Fitting new CU to TT system and old BS fuse box

[Chris Churchman]

Hi there, new to this site and just after a bit of advice. I am part qualified as a Domestic Electrical Installer and am just waiting for the course to complete my Part P.

I have been asked to install a new Consumer Unit on a TT system in a very old cottage. As seen in the photographs, the current system is the old BS, but I am a little concerned that on examination, the CPS appears to be only 2.5mm where I would have thought it should be 10mm minimum.

Any advice would be greatly received.

Chris

 

[Darkwood]

Welcome to the forum chris, if your still on a training course you might be interested in applying for our Trainee section where we have selected mentors to help with the level of question a trainee asks.

Regarding the pic ...are they exposed live terminals as you found them?????

 

[assegayer]

if its TT then just upgrade it yourself?

 

[Chris Churchman]

Welcome to the forum chris, if your still on a training course you might be interested in applying for our Trainee section where we have selected mentors to help with the level of question a trainee asks.

Regarding the pic ...are they exposed live terminals as you found them?????

Many thanks for the advice. I took the covers off the terminals so they are not normally exposed like that.

Regards,

Chris

 

[Chris Churchman]

Thanks a lot. I'll upgrade it to 10mm. Thanks guys

 

[Simonslimline]

Hi there, new to this site and just after a bit of advice. I am part qualified as a Domestic Electrical Installer and am just waiting for the course to complete my Part P.

I have been asked to install a new Consumer Unit on a TT system in a very old cottage. As seen in the photographs, the current system is the old BS, but I am a little concerned that on examination, the CPS appears to be only 2.5mm where I would have thought it should be 10mm minimum.

Any advice would be greatly received.

Chris

Why would it need a 10mm2?

 

[Chris Churchman]

Just always seen 6mm or 10mm CPC running from the CU in training. Do you not think it needs upgrading?

 

[Andy78]

Welcome to the forum chris, if your still on a training course you might be interested in applying for our Trainee section where we have selected mentors to help with the level of question a trainee asks.

Regarding the pic ...are they exposed live terminals as you found them?????

Agreed .... Possibly the most immediately important question that the OP throws up ...this needs answering first before any of the many other questions regarding this potential work !!!

 

[davesparks]

Thanks a lot. I'll upgrade it to 10mm. Thanks guys

Why 10mm?

First get the DNO out to see if an earth connection can be made, it usually can these days.
If not then you'll need to install new earth electrodes (the little twig that goes with the voelcb is unlikely to be any good) to achieve a stable and low Ra.
Calculate the required size of protective conductor using the adiabatic equation, assess the required size of main bonding and then select a suitable earthing conductor.

 

[davesparks]

Just always seen 6mm or 10mm CPC running from the CU in training. Do you not think it needs upgrading?

Are you sure you are far enough through your training to be able to do this job? Protective conductor sizes are not guessed based on what you've seen in a classroom, they must be calculated and selected on accordance with the regulations.

 

[Sparky Pat]

Voltage operated ELCB installed, regular practice to fit 2.5mm earth wire for them. Different principle to RCD's only needs a 40 volt leakage voltage to operate.

 

[Leesparkykent]

http://www.napitonline.com/downloads/CP 3 07 P 10-11 16th Cable Sizing 2 of 2.pdf

 

[Chris Churchman]

I really appreciate all the advice and comments which I shall take on board. I'll be using a Fluke 1652C 17th edition multifunctional tester to establish impedance etc.

 

[davesparks]

I really appreciate all the advice and comments which I shall take on board. I'll be using a Fluke 1652C 17th edition multifunctional tester to establish impedance etc.

And what will you use to measure Ra?

 

[Chris Churchman]

When doing my Ra, test my understanding is that I need to do it from the mccb / RCD and the incoming earth bar connection. The definition of Ra is given as - the sum of the resistances of the earth electrode and the protective conductor(s). I know we don't use the Zs reading although I have been told it the same. My understanding is that RA is the sum of the resistances of the earth electrode and CPC multiplied by the RCD operating current. This needs to be 50V or less. As you can tell, I'm very much in the early stages and appreciate all the advice and guidance.
Many thanks.

 

[davesparks]

No, Ra for an installation is the combined the resistance to earth of all connected earth electrodes.
Ra is also used with reference to a single electrode.

Testing can be carried out in a number of ways, which generally require an earth resistance tester of some type. The most common method being the 3 wire fall of potential method.
It is possible to obtain an approximation of Ra through use of an earth loop impedance tester where the measured Ze minus half the measured L-N loop impedance is used.

 

[HandySparks]

...It is possible to obtain an approximation of Ra through use of an earth loop impedance tester where the measured Ze minus half the measured L-N loop impedance is used.

And since it's the Ze (& Zs) that really matters with regard to earth fault disconnection times, if you have a supply already on site it's not a bad way of doing it (unless you really need to know an accurate figure for Ra).

 

[Chris Churchman]

YOU are a star! I really appreciate all of this expertise and advice. As you can tell, as much as I am fairly confident in my abilities, there is an awful lot to learn and experience is the greatest of them all. Thank you for all your help.

Kind regards,

Chris

 

[davesparks]

And since it's the Ze (& Zs) that really matters with regard to earth fault disconnection times, if you have a supply already on site it's not a bad way of doing it (unless you really need to know an accurate figure for Ra).

I guess it depends on whether you are happy with an approximation which can be affected by external influences or if you prefer to measure it properly.

 

[davesparks]

YOU are a star! I really appreciate all of this expertise and advice. As you can tell, as much as I am fairly confident in my abilities, there is an awful lot to learn and experience is the greatest of them all. Thank you for all your help.

Kind regards,

Chris

Yes it is clear you need a lot more experience. You are obviously not ready to be working away from your mentor yet. How far through your apprenticeship are you?