Bright Spark

The answer is simple. Three words.

"Use Ohm's Law".

You need to do the rest on your own. If the question is still to complicated, simplify the words down to their basic meaning and draw a circuit.

LET THE LEARNING BEGIN!

Since a normal 240 VAC circuit is actually a [120V - Neutral -120V] circuit, the line potentials are only 120 VRMS above or below ground Thus for a zero ohm fault to ground, the ground impedance would be the current limiter were it not for the fuse.

Neither phase wire of the 240V circuit can supply more than the 15A fuse rating without the fuse blowing. Since the ground impedance would allow 120V/1.9ohms or something more than 60 amps to flow, the fuse will limit the current and blow at a current nominally around 15amps.

Dave Cooper

Top Reply! Just what I was hoping for Thanks

Not really a good answer, only if you live in a 110 volt 60 hz zone, USA or similar.

I guess you live in the USA....or similar!!

Sorry, not a good answer! In fact a really bad one!

Many countries (those not affiliated with the USA power wise) have 240 volts between phase and neutral, at generally 50Hz......380 volts phase to phase!!!

As you do not have a clue who the guest is, or where he lives in the world, on the grounds of safety alone, you should moderate your answer accordingly, so as to not chance someone getting killed because of your ignorance of other world power systems........

well said Andy. far too many contributors quote their local conditions or rules as if they applied all over the world,

regards

Chas

I (like you and severaal others) are first for safety in all of my answers. Not knowing where the guest lives, makes it slightly more difficult to be accurate.

Sadly, some people cannot think beyond their own 4 walls.......

The answer 126.3 Amps. Ohms law simple as that. I thought that being a question for an advanced electrical engineering course it would be a bit more tricky by using the max earth-fault impendance of the 15A semi enclosed fuse somwhere (which by British standards would be 2.67 Ohms) but no my lecturer assures me its correct at 126.3.

Sorry larged up the wrong person by mistake earlier should have been Jack even though he did belittle me!

Let the learning continue!

Hi guys.

The figure mentioned sounds right.

A 15A fuse will not change the earth fault current

It will still be 126.3A

It's just this will cause the fuse to blow very very quickly! And isolate the circuit.

Surely the fuse in your question must be a red herring.

Good luck with your course!!

No belittling was intended, the answer was a simple one, it was the original question that was confusing. It is no wonder you had trouble trying to figure out the answer, it looked like a trick question the way it was asked (lots of additional information not needed to answer the actual question asked).

Still the question is a good indication of what happens in the real world. Lots of information available but only some of it is relevant to the problem at hand.

Hello again -

My original answer is correct as given. The maximum current will be the rated current allowed by the 15 amp fuse.

This does NOT depend on the line voltage or the frequency of the specific power system. It IS CORRECT in Germany or Britain, or whatever distribution voltage you are using.

The "Lecturer" (British?) is incorrect. He has calculated the ground fault current for the circuit NOT protected by the fuse.

The problem stated "If a 240V circuit is protected by a 15 Amp fuse...." A fuse is used to protect the energized line.. and thus its rating must be lower than the ground fault condition.

Dave Cooper

I think you are getting confused by the question.

"If a 240v circuit is protected by a 15A semi enclosed fuse & has an earth loop impedance of 1.9ohms determine the earth fault current in the event of a zero impedance earth fault?"

Here is what I think it actually simplifies to

If a 240v circuit (240V phase to earth potential) is protected by a 15A fuse and has a phase to earth fault (where the total impedance to ground is 1.9 ohms) determine the MAXIMUM earth fault current that will flow (assume the total impedance limiting the current is 1.9 ohms).

Given a MEN or similar electrical system where the earth is connected back to the neutral (rather than isolated, like commonly found in USA) the answer given by the lecturer using standard ohms law makes sense and is correct.

As no other information such as fuse curves, fault duration or other information was given to try and calculate the current at the specific time the fuse operated then the maximum instantaneous current found using ohm's law is correct in this case (assuming no other data has been left out of the question originally supplied). The fuse value itself becomes a red herring and plays no part in the question (no mention was made of fuse operation time, etc) as there is not enough information provided to calculate fuse operation time, etc.

Here we have a question of technical semantics. Logically as Dave Cooper states the fuse will blow at 15A and then this will be the max current in the circuit but anyone who has looked at current on an oscilloscope will can verify that the current through a fuse will reach many times the fuse rating before the fuse disconnects the circuit. Take a look at a fuse manufactures curves for their products.

see http://www.schurterinc.com/pdf/usa/c_spt5x20.pdf

http://www.littelfuse.com/data/en/Data_Sheets/217P.pdf

1000% 15A .04 second, Maximum

INTERRUPTING RATING: 35 amperes or 10 x rated current;

It was still an obvious homework question which are not allowed on CR4.

I think we can let this one slide as the original "Guest" poster actually came up with the answer without help here, it was the slightly confusing question that caused the problem (and still is with others).

Hi.

Sorry but being a guest & not reading the terms & conditions, I had no idea coursework questions were not allowed. The question was worded exactly. Thank you all for your replies. I will refrain from posting any more.

Thanks A

While homework questions are are not allowed this qestion has shown up a basic misconception which is prevalent even among proffesionals.

I don't think not allowed is completely correct. More like ethically discouraged. If we do the home work and someone is killed or even harmed by someones using us for cheating, our wrath would be inward as well as outward.

Brad