Ze, Zs, R1+R2 etc

[briwire]

I'm due to attend a testing and inspection course but would like a bit of prior knowledge.

Loop impedance Ze, where exactly is it to be measured? Is it at the earth terminal of a DB? Do I have to disconnect any earth connections to pipework, steel framework etc? So the reading is purely the cable.

Zs, similar question.

R1+R2. I believe it is the resistance reading of the phase and earth conductor in series. What is done with 3 phase work. Are 3 seperate readings required?

PFC. Where is that measured? At the furthest point or at the DB earth terminal?

 

[intrinsicallysafe]

Good luck,

Think you're gonna need it!

Try Guidance note 3, that should help.

 

[briwire]

why's that then intrinsicallysafe, is it difficult or do you just think I'm thick?

 

[intrinsicallysafe]

Sorry,

If you're going on the course you will need GN3 anyway.

Have a look and you will see, there are clear diagrams for all tests.

Suggest you borrow a mates tester and do a bit of practising. If it's 2391 you're doing then the course will not teach you how to test, you are expected to know this already.

Practice makes perfect as they say!

 

[Spark123]

R1+R2 is the resistance of the circuit phase conductor and the resistance or the Circuit protective conductor. This one of the dead tests i.e. pre-energization, connect the circuit phase conductor to the earth rail either by removing it from the MCB or by using a short linking wire. At the furthest most point of the circuit measure the resistance between phase and CPC using a low resistance ohm meter. This is your R1+R2 value.
The R1+R2 for Ring final circuits is done slightly differently.

Ze is the external earth fault loop impendance to the installation, it is measured in isolation i.e. you need to isolate the supply, disconnect the earthing conductor from the MET and measure the impedance between phase and the earthing conductor using an efli tester. Once you have completed the test, remember to reconnect the earthing conductor before you re-energize!!

Zs is the total loop impedance of the circuit, it can be done by measurement at the furthest most point of a circuit or by Zs=Ze+(R1+R2).

PFC is basically the MAXIMUM current which can flow in the system. It needs to be conducted at the origin. For this you need to conduct a PSCC (Prospective short circuit current) test which is between each phase and neutral or phase to phase (if 3phase) and also a PEFC which is between each phase and earth. The highest value is the Prospective fault current PFC.
If your tester cannot do a 415v measurement then instead of doing a phase-phase test you would take the highest phase-neutral value and double it.

Briwire you put your email addy in your profile or drop me an email?

 

[briwire]

Thanks for the info. I should have posted that I have access to an all singing Fluke 1653.

What about the R1+R2 for 3 phase?

 

[Spark123]

If all phase conductors are identical then the R1+R2 is just one of the phase wires to the CPC.

 

[jujubean]

why's that then intrinsicallysafe, is it difficult or do you just think I'm thick?

Noooo you aint thick
We all have had to do the 2391 and many have failed...The actual course is a bit of fun and I am sure you will get into it
I attended the course over 10 nights at one day per week This I feel is better than doing a block week as you can read over and digest it better.
Sadly though the exam can be a pain It seems the examiners are out to trick you with the wording of the questions. This is supposed to be an electrical examination ...not an English exam
It would be a good idea to take a look at past papers just to get the feel of what lies ahead
Good luck

 

[ebee]

The examers are not out to trick you but are out to test you know your stuff.
Ze is the Z at the external part of the installation, that is immediately after the meters as far as practicable.
Ze, power off, at your main (or only) isolator with all bonds disconnected.
Also check the Prospective current here.
With bonds connected then check prospective current again (it is usually higher due to para paths), this highest reading is the PFC.
Check R 1 + R2 on each circuit and add to Ze then this gives the calculated Zs.
When energised (with bonds reconnected of course) then recheck that Zs = Ze + R1 + R2 or less than (Due to para paths) , this confirms Zs actually does exist.
Sorry also Ph to Ph in 3 phase for a PFC or double the single phase reading

 

[breaker1]

Dont worry too much mate , I have been tutoring these courses for some time and its not as difficult as think, most people get through first time, listen to your tutor not the rest of the class and dont be afraid to ask questions,we are there to help,we want 100% pass rates, a lot of your answers are in the regs,buy guidance notes 3 from the iet if you dont have one already, digest that and youre almost there, brush up on your maths if needed to help with the equations, there are no trick questions, just read them twice before diving in, the pass mark is 85%