Help with tt system certification

[Simon81]

HiCan anyone help me with regards to filling in a niceic part p cert with regards to the part were it concerns tt.This is on page 2 I need help with the parts electrode resistance ra , is this the measured ze result I have taken ?And also method of measurement ? Also what is the maximum ze for a tt system , I no it's 0.35 for tncs and 0.80 for tns. Any help is appreciated Thanks

 

[telectrix]

i would just get the Ze. a TT system can be almost anything, but the brb says anything over 200ohms can be unstable. niceic say 100ohms. there is no stipulated maximum as even with 23ohms, your fault current would only be 10A, so RCD is used to achieve disconnection times. max. value of Zs is then entered as 1667ohms.

 

[Guest123]

Ra & Ze are essentially one and the same in that they are the resistance of the earth electrode & the main earthing conductor connected to it..


Method of measurement would either be a proprietory earth electrode tester with spike kit (prop test) or using an earth fault loop impedance tester (EFLI tester). The bracketed wording is what I put in the boxes obviously depending on which method is used.

BS 7671 states that the maximum measured Ra/Ze on a TT system should not exceed 200ohms, whilst the NICEIC require no more than 100ohms.

GN3 has a diagram on on both methods plus other info.

 

[telectrix]

Snap......

 

[beaver74]

Ra reading can be taken using meathod 2 by an earth loop impedance tester
the max reading is in theory 1667 ohms but you should try to get bellow 200 to allow for sesioal changes

 

[Simon81]

Thanks formal your replys got a ze of 4.69 just had a nightmare with it all

 

[Guest123]

4.69 is a jaffa. Must be particularly good ground or lots of copper!:goofy:

 

[Simon81]

Thanks formal your replys got a ze of 4.69 just had a nightmare with it all. Also I have been told to enter 0.2 in the disconnection time and have been given the following maximum zs for the following mcb 6amp 38.3316amp 14.3820 amp 11.5032amp 7.19Does anyone no if this is right?

 

[Deleted member 9648]

Thanks formal your replys got a ze of 4.69 just had a nightmare with it all. Also I have been told to enter 0.2 in the disconnection time and have been given the following maximum zs for the following mcb 6amp 38.3316amp 14.3820 amp 11.5032amp 7.19Does anyone no if this is right?

Oh dear.....you need to take the 2391.

 

[Simon81]

So do you no the right answer wirepuller or do you just leave smart arsed comments to try and look like you do. I no electricians who got every qualification you can name but when it comes to tt systems they haven't got a clue !!!

 

[IQ Electrical]

Oh dear.....you need to take the 2391.

Absolutely, are you installing work as a competent person?

I'm all for helping here but your questions are certainly not inspiring any confidence whatsoever in your ability to install, test and certificate a TT system!

 

[Deleted member 9648]

Apologies for being blunt but given your questions I would not want you testing my installation.....anyone testing and filling in a cert should not have to ask.
If you are learning testing and your questions relate to a hypothetical installation fair enough,I apologise.
If you are testing an actual installation for someone you shouldnt have to ask questions like this.

 

[Simon81]

I have never worked on a tt system that all and I don't no any other person who has ,

 

[IQ Electrical]

I have never worked on a tt system that all and I don't no any other person who has ,

But why didn't you read up on the requirements for TT before getting involved in someone's installation?

 

[Guest123]

I have never worked on a tt system that all and I don't no any other person who has ,

I think GN8 would be a sensible purchase for you if you will be coming up against TT systems again.

Guidance Note: No. 8: Earthing and Bonding Standards and Compliance: Amazon.co.uk: Geoffrey Stokes: Books

 

[Octopus]

I've got some sympathy for the OP here.

The OSG defines limitations for TNCS and TNS very well but when it comes to TT is less than clear having 200 ohms or 100 ohms for the Re/Ra doesn't help. When it comes to main bonding its the same ambiguity.

Lets hope such "details" are clearer when the next edition of the OSG comes out.

For what its worth I'm with the NICEIC to aim for sub 100 ohms, and I'd like to see main bonding brought into line with TNCS and TNS too.

Tin hat on!

 

[Des 56]

I dont think its unreasonable to expect an electrician to have an understanding of most of the requirements of a TT, even if they come upon them only occasionally

 

[Deleted member 9648]

Once again Simon I apologise for the tone of my post,although I stand by the gist of what I was trying to say.
At least you have asked for the correct information,rather than fabricating a cert in the pub which is what a lot of blokes would do if they didnt know....(and even if they do know)

 

[telectrix]

Thanks formal your replys got a ze of 4.69 just had a nightmare with it all. Also I have been told to enter 0.2 in the disconnection time and have been given the following maximum zs for the following mcb 6amp 38.3316amp 14.3820 amp 11.5032amp 7.19Does anyone no if this is right?

trhe bit in red is gibberish. ok.?

 

[IQ Electrical]

I've got some sympathy for the OP here.

The OSG defines limitations for TNCS and TNS very well but when it comes to TT is less than clear having 200 ohms or 100 ohms for the Re/Ra doesn't help. When it comes to main bonding its the same ambiguity.

