How does fault current loop? "Earth Fault Loop"

[Lainan01]

Confused as to what the earth fault loop is. I'm aware it's the path that fault current takes (Line-Earth) but how many times does fault current "loop"?

 

[newfutile]

It’s referring to the loop from the line (live) back though the transformer then via the earthing path to return (loop) to complete the circuit to your meter

 

[Dustydazzler]

Its doesn't 'Loop' round and round and round ( well it shouldn't anyway ) otherwise you have a serious issue with disconnection of supply

 

[Rockingit]

To answer this fully, you first need to appreciate that what we call 'earth' (in all it's various forms and terms) is simply in effect another Neutral. It's a back-up plan if things go wrong with the normal operation of current to ensure that energy still flows back to the origin and in this particular case can create a large enough flow of energy to activate a protective device siuch as a fuse or MCB.

Imagine if you will, a rubber band looped between your two index fingers where your left hand is the source of energy and your right hand is the device that's using it. So long as that rubber band is intact you have a working and safe system, it just keeps on revolving, current flowing from source and back again (and your fingers get hot which is the device using energy!. Now imagine that the rubber band snaps somewhere - you still have a link to the source of the energy (your left index finger) but now it's flapping about, able to connect to anything around and energise it, potentially dangerously. So what we do is we create another route for that energy to get back to the source, a duplicate return path which we call Earth. Except now, we don't have the current using device involved, we basically have a direct short circuit. The only 'load' which is left is the resistance of the conductors involved - literally (normally) the Ohms per metre of the copper in the wires, which is very low. So now we have Ohms law which says that the amount of current which can flow is the voltage divided by the resistance. If the remains of our rubber band plus the alternative earth connection are say 1 Ohm then we get 230 divided by 1, which equals 230, which equals 230 Amps of current will flow instantaneously. Obviously this is significantly more than say a 20A fuse so the fuse will blow, cutting off the energy and making the circuit safe again.

Hence, we use the term 'Earth loop' and in normal terminology we call it Earth Loop Impedance - the impedance refering to the resistance of our emergency rubber band. The lower the number the greater the fault current (V / R = I) so the faster the circuit disconnects. Expressed as Z in most applications - Ze refers to the loop resistance that exists between the supply transformer down the road and then (confusingly) INSIDE the installation we use the term R1R2 which is the sum of the live conductor resistance and the Earth (CPC) conductor. We add those two together Ze + R1R2 to get the total resistance from the socket outlet in a front room right back to the supply transformer and we call is Zs, where the 's' stands for Sum.

This overall system we call Automatic Disconnection of Supply - or ADS for short.

 

[pc1966]

how many times does fault current "loop"?

As above, the term 'loop' really means a closed circuit.

What appears counter-intuitive at first is you usually want a high fault current to flow, as that results in a faster disconnection time and less exposure to elevated voltages on any metalwork. Also, and depending on the type of protective device, often a lower total fault energy and so less heating of conductors and risk of insulation damage from that.

Hence you want a low loop resistance for that to happen.

Occasionally the prospective fault current can be too high, but that is unusual in most cases (domestic at least), and a whole other debate to be had on dealing with it.

 

[Lainan01]

Imagine if you will, a rubber band looped between your two index fingers where your left hand is the source of energy and your right hand is the device that's using it. So long as that rubber band is intact you have a working and safe system, it just keeps on revolving, current flowing from source and back again (and your fingers get hot which is the device using energy!. Now imagine that the rubber band snaps somewhere - you still have a link to the source of the energy (your left index finger) but now it's flapping about, able to connect to anything around and energise it, potentially dangerously. So what we do is we create another route for that energy to get back to the source, a duplicate return path which we call Earth.

Great answer thank you!

 

[Rockingit]

Great answer thank you!

You’re welcome, hope it helped.

 

[Intoelectrics]

Great answer thank you!

Just to add, the safety circuit (earthing in this case) when required, is along with any safety device the most important part of an installation.
Selecting the correct equipment for safety in an installation is essential, and this includes cable, connectors, devices...etc .
It is also essential that any safety system is functioning correctly and that all the test results are met as stated in the regs.

Under fault conditions you want the confidence that the system will operate as designed and that in such an event it may save a life.

So, good question and well done for enquiring. Keep asking and keep learning!