Ze & Zs

[Wontdothatagain]

Please help.
I am a second year 2330 student and our lecturer has been sacked. A temp has been brought in but he is 200 years old and keeps getting to complicated.
Can someone please explain earth loop impeadence in a language i can understand and how to measure it.
I only have 2 weeks to my exam and am pulling my hair out.

MANY MANY THANKS

 

[Spark123]

Impedance basically is a component in AC electrical circuits which opposes the current. It is made up of resistance, inductive reactance and capacitive reactance.
In day to day life efl impedance is measured using an earth fault loop impedance tester. The Ze is the earth fault loop impedance with is external to the installation. Zs is the total earth fault loop impedance at the furthest most part of the circuit.
Ze is measured in isolation i.e. between the incoming phase wire and the main earthing conductor with the installation isolated and the main earthing conductor disconnected from the MET (this is to avoid parallel paths).
Zs may be calculated using the Ze+(R1+R2) = Zs.
It is important to know the efli of a circuit to ensure enough current is flowing in every part of the circuit to disconnect the MCB/fuse quickly enough in the event of a fault.

 

[Wontdothatagain]

Many thanks
Could you explain efli.

Sorry Added in error

 

[GaryMo]

http://www.tlc-direct.co.uk/Book/5.3.1.htm

 

[Wontdothatagain]

Thank you both.
One last question.
How do i relate this to short circuit capacity?

 

[Spark123]

The short circuit capacity of a protective device?
The Prospective Fault Current (PFC) should not exceed the short circuit capacity of the protective device (unless backup protection is present).
PFC is the greater of the Prospective Earth Fault Current (PEFC) or the Prospective Short Circuit Current (PSCC).
These are the maximum currents which can flow in the installation i.e. generally at the origin. To check PEFC you need to test each phase to earth. To test PSCC you need to test each phase to neutral and each phase to each phase if your tester allows it. If your tester doesn't allow phase to phase then you can take the largest phase to neutral figure and double it.

 

[Wontdothatagain]

So, if i have a lighting circuit with a resistance of 40 Ohms with a supply of 230v, would i just use ohms law to work out the PFC and then check that against the rating of the MCB or is there a graph to show the maximum for each different type of circuit.

 

[Spark123]

The maximum should be printed on the MCB, usually nowadays it is a figure in a box i.e. 6000 is a 6KA device. Older ones had it as an M number i.e. M3 was 3KA.
You shouldn't see 40 ohms on a lighting circuit unless you are measuring across a lamp or something similar. The PFC is measured at the origin of the installation as that is the MAXIMUM current the MCB (or fuse) will ever have to break.
Modern day efli testers normally have a range for PSCC which you can use to measure the PSCC and PEFC. You can also calculate PEFC using ohms law.

 

[Wontdothatagain]

i was just using 40 ohms as a example

 

[Spark123]

Yeah, it is a big figure!
Talking of TN-S you shouldn't see a Ze exceeding 0.8 ohms or TN-CS 0.35ohms. In a TT system it can be bigger.

I should also mention, the big reason for not wanting to exceed the short circuit capacity of a protective device is it may explode! Say if you install a 3kA device in a situation where in a short circuit 35kA can flow, the 3kA device can go with a bang!!

 

[Wontdothatagain]

Thank you very much.
I can finaly get my head around it now.

 

[briwire]

The eli includes all of the resistance in the path of a fault current. Right from the source of the supply, through the installation and back down the PE path.

A tester puts a measured fault on a circuit and calculates the ELI as it knows the voltage and current.

The tester then has to ensure that this fault currnet will operate the protective device (MCB or fuse) withing the correct time.

 

[Wontdothatagain]

THANK YOU ALL. I took the exam yesterday and had 3 questions on efli and got them all right. Only 2 wrong on the whole exam.

Many Thanks. I was pulling my hair out. The lecturer we had was next to useless and did not explain thing very well unlike you lot.

3rd year hear we come. The final stretch

 

[empip]

TLC 'book'

I wonder if they will update??

-0-

 

[securespark]

Update what pip?