The value of Ze on a TNS system useing an earth loop impedance test Instrument

[lofty84]

The value of Ze on a TNS system useing an earth loop impedance test Instrument

list the steps in the procedure and the reasons for each step, I think this is right ?

1.Select appropriate test Instrument - It needs to be able to read Ze (low ohms resitance metere ?)
2. Perform safety check ( we dont want to be getting a bang)
3. Isolate the supply and prove dead (We will be removeing the earthing conductor so the system will be left without a earth and therefore dangerous)
4. Remove the earthing conductor form the MET and clip test probe (crocidile clip) to it ( this is to get rid of the paralel paths )
5. Measure between incomeing line and protective earth. ( can anyone tell me why)

If the system was a TNCS what would I need to do ?

 

[malcolmsanford]

The value of Ze on a TNS system useing an earth loop impedance test Instrument

list the steps in the procedure and the reasons for each step, I think this is right ?

1.Select appropriate test Instrument - It needs to be able to read Ze (low ohms resitance metere ?)Earth Fault loop impedance tester
2. Perform safety check ( we dont want to be getting a bang) What Safety Check? Isn't this a part of a answer 3
3. Isolate the supply and prove dead (We will be removeing the earthing conductor so the system will be left without a earth and therefore dangerous)As part of safe isolation lock off so that the supply can not be re-energised by accident.
4. Remove the earthing conductor form the MET and clip test probe (crocidile clip) to it ( this is to get rid of the paralel paths ) OK though perhaps eliminate parallel patrhs would be worded better
5. Measure between incomeing line and protective earth. ( can anyone tell me why) It is called an earthing conductor, some call it main earthing conductor, but just earthing conductor is ok.

If the system was a TNCS what would I need to do ?

Do you need to do anything different if it was a TN-S

We measure the Ze to ensure that we have an external earth fault loop path as this is critical to ensure we can have a Zs low enough to operate the protection devices in the required time.

 

[Deleted member 26818]

You would do the same TN-C-S.
With TT the measurement would be called RA instead of Ze.

 

[pushrod]

Think No 2 is a visual check of the condition of the test meter and leads ?

 

[malcolmsanford]

Think No 2 is a visual check of the condition of the test meter and leads ?

Ahhhhhhhhhhhhh thank you push

 

[lofty84]

so if it said list the procedure for measureing ze on a tncs system or tns system it would be exactly the same or is there a key difference that under exam conditions they may be specifically looking for me to state

 

[lofty84]

we had this question follow on from the original

explain why it is acceptable to record the external earth loop impedance as the electrode value on a tt system

it is acceptable as most of the impedance will be between the 2 earth stakes, can anyone explain this to me so that I understand it and not just the answer

thanks

 

[Deleted member 26818]

The differences would be that with TN-C-S, you would expect a value of about 0.35Ohms and TN-S about 0.8Ohms.
When conducting PFC tests,you would expect the PSCC and the PEFC to be the same with TN-C-S, whereas the PEFC is usually lower with TN-S.

 

[lofty84]

cheers mate, i think i learn more doing it like this then in class

 

[malcolmsanford]

we had this question follow on from the original

explain why it is acceptable to record the external earth loop impedance as the electrode value on a tt system It is acceptable to take the Ra of your earth electrode by using earth loop impedance tester by doing a Ze test. The other way is by using an earth electrode tester and and the 3 stakes method.

it is acceptable as most of the impedance will be between the 2 earth stakes, can anyone explain this to me so that I understand it and not just the answer have you wrote this down properly?

thanks

Have to admit not sure if that is what your asking

 

[pushrod]

we had this question follow on from the original

explain why it is acceptable to record the external earth loop impedance as the electrode value on a tt system

it is acceptable as most of the impedance will be between the 2 earth stakes, can anyone explain this to me so that I understand it and not just the answer

thanks

On a TT system the external earth loop impedance value (Ra) includes the resistance of the earth electrode and the resistance of the earthing conductor added together.

However because the resistance of the earthing conductor will be tiny(probs less than 0.05Ω ) and insignificant in comparison to the resistance of the earth electrode (possibly 50Ω) it is acceptable to say that they are the same.

If they did not allow this you would have to go and test it where the rod/electrode enters the ground. Or subtract the resistance of the earthing conductor from the value.

I remember being asked the exact same question when i was studying

 

[malcolmsanford]

On a TT system the external earth loop impedance value (Ra) includes the resistance of the earth electrode and the resistance of the earthing conductor added together.

However because the resistance of the earthing conductor will be tiny(probs less than 0.05Ω ) and insignificant in comparison to the resistance of the earth electrode (possibly 50Ω) it is acceptable to say that they are the same.

If they did not allow this you would have to go and test it where the rod/electrode enters the ground. Or subtract the resistance of the earthing conductor from the value.

I remember being asked the exact same question when i was studying

Well that blew my attempt out of the water ..............................

 

[pushrod]

it is acceptable as most of the impedance will be between the 2 earth stakes,

Have to admit not sure if that is what your asking

The 2 earth stakes bit is referring to the resistance between the installations electrode and the transformer's electrode - see my last answer. The resistance of the earthing conductor from the installation CU to the installation's electrode is negligible.

 

[pushrod]

Well that blew my attempt out of the water ..............................

But i have only fairly recently been through all these exams, ask me a practical question requiring some of your experience and i am stuffed

 

[Deleted member 26818]

Earth electrode resistance testers are usually used where the electrode is being used as an earth point for PME.
They provide greater accuaracy when getteing the resistance down to 20Ohms and below.
An EFLI tester is acceptable, if the electrode is being used in conjunction with RCD protection and accuaracy is not particuarly important.