TT System - RCD protection

[PC123]

Hi All,

New to the forum so thanks in advance for replies and assistance.(My background is in IT but am currently studying to hopefully add electrical qualification in the future. I have passed part P, registered with NICIEC but have not had assesment as yet)

OK, I have recently moved into a new property which has a TT system installed. The CU is a split with main isolator switch and 6 circuits (lights, cooker, boiler etc) then 30mA RCD with 2 ring final circuits.

Through reading OSG I dont think that this set up is appropriate. Can anyone confirm this. I believe I need to have both sets of circuits RCD protected utilising a 100mA RCD next to the main isolator switch if I am to remain with the TT system.

The overhead cables in the village have been changed recently to 3 twisted cables (PME system?) rather than 3 seperate cables therefore I have the possiblilty of ditching the TT system and earthing through the supply (TN-C-S).

My first question is would it be preferable to change my earthing arrangements rather than my CU setup. TT systems seem to have thier own peculiarities (high earth loop impedance results etc) so this may also make sense for the future.

Sorry for the long post, hope it all makes sense

PC123

 

[lazygit]

17th requires RCD protection on all circuits now, unless your house is wired in earthed metal conduit.

 

[PC123]

Of course (how often are they gonna change these regs!!!)

therefore would installing a 100A 300mA Type S (Time Delayed) RCD Main Incomer do the job?

Initial question still remains - would it make sense to move away from the TT system

thanks for the reply

 

[GaryMo]

Through reading OSG I dont think that this set up is appropriate.

If you're reading an OSG then it'll be for the 16th edition as the 17th edition version isn't available yet.
You need to be reading regulations or guidance based on the 17th edition (BS7671:2008).

 

[PC123]

thanks GaryMo

I have got a copy of John Whitfield Electricians Guide to 17th Edition. Unfortunatley Earthing section so badly printed I cant read it so am waiting for replacement to be delivered

 

[GaryMo]

I believe you don't always have the option of converting from TT to TN-C-S. I've never been involved in a conversion so not sure who you would contact - the DNO or the company you pay your bill to.

 

[PC123]

Thanks Gary Mo,
Interesting stuff, and seeing as getting the DNO or my supplier to come out and change anything will probably take a few days/weeks sticking with the TT system may be the way to go.
Is there any need to replace my current CU with a 17th edition compatible one or would it be ok to upgrade my current CU with a 100Ma RCD (S-type) so that it complies.

Have I got the terminology and suggested course of action correct?

thanks again

 

[Spark123]

How can you pass "Part P" - it is a building regulation- not a qualification?
Do you own a copy of BS7671:2008? Take a look at the sections 411.3 and 411.5.
Does your installation comply with this? Would it comply if you installed a 100mA (S) RCD? I doubt it.
Also take a look at 522.6.7.

 

[PC123]

Hey Spark123 sorry for the sarcasm but did you get up on the wrong side of the bed today?
As i said in my opening post Im new to all this and I was hoping to get some friendly advice and help!
Part P, qualification/non qualfication discuss... Does it really matter, you have to be conversant in that side of the building regs, you have to be tested to prove you are, so thats what ive tried to do - I am under no illusions about what it provides and what additional training and quals I need to pursue this further. However if the use of 'passed Part P' offended you then please accept my apologies.
I will look up the sections you mentioned so thanks for the nudge in the right direction.
I am currently doing a visual inspection on the installation in my house and these are some of the 1st things i've noticed and wanted to get some guidance. Call me a bit thick but sometimes its a good idea to check with others as well as reading regs which sometimes are not the easiest to understand.
All the same thanks for the post - all comments always welcome

 

[Spark123]

You can read about part P here: //www.diynot.com/wiki/electrics:diy-electrical-work-and-the-law.
I find it somewhat worrying that you are applying for NICEIC membership, presumably to do this sort of work for reward and you haven't the foggiest what the current regs require.

 

[PC123]

Well thats what learning is all about!

I am doing the visual inspection to help fully understand the regs and in the frst place and how they relate to real life situations and what factors need to taken into account in different situations.

When I am doing this type of work for 'reward' then, im sure I will be fully conversant with all of the regs. Until then I wont pretend to know everything and I will continue to get education and training where necessary until that time. The NICIEC is great source for both of these as well as being the organistaion that will eventually assess my competence and work.
I thought this forum would also be a good place to get some extra advice - maybe I was wrong.

Anyway thanks again.

 

[Spark123]

I'll suggest you start by reading the current version of the regs.
It doesn't matter what you background is, if you are going to be working on anybodys electrics you need to be competent, which is why you need the technical knowledge. The information you are after is quite basic which is why I am somewhat confused why someone who considers themselves ready to apply for NICEIC membership would need to ask.

 

[Pensdown]

Interesting stuff, and seeing as getting the DNO or my supplier to come out and change anything will probably take a few days/weeks sticking with the TT system may be the way to go.

As your existing installation does not meet the requirements for a TT installation in the first instance I would ask if it can be converted to TNC-S. It's quite straight forward and it is by far your cheapest option. Call your supplier and they will send someone out to measure your Ze. If it's low enough they normally convert it there and then. Some suppliers charge but the last one we had done was FOC.

Is there any need to replace my current CU with a 17th edition compatible one or would it be ok to upgrade my current CU with a 100Ma RCD (S-type) so that it complies.

If you decide to stay with TT you would need to replace your consumer unit. And because your replacing the CU it must be done in accordance with the current regs.

Unless your concealed cables have been installed in accordance with one of the five options (very unlikely) you will need to install 30mA earth leakage protection to every circuit that has concealed cables. So a 100mA type S as a mains switch is unsuitable. Also, it will not meet 314.1

To comply with the regs, including 314.1 you would need to fit a 30mA RCBO to every circuit.

Try for a conversion first.

 

[GaryMo]

To comply with the regs, including 314.1 you would need to fit a 30mA RCBO to every circuit.

Pensdown, have a read of this topic on the IET forum which seems to object against the use of SP 30mA RCD's without a 100mA S type RCD up front.
I think some valid points are raised.

Thread on IET forums

 

[Pensdown]

I totally agree and should have added that RCBO's for TT installations need to be double pole.

Having tried it in my own place, the 100mA TD RCCD solution is a non-runner.

For starters, every time I worked on an isolated circuit it would trip because of the N-E problem and when lamps blew more often than not they would take it out.

Because of the above I can't see how their use meets 314?