Ze too high

[LarkySparky]

Hi all need some advice please,

Have a Ze reading of 4,6 Ohm on a TNS supply in a Terrace house, informed the DNO of this and they have since come out to investigate and install a shaved neutral clamp, same as TNC-S. On visiting the property they took one look and informed my client that this could not be carried out as their supply is through steel pipe (which the main earth is currently clamped to) and advised that i should install an earth rod - no probs there.

What is the problem is that the house currently has a very old MEM board (type 3 M9 breakers) which serves the property. The new consumer unit which i will install will be RCD protected and have all NEW circuits for his extension.

Would putting in a Main RCD Switch (30mA) to feed the current CU be acceptable? RCBO's throughout the old board would be the most correct option but isn't possible. Nuisance tripping may be a problem..

I ask this as i have not delt with TT alot (all TNS or TNC-S) and the cost of replacing the old CU is too costly for the client.

Cheers Guys

 

[Zeespark]

Dont use a 30mA RCD to protect that many circuits, use a 100mA RCD.

Andy

 

[steptoe]

Dont use a 30mA RCD to protect that many circuits, use a 100mA RCD.Andy

HE cant,

once he starts changing stuff like that he will need to provide protection equal to or greater than is asked by 7671/2008 which will mean 30mA RCD.

can you do an IR test across L/E & N/E and see if the readings are OK first.

you could install a front-end 30mA RCD if cost really is that much of an issue and record deviation of no segregation,

but you can pick up a split load board with MCBs for about

 

[Zeespark]

My apologies Steps.

Andy

 

[gordy71]

Surely its the DNO responsibility to get this sorted as its an emergency situation. I had this once (8 ohms i think) and they were out in half an hour because they were so concerned. Putting an earth spike in isnt ideal.

 

[gselectrical]

They don't have to provide an earth. Therefore TT would be the best option in this case.

GS

 

[steptoe]

My apologies Steps.Andy

no worries Andy.

---------- Post Auto-Merged at 21:22 ---------- Previous post was made at 21:19 ----------

My apologies Steps.Andy

Surely its the DNO responsibility to get this sorted as its an emergency situation. I had this once (8 ohms i think) and they were out in half an hour because they were so concerned. Putting an earth spike in isnt ideal.

they can withdraw their earth as they see fit,

although if they do provide an earth they are responsible for maintaining it,

make of that what you will.

the bottom line is they are NOT obliged to provide you with an earth, they only do so if it is of no inconvenience to themselves.

hence why I prefer TT, at least you know its down to you to maintain it and its very unlikely to just disappear and you wouldnt know till you had a fault that you were underprotected.

 

[spinlondon]

No reason why a 100mA RCD can't be used.

Just make a note of the lack of 30mA protection to whatever requires 30mA protection.

 

[steptoe]

My apologies Steps.Andy

No reason why a 100mA RCD can't be used.Just make a note of the lack of 30mA protection to whatever requires 30mA protection.

please explain to me how you can ADD 100mA protection to a circuit which still leaves it in NON-COMPLIANCE with 7671/2008 ?

unless you fill out the deviations then any changes you make should comply with the regs in force at that time.

 

[spinlondon]

To what are you refering when you use the term deviations, are you refering to intended departures from BS7671 in the work you are conducting, or to defects and non-compliances in the existing installation?

You are required to ensure your work complies with BS7671, there is no prohibition in BS7671 against installaing a 100mA RCD.

Any 'deviations' in the existing installation should where reasonably practicable be noted in the section of the certificate for comments on the existing installation.

 

[steptoe]

yes, and when you install an RCD you are changing the installed circuits protective device characteristics,

(we have had this discussion elsewhere)

so you need to provide an EIC to show compliance with 7671, or note down any deviations you have made,

just shoving a front end 100mA RCD on to compensate for a poor Ze is a real bad John Wayne approach to anything,

dont you have any pride, or is it simply competence and knowledge you lack.?

 

[Sidewinder]

Well,

We have had a few answers, this would be mine, it would go along the lines of "MR. DNO operative" can I have your name please for the record.

Can you please wait here as I have the mobile number of the HSE chief electrical inspector on my laptop I'd like to ring him so that he can have a chat with you.

Your company has provided an earth connection to this premise. Under ESQCR, you have a statutory responsibility to maintain this, as you are now refusing to do this you are proposing that you leave the premises in a seriously dangerous situation which is liable to cause electrocution, this is by the action or inaction of your company and yourself.

I'm sure he would like to know about this just in case someone gets killed by your failure to maintain your companies equipment.

You are entitled to withdraw this earth connection at any time, however, I'm sure that the supply legislator would also be interested in your companies attitude, & I'm sure that the HSE guy will have their number too.

Now are you going to rectify this fault or am I going to make that phone call...

 

[steptoe]

SW , I would have agreed with that up until a few years ago,

up until then anytime I had ever rang DNO about a high earth they have been out within an hour at most,

then on one occasion they came out, ****ed about for an hour or so, got the earth to 0.7 and issued the householder with a notice serving disconnection of earth after 14 days,

dunno what it was all about but tenant apparently sought advice on it and anyway we were back the following week to TT it.

 

[spinlondon]

'yes' what?

How exactly are you changing the CPD characteristics if you are not changing the CPD?

What does it matter if you do change the CPD characteristics, is there a Regulation that prohibits you from doing such in BS7671?

Are we now according to you not allowed to replace BS3036 with BS60898 devices because that would be changing the CPD characteristics?

Whilst I agree that in this instance, an EIC will be most appropriate, I do not agree with your comment regarding deviations.

There is nowhere in BS7671 where any reference to deviations is made. There are references to departures, defects and non-compliances; however not to deviations.