Lets hope such "details" are clearer when the next edition of the OSG comes out.

For what its worth I'm with the NICEIC to aim for sub 100 ohms, and I'd like to see main bonding brought into line with TNCS and TNS too.

Tin hat on!

But look what he posted above regarding maximums for devices, that was just a lack of basic knowledge, never mind TT specifics!

I've got no sympathy whatsoever, he could be installing this at my mother's house!

How can we all bang on about competency and sliding standards then defend posts like this one?

If the post had started "I'm doing a TT system next week, I have a few questions.......

 

[bozobro0]

maximum zs values for 60898 type B mcbs, 6A=6.13
10A=3.68
16A=2.30
20A=1.84
25A=1.47
32A=1.15
40A=0.92 Ohms

 

[telectrix]

when you got the Ze of 4.69, was that with the main eaerth disconnected from the CU, to avoid parallel paths?

 

[mcs.serv]

But why didn't you read up on the requirements for TT before getting involved in someone's installation?

Or any other system for that matter!

 

[Simon81]

I am a compotent electrician I no the maximum zs for mcb 32 1.44 6 7.67 etc but I have been told by a fully approved niceic member that the values change when the system is tt so is he wrong is that what your saying

 

[Octopus]

I am a compotent electrician I no the maximum zs for mcb 32 1.44 6 7.67 etc but I have been told by a fully approved niceic member that the values change when the system is tt so is he wrong is that what your saying

Not wishing to be too critical, but your posts would benefit from some punctuation, and editing, before you hit the "post" button

 

[bozobro0]

As far as im aware the max zs of the device never changes,but with it being a TT system you are more than likely going to be over the max zs when testing.There are tables in the brb that you can look at to see if they are ok.Think its table 41.5,i may be wrong as i dont have my book handy at the moment,but someone may point you in the right direction.

 

[IQ Electrical]

But you must be competent on the system that you are installing!!

 

[Simon81]

Them values that I have been given are from nic software so who wrong?If the values don't change how can a ze of 4.69 and a r1 + r2 of 0.68 which gives a zs of 5.37 be within the maximum zs of a 32 amp mcb 1.44 ????

 

[Guest123]

I am a compotent electrician I no the maximum zs for mcb 32 1.44 6 7.67 etc but I have been told by a fully approved niceic member that the values change when the system is tt so is he wrong is that what your saying

Not wrong no but, it's the introduction of RCD's to cover earth fault protection due to the high Zs values that effect the changes in maximum permitted values i.e. 1667ohms for a 30mA RCD etc etc.

 

[Simon81]

I have been misinformed it seems that all an easy mistake I have been put in this position by the electricity board who wouldn't provide an earth to the property

 

[bozobro0]

Have you got a 17th edition regs book,there is a table in there which deals with this type of situation.

 

[mcs.serv]

As far as im aware the max zs of the device never changes,but with it being a TT system you are more than likely going to be over the max zs when testing.There are tables in the brb that you can look at to see if they are ok.Think its table 41.5,i may be wrong as i dont have my book handy at the moment,but someone may point you in the right direction.

Correct. Table 41.5

I think what his mate is try to tell him is, what is written in the max Zs changes with TT

Max Zs = ohms1667

 

[Simon81]

He has got them values from nic software he does them on the computer and when it ask for kind of system and he enters tt them are the values for the mcb so maybe he needs to contact the nic regarding this

 

[Simon81]

Thanks for your reply bozobro also the zs you gave were slightly wrong them are at 80 percent if you look in bs7671 they are 32 1.4416 2.876 7.67

 

[bozobro0]

Iv used the NIC software before and it does automatically put the values for max zs of a device in the box for you,but the box for your actual reading goes in the measured zs.If i remember right,when you finaise the cert you will get a warning telling you that your values are high,ignore it and complete the cert.

 

[bozobro0]

Im going of the charts you get with nic certs mate.

 

[Simon81]

Yeah I no in the back I got pulled on it last year though

 

[Deleted member 9648]

He has got them values from nic software he does them on the computer and when it ask for kind of system and he enters tt them are the values for the mcb so maybe he needs to contact the nic regarding this

.....No,ignore the software and use the maximum value of Zs for the RCD protecting the final circuits. It is rare on a TT for the Zs to be within the limits for an overcurrent device,the RCD provides earth fault protection...see table 41.5 and regs 411.5.2....411.5.3.
I would suggest given your Ze that you have not disconnected the main earth lead prior to testing and parallel paths have lowered the reading.A single rod TT would normally have a Ze in the region of 50-150 ohms,or at least higher than you have recorded.


Edit....just a thought....have you provided RCD protection in accordance with the requirements for a TT?

 

[Simon81]

Just goes to prove not everyone no everything fully niceic approved contractor got his 2391 and he made me look like I don't no what I'm on about , I'll remind him not to come on this site

 

[bozobro0]

Nobody knows everything ,and if they think they do they are normally full of .I think everyone feels a bit daft asking questions and being unsure in case they get a trolling,but its better to ask and get it right than to not ask and get it seriously wrong.