test questions

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gunpowder

Member
Location
Glen Rock, Pa
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
 

kwired

Electron manager
Location
NE Nebraska
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?

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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

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
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

Electron manager
Location
NE Nebraska
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

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
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

Electron manager
Location
NE Nebraska
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

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems

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

Electron manager
Location
NE Nebraska
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

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
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

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
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.
 

infinity

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Staff member
Location
New Jersey
Occupation
Journeyman Electrician
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. :)
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
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

Senior Member
Location
Earth
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

Member
Location
Glen Rock, Pa
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.
 

domnic

Senior Member
Occupation
Electrical Contractor
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.
 
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