ECN Forum Forums General Topics of the Trade General Discussion Area Class stumped?

I agrre. The question is so vague it causes more confusion than it teaches. I mean hey it confused a whole class. Sounds more like an author is nitpicking about semantics.

Are you kidding, they sue Jesus himself if they could drag him to court.



[This message has been edited by IanR (edited 10-05-2005).]

I agree, it's a very unfair question in my mind.

Option D doesn't even cover an open circuit, one of the two basic categories of main faults (shorts and opens).

In an ungrounded system you can have verrying degrees of ground faults. When we thump grounds we have a ammeter and the thumper generates a thump in relation to the quality of the ground fault. I have seen 2amp grounds which are hard to find then I have seen 10 amp grounds that are easy to find.

Option D doesn't even cover an open circuit, one of the two basic categories of main faults (shorts and opens).

A fault by definition is any situation where the resistance between the current carrying conductors themselves and/or the neutral and/or ground decreases to where current begins to flow through the resistance. A dead short is one example, a squirrel (also dead) is an example of a partial fault.

Notice that an open would not fall into this definition and should not be referred to as a fault.

Answer "D" is correct.

-Hal

There are always ground faults according to our LIM's (Line Isolation Monitors)We use them instead of GFCI in operating rooms.Basically they will read from .2 to .8 ma with special silicon gel low leakage appliance cords etc in perfict condition.The LIM will go into warning at 1.5ma and full alarm at 2ma.Dust between the receptacle screws or even wiping down the OR table with a damp cloth will set it off.
I would say partial ground faults are everywhere its just where and what point it becomes a problem.

WFO hit it right, it has to do with a high impedance grounding system.

it has to do with a high impedance grounding system

No, a high impedance grounding system would not be a fault since it is intended to operate that way.

-Hal

Quote:
"No, a high impedance grounding system would not be a fault since it is intended to operate that way."

Re-read my post. I was referring to unintentional grounds; specifically those involving conductors whose current flow is only limited by the impedance of the soil they lay in.

Quote:
"Notice that an open would not fall into this definition and should not be referred to as a fault."

This got me to thinking about the example I referred to above. We once had a 7200 volt line down that didn't trip the breaker in the substation. We put an ammeter in the metering circuit and could not get a descernable reading. So even though the line was physically in contact with the earth, it was technically an open circuit.

A fault or not?

On the other hand, I've seen a damaged underground 240 volt feed to a barn spin the KWH meter like a helicopter, but never trip its' breaker because the earths' impedance kept the current below it's trip point.

So let's look at one other example (just to play devils' advocate). The 240 volt heating element (4500 watt)in a hot water heater loses its insulation at precisely the midway point of the heating element. Since each half of the element is is now at half voltage (120 to ground through the tank) through one half the resistance, the current flow has not changed and the water still heats. Is it a fault?

"is it a fault"

Technically yes because the system is not operaring as designed. Additionally now you have a 120 volt load using the EGC as a current return path. But I can almost see what you are getting at.