test questions

[gunpowder]

Just passed md contractors exam & couple of questions are still stumping me:
1.What % of secondary current is protected by primary ocpd?(I forget trx setup or %s)
2. Whats minimum times current is a fuse allowed to protect >600v service? Smallest choice was 1.
3. What is voltage to ground for b phase on 240v ungrounded(?) delta? I chose 208, but what's reference ground, primary ground?
ty

 

[gunpowder]

4. What's smallest size conductor allowed in parallel? I chose 1/0, but couldn't also be 6awg for gec?

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[kwired]

Just passed md contractors exam & couple of questions are still stumping me:
1.What % of secondary current is protected by primary ocpd?(I forget trx setup or %s)
2. Whats minimum times current is a fuse allowed to protect >600v service? Smallest choice was 1.
3. What is voltage to ground for b phase on 240v ungrounded(?) delta? I chose 208, but what's reference ground, primary ground?
ty

4. What's smallest size conductor allowed in parallel? I chose 1/0, but couldn't also be 6awg for gec?

Sent from my SAMSUNG-SM-G891A using Tapatalk

1. Not sure the actual question here, but thinking you are looking for primary to secondary ratio to be involved here?
2.
3. If it is an ungrounded system the voltage to ground can be fairly unpredictible as there is no ground reference until there is a ground fault on something.
4. 1/0 is the correct general rule answer. You can't parallel GEC's or EGC's for the purpose of effectively making a larger capacity conductor out of smaller conductors. You still end up with parallel paths in many instances though.

 

[infinity]

1) Look at 450.3
2) Not sure the way the question is written.
3) Ungrounded=0
4) Forget GEC's, #1/0 look at 310.10(H).

 

[kwired]

1) Look at 450.3
2) Not sure the way the question is written.
3) Ungrounded=0
4) Forget GEC's, #1/0 look at 310.10(H).

Your #3 answer vs my #3 answer is all about capacitive effects, otherwise the theoretical answer is 0, if there are no ground faults in the system.

 

[ggunn]

Your #3 answer vs my #3 answer is all about capacitive effects, otherwise the theoretical answer is 0, if there are no ground faults in the system.

I would say the answer is "undefined". If you put a meter from the B phase to ground, you probably won't see 0V, but rather some nonsense number. If "none of the above" is a choice, then it's the right one no matter what the other choices are, IMO.

 

[kwired]

I would say the answer is "undefined". If you put a meter from the B phase to ground, you probably won't see 0V, but rather some nonsense number. If "none of the above" is a choice, then it's the right one no matter what the other choices are, IMO.

I agree.

 

[ggunn]

I agree.

Some test questions are designed to make you burn up test time trying to do calculations if you do not understand the underlying principle that makes the calculations unnecessary or futile. Sneaky.

In a multiple choice test the first thing you should do on every question is to see which of the answers you can rule out without doing much if any calculation.

 

[kwired]

Some test questions are designed to make you burn up test time trying to do calculations if you do not understand the underlying principle that makes the calculations unnecessary or futile. Sneaky.

In a multiple choice test the first thing you should do on every question is to see which of the answers you can rule out without doing much if any calculation.

Also agree.

 

[infinity]

I would say the answer is "undefined". If you put a meter from the B phase to ground, you probably won't see 0V, but rather some nonsense number. If "none of the above" is a choice, then it's the right one no matter what the other choices are, IMO.

I would guess that the answer undefined wasn't on the test so you would need to look for the theoretical answer which should be 0 since it says that it's ungrounded.

 

[ggunn]

I would guess that the answer undefined wasn't on the test so you would need to look for the theoretical answer which should be 0 since it says that it's ungrounded.

That's not the theoretical answer. 0V says that they are at the same potential, i.e., the conductor on which you are measuring the voltage to ground is grounded.

 

[Ingenieur]

Question 3, this may help
pg 4, fig 6-6
http://static.schneider-electric.us/assets/consultingengineer/appguidedocs/section6_0307.pdf

 

[infinity]

That's not the theoretical answer. 0V says that they are at the same potential, i.e., the conductor on which you are measuring the voltage to ground is grounded.

So if I take my Wiggy and go from any phase to ground it will read what? In my mind the author of the question was thinking ungrounded = 0 volt to ground from the B phase. If we knew the answer choices that would clear it up.

 

[kwired]

So if I take my Wiggy and go from any phase to ground it will read what?

Very likely a different reading then if you had a high impedance meter.

 

[GoldDigger]

Normal stray capacitance will cause the ungrounded three phase to float very close to ground.
For an ungrounded 240V delta, A, B, and C will be about 139V to ground on a high impedance meter.

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[Ingenieur]

Normal stray capacitance will cause the ungrounded three phase to float very close to ground.
For an ungrounded 240V delta, A, B, and C will be about 139V to ground on a high impedance meter.

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:happyyes:
The link I posted gives a good explanation

 

[gunpowder]

So if I take my Wiggy and go from any phase to ground it will read what? In my mind the author of the question was thinking ungrounded = 0 volt to ground from the B phase. If we knew the answer choices that would clear it up.

Cant recall exactly, but i do remember 0, 240 & 208 being choices. I went back and forth between 0 & 208, finally choosing 208 because there would have to be some kind of grounding system, whether it terminated in xo of that secondary, the primary, or ges to the service. At least that was my thinking, under the gun, at the time. :? Thanks for the great responses.

 

[gunpowder]

240/sqrt3=139.
cant recall if that was a choice.
thanks ingenieur!

 

[domnic]

conductor parallel . queston #4

conductor parallel . queston #4

4. What's smallest size conductor allowed in parallel? I chose 1/0, but couldn't also be 6awg for gec?

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The smallest conductor allowed in parallel is 20 awg.

 

[jumper]

The smallest conductor allowed in parallel is 20 awg.

Maybe some esoteric rule or exception says that, but most tests would want 1/0 per 310.10(H).