why a low earth fault loop impedance

[Ryan Kavanagh]

hi guys,

Just wanted some help with some course work for college. Any help is greatly apprecitaed.

Why should a low value earth fault impedance should be achieved for all circuts?

thanks

 

[a1guvner]

Have a think, if you had a high reading how well would the current flow

 

[SPARTYKUS]

......and, what then happens when a high current flows in a circuit?

 

[malcolmsanford]

.............and what will be the end result

 

[a1guvner]

Home by 5?

 

[Ryan Kavanagh]

i thought it was on the lines of it the reading was high it shows they a earth fault on the circut.

 

[SPARTYKUS]

i thought it was on the lines of it the reading was high it shows they a earth fault on the circut.

No. You've got some tech to learn.

 

[Ryan Kavanagh]

i know,

thanks why im trying to get the the bottom of a few points i dont fully understand!

thanks

 

[telectrix]

OK< in simple terms. think ohm's law. I=V/R. V is fixed at 240v (single phase) . the value of R ten determines the fault current. the lower the vale, of R, the larger the fault current. if you look at the time/current curves in app.3 of BS7671, you will see that the higher the fault current, the quicker the fuse/MCB will trip, thus isolating the circuit. the quicker it does this, the better.

 

[SPARTYKUS]

Telectrixs answer was a bit more helpful than mine, sorry :-(

 

[Ryan Kavanagh]

so its importnat to have the low value earth fault impendance so the current is flowing freely and will be higher. it will then trip the circut breakers quicker under fault conditions as its got less current to increase to that trip point!

am i getting it now?

 

[telectrix]

think you are there. apart from the last bit .

 

[Ryan Kavanagh]

oh okay,

so its basically it can increase quicker and trip the breaker

 

[IQ Electrical]

oh okay,

so its basically it can increase quicker and trip the breaker

Not increase quicker, but a higher value of current can flow, tripping the protective device in the required time.

Don't limit your answers to 'circuit breakers' etc.

 

[trev]

Be careful of the terminology too. You need to refer to "Causing the operation of the protective device"
In an exam like 2391 terminology can get you a fail. Obviously if you're at a low level they may let you by because they know what you mean but it's good to get into the habit ofusing the corrct terms