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

 

[leonard]

The EFLI have to be low enough to allow more current to flow during a fault
so the protective device operate on specified time.

 

[Guest 004]

If your earth loop is too low, then the overcurrent device will NOT be able to stop the fault the current, and in effect weld the MCB.

Very rare I might add, but might make you understand better.

 

[IQ Electrical]

If your earth loop is too low, then the overcurrent device will NOT be able to stop the fault the current, and in effect weld the MCB.

Very rare I might add, but might make you understand better.

Or as we usually put it 'a high PFC'

 

[SirKit Breaker]

Its like this. Ohms law.

230v with an earth loop impedance of 0.1 ohm. (Good)
230/0.1=2300A.

So 2300a of fault current will flow in the circuit.

Lets assume the circuit is protected by a BS60898 16a B type MCB

2300a in this particular circuit will disconnect almost instantly, and so comply with the 0.4 second disconnection time. The difficult to understand tables in the BGB will give you an exact time.

Now lets look at poor EFLI

230v/10ohm=23a.(Bad)

23a fault current in this circuit, again from the difficult to understand tables in the BGB, we can see that this circuit will disconnect in about 10,000 seconds, thats about 2.5 hours, so it does not meet the 0.4 second disconnection time. Bear in mind that current of about 80mA can kill you, then 23a for 2.5 hours wont be much of a laugh.

This is why we need a very low loop impedance and this is why the suppliers state TN 0.8ohm Ze, and TNCS 0.35ohm Ze as a maximum.

The figures i quote from the BGB may not be accurate, as i have done this off the top of my head, but you get the picture i hope.

Cheers..........Howard

 

[pencilpusher]

Or as we usually put it 'a high PFC'

How beautifully explained!