why a low earth fault loop impedance

Ryan Kavanagh · Mar 14, 2012

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

leonard · Mar 14, 2012

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 · Mar 14, 2012

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 · Mar 14, 2012

MrEnigma said:
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 · Mar 14, 2012

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