circuit) then the CT will still attempt to drive secondary current, hence the terminal voltage will rise.

Paul
 
He did shut down the power before connecting to the CT. But there were still a number of things that could go wrong with the setup that would quickly take the voltage well over the meter's rating or the alligator clip lead insulation rating. The alligator cips could pop off, (which is very likely if the jaws have to be opened all the way to get around the stud) or the meter dial could be changed or the leads bumped or the fuse blows, etc. Any of these would virtually instantly expose the set up (and by extension, any one holding it) to voltages that far exceed the instrument ratings.

Always ask yourself "What could go wrong and what will that do?"
 
"Tell me whats wrong with this situation.."

You calling the guy an engineer. Thats the only error I could find in the situation.
 
If he did the following:
Removed power.
Attached multimeter with the range set to the same as the CT.
Reconnected power.
Watched the meter display from a distance.
The whole time making sure noone else can come close to the leads.
When done, remove power and reset to normal.
Then it would be OK by me. The worst that can happen is that the fuse blows and the meter gets toasted.

If you know what you are doing, and you think safety all the time it is OK to do many abnormal things. Sometimes you have to.

I will do the same if I am desperate enough. But only if a clamp am-meter is unavailable.

PS. I am an engineer.
 
Y'all get worked up over some silly things at times. The voltage is what the voltage is, usually 480vac here in the US, I did not understand all that stuff about voltage rising.

For almost 25 years I have used a Fluke 87 multimeter; which is rated 1000v and 10A, so I would assume his meter has the same ratings if not higher. I have the test lead kit which has the wide jawed alligator leads, they are designed to fit on screw terminals but do need to be fully opened but in my younger days I did make leads with plain ol alligator clips. I do not think anything would happen to the meter, as long as standard low voltages are involved.

The method described is one way to test the panel meters readings, if it was not then why have the meters capable of doing it? There was a time when I did this a lot.

I do not know what kind of meter he has but if it is a Fluke then he can get a current clamp-on attachment for the meter that may make situations like this easier and possibly safer: http://us.fluke.com/usen/products/category.htm?category=PRCU(FlukeProducts)&parent=ACC(FlukeProducts)
 
A CT is a transformer, so the voltage it generates is not directly related to the common supply voltage.
I dont remember offhand, but I think you can achieve secondary winding voltages above 1 kV. This voltage is only achieved with open leads. Even at this high voltage you should be safe even if you hold the meter in your hands. Reputable multimeters are double insulated from its outside. This to make them safe to work with even at high voltages. But they may destruct at a lower voltage.

Still, I prefer to be double safe and would avoid touching the entire setup during the test.
 
I am also an engineer, and as stated above i would also have cerried out this work if i felt it was necessary following as many safe practices as possible.
While working on an open CT the voltages can rise into the KV range. The math for this is simple.

V= I x R

it is a current transformer so say it will induce 2.5A in the meter circuit.

if the meter circuit is a closed loop of low impedance, say 2 ohms

v = 2.5 x 2
= 10v

safe enough

bad connection 100 ohms

v = 2.5 x 100
= 250V

getting there

no connection infinite ohms

v = 2.5 x infinity (use 10Mohms)
v = 25000000

it wont get to this the CT will go on fire

i have never seen this happen only studied it in a classroom.

anyone seen it happen? some good stories?
 
I think I knew this but getting old and CRS.

Whether it is 120, 240, 380, or 460 VAC, then I made sure not to touch the wires or hopefully not allow them to touch metal etc.

I think I get the rising voltage thing, had to do some reading, naturally many debate this subject. Anyway, my interpretation is a CT will have a flux field that is low (ckt has low impedance) when in ckt but when open circuited this flux field will see a high impedance which increases the voltage.

NOW that happens with standard CT's but multimeters ususally use the Hall Effect which changes the risk factor, I forgot the fact that the panel meter is probably working from a CT which if open circuited could apply higher voltages to the meter.

I stopped doing this long ago and got one of the clamp on accessory units that works with my Fluke. It just saved an enormous amount of time plus I did not have to handle live wires.

Technically I guess I agree with Alaric, even though I have done it I probably would not these days.
 
A CT is a transformer, so the voltage it generates is not directly related to the common supply voltage.
I dont remember offhand, but I think you can achieve secondary winding voltages above 1 kV.
I believe the voltage in the CT will depend on how much current is flowing in the conductor it is around. How else would get a 5 to 300 or what ever ratio?
 
Thomas Sullens said:
I believe the voltage in the CT will depend on how much current is flowing in the conductor it is around. How else would get a 5 to 300 or what ever ratio?

Yes, when it's connected, when you break it under power, you get something like an automotive ignition coil discharge spike in volts...
 
Remember all transformers have 2 ratios. Tyically they are rated in voltage. 4 to 1 as in 480 to 120 volt transfrmers. The current ratio will be 1 to 4.

With a CT you get the CURRENT ratio. 300 to 5 makes the voltage 5 to 300. If you have 480 on the primary, having the secondary loaded with a burden you get the small voltage desired. If it opens, the secondary voltage rises to the turns ratio.

Multiply 60, which is the actual turns number and 480 and you get 28,800 volts. That is why CT's tend to burn up when they are not burdened. The high voltage breaks down the insulation. It aint nice when a ct lets you know it is not burdened.

If you insert a multimeter in series with the ct secondary to measure the amps, no harm no foul provided the circuit remains closed. I personally will not do that as I prefer to use a clamp on and read it a whole lot safer.
 

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