2391 need help on how to answer question

[bobby101]

hi guys came across 2 questions in a past paper but not sure how to give a correct responce for it in an exam i am probably thinking about this one a bit too much and i know the answer is simple but putting the answer on paper is not happing for me any help wil be appreciated thanks in advance

Q. explain the effects on earth fault protection of a circuit if the circuit breaker is installed in the neutral conductor only

Q. the maximum test current to be applied on a RCD giving protection against fire

got the exam on thursday nite not looking 4ward 2 it

 

[brucegenerator]

a IT WONT OPERATE b LESS YHAN 300mA

 

[lee1002]

hi guys came across 2 questions in a past paper but not sure how to give a correct responce for it in an exam i am probably thinking about this one a bit too much and i know the answer is simple but putting the answer on paper is not happing for me any help wil be appreciated thanks in advance

Q. explain the effects on earth fault protection of a circuit if the circuit breaker is installed in the neutral conductor only

Q. the maximum test current to be applied on a RCD giving protection against fire

got the exam on thursday nite not looking 4ward 2 it

i have my exam as well m8 im not looking forward to it either.
1) if it was to be in the neutral conductor then there will be no R1 + R2 reading. This in turn means there is no Earth fault loop path, meaning should there be a fault the cb will not trip at all.
2) rcd for risk of fires is 300ma so the max is 300ma. you do not do a 5x test on anything over 30ma

 

[Des 56]

Always hard for me explaining something in the written form as well

Question 1
If you think about the practical situation,the overcurrent is in the neutral conductor
An earth fault develops,the fault path from line through to the fault to earth and back to the supply transformer,the circuit is made without the neutral

The fault path doesn't involve the overcurrent thats in the neutral,so its a waste of time having it
This means there is no effective local overcurrent protection other than the next overcurrent upstream,which may be the suppliers

A better worded reply will soon come,but thats my take on the answer they look for

Question 2 I'm not going to write out a reply,but you will find what you need on page 184 of the brb

 

[bobby101]

any ideas on this one

Q. an earth electrode resistance test has been carried out using an earth electrode tester, and the following results were obtained: 154 ohms, 158 ohms and 160 ohms determine the value of Ra to be recorded on the PIR

 

[Des 56]

[QUOTE=lee1002;128291
1) if it was to be in the neutral conductor then there will be no R1 + R2 reading. This in turn means there is no Earth fault loop path, meaning should there be a fault the cb will not trip at all.

Lee
The R1+R2 is the measured resistance of the live to the cpc and back to the origin
You will still have an R1+R2 reading because the circuit is still intact

There will be a fault path,but there is no overcurrent in the part of that circuit for it to operate

Best way is draw out the fault path and position the overcurrent in different parts and see the path of that fault current and its effect on the overcurrent

Good luck to you both

 

[pushrod]

i have my exam as well m8 im not looking forward to it either.
1) if it was to be in the neutral conductor then there will be no R1 + R2 reading. This in turn means there is no Earth fault loop path, meaning should there be a fault the cb will not trip at all.
2) rcd for risk of fires is 300ma so the max is 300ma. you do not do a 5x test on anything over 30ma

you could get an R1+R2 reading. The earth fault loop path is still there but when an earth fault current flows it will not involve the neutral where the circuit breaker is so as you say the protective device will not operate on an EARTH fault.(It is not true just to say a fault , eg with a short circuit fault the neutral would be involved and so the breaker would operate)

 

[Sintra]

any ideas on this one

Q. an earth electrode resistance test has been carried out using an earth electrode tester, and the following results were obtained: 154 ohms, 158 ohms and 160 ohms determine the value of Ra to be recorded on the PIR

The average of the three readings is taken as the electrodes resistance (as long as all three readings are within 5% of one another). If they exceed 5% the test must be repeated with the distance of test and auxiliary electrodes increased.

Answer 157.33 ohms

 

[Des 56]

You can find information on this test Test method 1 in guidance note 3
The 3 results are 3 readings taken with the auxillary probe moved closer and further away

Its then an average of the readings obtained if they are simillar

 

[pushrod]

so correct me if iam wrong there is no return path for an earth fault current in the event of a line to earth fault

there is a path it just doesn't involve the neutral.

 

[bobby101]

thanks guys i wasnt sure with the Ra calc i was doing more calcs than was required, the way the question was worded threw me, many thanks once again

 

[Sparkypaul11]

From the way I interpret the question, I read it as if the neutral conductor only was protected by the circuit breaker then as the circuit breaker operates, disconnecting the neutral conductor only therefore leaving the line conductor 'live'.

It is for that reason that neutral conductor is no longer fused at the cut (as the neutral fuse may disconnect the neutral and prevent the circuit from functioning, however the circuit will still be 'live'

 

[pushrod]

i just reread the question it refers to the ciruit being connected to the neutral conductor ONLY hence rendering any overcurrent or earth fault protection useless as their is no connection to the line conductor apart from that their would be no live circuit just a neutral loop for the circuit in question DOH!!!

No.

Did you read my previous replies. It is only earth fault protection that is disabled . An overcurrent resulting from a short circuit fault will still operate the circuit breaker as the fault current in that case does take the neutral path.

edit : there is live and neutral but the protective device is in the neutral.

 

[Des 56]

Quote
From the way I interpret the question, I read it as if the neutral conductor only was protected by the circuit breaker then as the circuit breaker operates, disconnecting the neutral conductor only therefore leaving the line conductor 'live'.

It is for that reason that neutral conductor is no longer fused at the cut (as the neutral fuse may disconnect the neutral and prevent the circuit from functioning, however the circuit will still be 'live'

Thats fair commentSparkypaul11 but,

Quote
explain the effects on earth fault protection of a circuit if the circuit breaker is installed in the neutral conductor only

 

[bobby101]

cheers guys i hope this question comes i will be ready for it thanks for all the feedback i think the c n g examiner likes to make you struggle if i got that question on the day i would just laugh