Testing Blown Fuse

[erickench]

Here's another question:

In order to find a blown fuse on a three-phase 208/120 volt ac panelboard, it is best to use

A. fingers across each fuse
B. a small neon test lamp connected from the line side of each fuse to ground
C. a 50-watt 120-volt test lamp placed from ground to the load side of each fuse in turn
D. an ohmmeter across each fuse on the panelboard

Which is the correct answer?

 

[roger3829]

I always use "A"

 

[Cow]

Personally, I like using a voltmeter, rather than an ohmmeter. But I guess the test makers didn't think that was the best method....

 

[erickench]

I would've chosen 'D' an ohmmeter. But the answer key gives 'C' as the correct answer.

 

[Besoeker]

Here's another question:

In order to find a blown fuse on a three-phase 208/120 volt ac panelboard, it is best to use

A. fingers across each fuse
B. a small neon test lamp connected from the line side of each fuse to ground
C. a 50-watt 120-volt test lamp placed from ground to the load side of each fuse in turn
D. an ohmmeter across each fuse on the panelboard

Which is the correct answer?

Is the live side of the fuse live at this point?
If so, connecting an ohmmeter across the fuse wouldn't be a smart move.

 

[gar]

110415-0940 EDT

As with all multiple choice questions you have to analyze the answers for the probability of correctness.

A is a safety problem. Probability 0.
B produces a result independent of the condition of the fuse. Probability 0.
C may be a valid choice if there is no back-feed from somewhere. Probability high.
D may be valid if the fuse is removed. With the fuse installed, power and a load present, this won't work. First, it would probably destroy the ohmmeter or something in it. Second if no power was applied there is likely a low resistance DC path thru the load and source (transformer secondary of the source, neutral wiring, and the load.). Probability 0.

Back to C. With power applied I would first check the output voltage from fuse output to neutral, and maybe to ground. For a blown fuse I would expect near 0 volts. Second, I would check voltage across the fuse. Should be near full line voltage if a load is connected, and fuse is blown. Then, if it appears blown I would pull the fuse and check its DC resistance. If these results do not correlate, then I would study the system further for what maybe wrong.

Anyway on a probability basis answer C wins.

.

 

[don_resqcapt19]

I would say "none of the above" but if I had to make a choice of the available answers, the only one that is close to being valid is "C".

"A" was probably an answer in a very old electricians handbook...the one that said you could test for voltage using your fingers, but for that method to work you would have to be isolated from ground. If you are isolated from ground and place your fingers across a good fuse you will not feel the voltage, but if the fuse is open you would. .