please help with this question

paulthemagpie · Mar 18, 2012

hi guys,

i need some help with an assignment and id appreciate it so so much if anyone can help me.

A circut which is supplied from DB 3 is protected by a 16A type C circut breaker and has a measured earth fault loop impednace of 1.35 ohms. Please determine (with reasons were applicable)

a. if this value of earth fault loop impendance is acceptable
b. at least one method of achieving disconnection should the device provide unacceptable in (a.) above with out re-wiring the circut.
c. at least one method of confirming the integrity of the live conductors of that circut under short circut conditions.

thanks alot guys for any help

plugsandsparks · Mar 18, 2012

Do you have any reference books to work with? cos this could get tedious

malcolmsanford · Mar 18, 2012

Ok I'm not doing your home work for you bu I'll point you in a direction

a) look at pg 56 in the regs book table 41.3 and then look at appendix 14 regs book pg 424

b) Section 415 in the regs

c) Section 411 in the regs

paulthemagpie · Mar 18, 2012

hi,

i got the regs book!

what does the BD 3 stand for!

also thanks for your help guys!

Richard Burns · Mar 18, 2012

DB3 is distribution board 3, just a designation.

paulthemagpie · Mar 18, 2012

oh thanks,

that simplifis things for me! thats the thing with my material im working with it just throughs something like db3 in and expects a newbie to know what that is! another thats simple to you guys but not for someone who has came across it for first time. At least now il know! lol

thanks for the help

Brightspark2 · Mar 18, 2012

Whats the point in us just answering all the questions for you. You need books to read and get an understanding of electricity, how it works and whats being asked, then open up the regs and look for the answer.

You wont learn much doing it this way. What is this a assignment for? collage? is it an apprenticeship or quick course?...

Darkwood · Mar 18, 2012

You really should stop going out on the razz - day before college.... DB 3 (not BD) is probably a ref' to Distribution Board 3 in some notes or diagram you have..... this is basic shorthand terms and should be well locked into your grey matter long before your even tasked with this level of question.... lets just hope its just a one off oversite or your gonna struggle alot and we are not into doing your homework but will guide you to the area to look as others have.

paulthemagpie · Mar 18, 2012

its the first time my material has used DB instead of distrubution board so when i first saw it i thought it was going to be something alot more technical than the ovious simple distubution board

paulthemagpie · Mar 18, 2012

so can i just check,

should i be working it out like this:

0.8 x (230/16) = 11.5

so the 1.35 should be less than or equal to 11.5

meaning its acceptable

am i on the right track guys

telectrix · Mar 18, 2012

hint. look up the max. allowable Zs for the MCB in question.

fivestar · Mar 18, 2012

You need to determine the maximum value of earth fault loop impedance that will trip the 16A breaker in the required disconnection time. This information is all in the regs book and should be what you are learning, you need to work it out for yourself

plugsandsparks · Mar 18, 2012

No, not really. The current you are looking for is the one which makes sure the breaker trips in 0.4s, hint its much much higher than 16A, i.e. the rating of the breakers and if you think about it, it would be a pretty annoying breaker if it did trip in 0.4S at 16A.

Also there is a rule of thumb used here cos its hard to measure when its all running at max temp and current so you measure at ambient, often with low or no load.