fault currents, external impedance and current demand

[Benny boy]

Hi

could someone please tell me why i need the following information, relating to the characteristics of the supply during the design stages of an installation??

prospective earth fault current

the value of external impedance

new assumed current demand of the installation

disconnection time of an existing external bulkhead type luminaire....

if someone could answer this it would really help me out...

thanks

 

[Doc Hudson]

Hicould someone please tell me why i need the following information, relating to the characteristics of the supply during the design stages of an installation??

prospective earth fault current

the value of external impedance

new assumed current demand of the installation

disconnection time of an existing external bulkhead type luminaire....

if someone could answer this it would really help me out...

thanks

Welcome to the forum Benny boy. I have deleted your duplicate post and moved this copy to the Student area, as I assume this is a college question. You will get the best response for any of these sort of questions by suggesting your answers first to then get any misunderstandings clarified. It has been proven over many years that just giving out answers is of minimal value to the learning process.

Just to start the ball rolling. This is a design question and if you consider the first part Prospective Fault Current (PFC). You need to think, Can it be too high or too low and what affect would this have on any equipment you choose to install? If you put your understanding of these topics it will help others to post their answers at a suitable level for you.

Doc H.

 

[Benny boy]

Hi

Thanks again, i presume the reason for measuring the pfc at design stage is to make sure its suitable to carry the fault current with the new loads that are being installed and to make sure the main protactive conductor has a suitable csa for the job.. the value of external impedance is needed to calculate the pefc but would you also need to know the external impedance to make sure there is no fault before your installation which could be affected from this.. the new assumed current demand of the installation is needed to make sure you dont have to much loading on the installation which can cause faults which then reflect on the pfc.. im not to sure why i need to know the disconnection time of the external bulkhead luminaire other than to make sure its within regulation...

i think thats right but i would like someone else to tell me where im going wrong if i am..

thanks

 

[Technician]

You need maximum fault current so that you can properly

select the protective device.

Maximum demand permits you to choose the correctly rated

device.

You need to know the disconnection time because there are

different times for different voltages and ratings of protective

devices. Without that information on disc time, you cannot

realistically assess your circuits, further, the information is

needed for your EIC.

HTH

 

[SPECIAL LOCATION]

HiThanks again,

i presume the reason for measuring the pfc at design stage is to make sure its suitable to carry the fault current with the new loads that are being installed and to make sure the main protactive conductor has a suitable csa for the job..

the value of external impedance is needed to calculate the pefc but would you also need to know the external impedance to make sure there is no fault before your installation which could be affected from this..

the new assumed current demand of the installation is needed to make sure you dont have to much loading on the installation which can cause faults which then reflect on the pfc..

im not to sure why i need to know the disconnection time of the external bulkhead luminaire other than to make sure its within regulation...

i think thats right but i would like someone else to tell me where im going wrong if i am..

thanks

Hicould someone please tell me why i need the following information, relating to the characteristics of the supply during the design stages of an installation??

prospective earth fault current

the value of external impedance

new assumed current demand of the installation

disconnection time of an existing external bulkhead type luminaire....

if someone could answer this it would really help me out...

thanks

Hello Benny...

I think you are getting a bit muddled with your PFC's & load's

The PFC & PSCC are the respective maximum currents that will flow if you stick a direct short access the Line & Earth or the Line and Neutral respectively...

they are not related to the connected loads..

Are you aware that fuses/MCB's etc as well as having a current rating say 6A, 16A, 32A etc..

Also have a maximum breaking capacity in 1000

 

[NozSpark]

All I'll add is that your Ze will affect the cables that you use for final circuits,,, i.e. their CSA and length due to the fact hat you need to keep within the max Zs of the protective device that is protecting the circuit.

 

[Benny boy]

hi

thanks everyone for your comments.. i see it now, i think i was over complicating everything in my head too much...

yeah i have done a fair bit on Zs, i have actually covered everything i asked about but in a different context and not within design..and when i see it on one of my papers i went to overboard with it and like i said complicated everything... thanks for the responses really appreciate it.. i dont know why i havent joined this website already.. its a handy one... anyone who does commercial and industrial want to give me a job in the future so i can do my level 3?? would be much appreciated.. im doing my course now through new career skills and i have my exams for my 2382, 2377 and building regs, i then am doing my 2392 and PV.. i then want to attemp the 2391 which i have heard is bloody hard, and then i want to do my level 3... so i have everything....you cannot stop learning as they say..

 

[Zeespark]

Go to a college, avoid private training companies. My other piece of advice would be to finish your level 3 before attempting the 2391.

AndyGuinness

 

[philbas]

Go to a college, avoid private training companies. My other piece of advice would be to finish your level 3 before attempting the 2391.AndyGuinness

And dont bother with the 2392. the 2330 outranks it, but hurry, i'm finding it hard to get on the 2391 at the moment

 

[Sammy89]

Hi, I've got the same question but could do with some more info on why you need to know the disconnection time. Thanks

 

[Doc Hudson]

Hi, I've got the same question but could do with some more info on why you need to know the disconnection time. Thanks

Welcome to the forum Sammy, you would be better starting your own thread with new questions, possibly with a bit more detail about what level you do understand. All fuses and circuit breakers have rated currents at which they are designed to disconnect the power from a circuit. But the time from when a fault occurs to when the power is removed will not be instantaneous. If this disconnection time is too long either to much fault current could be flowing down the circuit cables causing the insulation to be irreparable damaged. Or a hazardous voltage could be present on an exposed metal part. As electricity can kill an adult in well under a second, it is not good having fuses taking several seconds to disconnect some circuits.

Doc H.

 

[Sammy89]

Welcome to the forum Sammy, you would be better starting your own thread with new questions, possibly with a bit more detail about what level you do understand. All fuses and circuit breakers have rated currents at which they are designed to disconnect the power from a circuit. But the time from when a fault occurs to when the power is removed will not be instantaneous. If this disconnection time is too long either to much fault current could be flowing down the circuit cables causing the insulation to be irreparable damaged. Or a hazardous voltage could be present on an exposed metal part. As electricity can kill an adult in well under a second, it is not good having fuses taking several seconds to disconnect some circuits.Doc H.

Thanks that's cleared it up for me, I'll make sure to add more detail in any future questions.

 

[Manator]

I will like to say well done to bennyboy for having a go and giving his thoughts. So many people try to ask a question and expect to learn by being told the right answer. It may get you through an exam but it will never teach you anything unless you understand the process.

So again well done benny for having a go, and thank you to all the contributors who gave some first class answers.