Departures should be intended during design, and should offer the same degree of safety as would be achieved by compliance with the Regulations.

As far as I'm aware, there is no requirement in BS7671 for distribution circuits to have 30mA RCD protection. If you can discover a Regulation that contradicts my understanding, please furnish the Reg. No. so that I will be educated.

The John Wayne approch (as you call it) is the very same approach that has been practiced and recommended (in the Regulations) for a number of years. It is still practiced and recommended today and wil be so for the forseeable future.

Rather than attempting to insult me, perhaps you should devote your time to better understanding the Regulations?

 

[kme]

But we aren`t talking about distribution circuits mate; we`re talking about final circuits.

You are proposing the O/P has a TT install, with a 100mA front-end RCD and 3036 OCPDs; and you`d be happy to sign an EIC for that?

I know damn sure I wouldn`t. AFAIC, that work would contravene not just the current edition of the regs; but previous ones, too.

Or have I missed summat here?

 

[steptoe]

My apologies Steps.Andy

'yes' what?How exactly are you changing the CPD characteristics if you are not changing the CPD?

What does it matter if you do change the CPD characteristics, is there a Regulation that prohibits you from doing such in BS7671?

Are we now according to you not allowed to replace BS3036 with BS60898 devices because that would be changing the CPD characteristics?

Whilst I agree that in this instance, an EIC will be most appropriate, I do not agree with your comment regarding deviations.

There is nowhere in BS7671 where any reference to deviations is made. There are references to departures, defects and non-compliances; however not to deviations.

Departures should be intended during design, and should offer the same degree of safety as would be achieved by compliance with the Regulations.

As far as I'm aware, there is no requirement in BS7671 for distribution circuits to have 30mA RCD protection. If you can discover a Regulation that contradicts my understanding, please furnish the Reg. No. so that I will be educated.

The John Wayne approch (as you call it) is the very same approach that has been practiced and recommended (in the Regulations) for a number of years. It is still practiced and recommended today and wil be so for the forseeable future.

Rather than attempting to insult me, perhaps you should devote your time to better understanding the Regulations?

you can change the PD with anything of your choosing as long as it meets the current regs,

deviations/departures, are they not one and the same in this instance.? or do you wish to give me a dictionary definition as to why they are different.?

if you add an RCD to a circuit are you not changing the protective characteristics of said circuit?

yes, you are now adding protection to a N/E fault, therefore you have changed the protective device characteristics, albeit by adding another PD with the existing one in series.

and people wonder why I dont like 5 day wasters.!?

 

[spinlondon]

KME yes it does appear that you have missed something.

The OP has asked if he can install a 30mA RCD switch to feed a CU. To my mind a circuit feeding a CU is a distribution circuit, not a final circuit.

Steps, I suggest you actually read BS7671 instead of as usuall making it up as you go along.

To start with, I have not suggested changing the Protective Device.

I have suggested installing an additional protective device to provide earth fault protection as per the requirements of BS7671 Regulation 411.5.2 . Something which is lacking at present.

It would be easier to understand your posts, if you were clear in the usage of terms.

For instance when you use the term deviations, are you refering to intended departures that will offer the same degree of safety that would be achieved with compliance with the Regulations, are you refering to defects and non-compliances in the existing installation, or perhaps even defects and ommisions in the work that has been carried out.

Yes you would be changing the protective characteristics of the circuit. In this case, providing protection where there is at present none provided. Your point is?

Protection to a N/E fault? Would that be anything similar to providing earth fault protection?

I still don't understand your point Steps.

Is there some Regulation that you believe does not allow the provision of earth fault protection?

As I have already suggested, devote more time to studying and understanding of the Regulations rather than making pointless or even insulting posts.

 

[kme]

okay:

lets start with 531.2.4

then 411.3.3(i)

also 415.1.1 refers

What must be considered is that the installation itself is being reclassified as a TT. Irrespective of that, socket outlets will require 30mA RCD protection to comply, as will special locations......under "normal" conditions.

You`ve HAD to alter the fault protection chacteristics, due to the huge increase in external earth impedance. How can you justify chucking a 100mA front end on, and calling it done?

 

[spinlondon]

Are you suggesting that selecting a 100mA RCD would not comply with 5431.2.4?

Do you believe that there is likely to be more than 100mA protective conductor curent during normal operation?

Are you suggesting perhaps a 300mA RCD be installed?

As far as I'm aware, there is no intention to install any socket-outlets.

As such the lack of compliance with 411.3.3(i) should be noted on the certificate under comments on the existing installation.

415.1.1 refers to the provision of additional protection.

I am not aware that provision of earth fault protection is classed as additional.

Yes socket-outlets, circuits of special locations and cables concealed in walls etc. will require 30mA RCD protection.

As none of these are being installed, they are existing, the lack of such protection should be noted as per the requirement of Regulation 633.2.

 

[steptoe]

My apologies Steps.Andy

Are you suggesting that selecting a 100mA RCD would not comply with 5431.2.4?Do you believe that there is likely to be more than 100mA protective conductor curent during normal operation?

Are you suggesting perhaps a 300mA RCD be installed?

As far as I'm aware, there is no intention to install any socket-outlets.

As such the lack of compliance with 411.3.3(i) should be noted on the certificate under comments on the existing installation.

415.1.1 refers to the provision of additional protection.

I am not aware that provision of earth fault protection is classed as additional.

Yes socket-outlets, circuits of special locations and cables concealed in walls etc. will require 30mA RCD protection.

As none of these are being installed, they are existing, the lack of such protection should be noted as per the requirement of Regulation 633.2.

are you for real?

they are installed,

you are changing the means of protecting them,

are you going to do that outside of 7671/2008 and then do a departure as top why you have used 100mA and not 30mA.?

surely your mistaken