TT earthing and supply

[frisbee]

I have a 1920s house with what i think should be a TT earth system. Basically, there are two cables from the overhead line (live / neutral) and there is no earth connection to the meter head which you would expect from a TNCS system. There is a 4mm earth cable that goes outside but not connected to anything - i think this was once connected to a earth rod and i will get this sorted.

I have a MEM metalclad consumer unit, it has a 100A DP isolator and some RCBO's (30ma) for sockets etc and some MCB for other circuits such as lighting. I intend to keep some of the MCB's so will sort out the wiring on these circuits to make it comply.

1. As its a TT system do i need to swap the 100a isolator for a RCD. With the new regs it mentions about not having a common RCD so not sure how i get around this. Can the RCD be installed after the meter but before the consumer unit (so outside the CU and separate from this)
2. What sort of RCD is needed (what rating and time delay)
3. What size earth cable is needed to connect to the earth rod

I was a electrician 15yrs ago so i have a basic knowledge just need to get back into the swing of it

thanks

 

[rattlehead85]

Firstly you need to get that earth sorted as you currently have no adequate means of earthing to your installation. You cannot rely on an RCD as a substitute for a lack of earthing. You could then replace the main switch with a 100mA rcd covering circuits that don't incorporate sockets.

 

[rattlehead85]

As far as the size of the earth cable is concerned it depends on how you plan to run it. The norm is to run it to an earth electrode inspection pit using plastic conduit or ducting as mechanical protection where buried. In this case you will need a minimum of 2.5mm2. If you plan to lay the conductor straight in the ground unprotected you will require a 25mm2 cable.

 

[ajelec10]

Go and buy an on site guide! Read it and then ask some questions!

 

[Engineer54]

Go and buy an on site guide! Read it and then ask some questions!

Now that isn't going to help too much either is it!! lol!!

 

[Engineer54]

The whole installation needs to be protected by 30mA RCD's, be that via a dual 17th edition CU or via all RCBO's
I would also swap-out the 100A isolator with 100A/100mA S type RCD as additional protection against any failing downstream 30mA RCD devices.

BTW with overhead TNC-S (PME) supplies, the main earth conductor is taken from the neutral in the service head, so your not going to see 3 cables coming from the pole into your house!!


Before you start driving in rods for a TT system, ask the DNO if your overhead supply has been PME'd, as many have been over the years!! Other than that, if you have to go the TT route, use at least 2 X 5/8'' rods coupled together for depth and stability, (as well as lower Ra values). Make your TT earth rod position at least 1.5m away from any external house wall, to be sure of missing any original builders rubble. And don't forget to install a propriety flush to ground earth pit!! Personally, i wouldn't use less than a 6mm for a rod earthing cable, unless your lucky and get -1 ohm, the a 10 mm would make sense!! lol!!

 

[rattlehead85]

Just a thought from the above post which is sound advice. The lighting will not require 30mA RCD protection if the Zs values recorded are low enough to allow the use of a 100mA rcd. Normally the max permissible Zs tolerance for compliant disconnection of a 100mA RCD is 500 ohms. However the type of RCD will be governed by how your cables are installed. You may need a 30mA RCD if you can't comply with 522.6.6 parts i,ii,iii or iv.

 

[jamie-spark]

Did anyone else read the article in January's professional electrician . it was an article by doepke, in it they said 100ma s type RCDs do not afford complete discrimination when upstream of a 30ma RCD, as the current/time tripping characteristics overlap. Instead a 300ma type s RCD should be fitted.
Looks like they are going to have to reprint the osg lol

 

[pete555]

Go and buy an on site guide! Read it and then ask some questions!

your being a bit harsh have you no patience for the unknowners

 

[ajelec10]

I just find the on site guide very help full with its nice little pictures, it's very straight to the point and easy to understand. Also If he did get one and read it we can then spend our time sorting out the questions after he understands the basics.

 

[ajelec10]

Also the op mentions a metal clad consumer unit, best practice to use an insulated consumer unit with a tt supply as normally the meter are not rcd protected and could chaff out on the metal making the whole installation earth live.

 

[Engineer54]

Did anyone else read the article in January's professional electrician . it was an article by doepke, in it they said 100ma s type RCDs do not afford complete discrimination when upstream of a 30ma RCD, as the current/time tripping characteristics overlap. Instead a 300ma type s RCD should be fitted.
Looks like they are going to have to reprint the osg lol

Do you or anyone else have a link to this article?? I'd like to see the full argument behind this claim!!

 

[Silly Sausage]

Eng, surely there must be loads of interesting places to go and check out over there, opposite HK?
Why do you waste so much time banging your head with these endless TT rod gibberish?
Give me a job as your boy and I'll save you from all this bull****!

 

[Engineer54]

Eng, surely there must be loads of interesting places to go and check out over there, opposite HK?
Why do you waste so much time banging your head with these endless TT rod gibberish?
Give me a job as your boy and I'll save you from all this bull****!

There is, and we do!! Go out exploring i mean. Thing is i have spent many years living and working in both HK and China now, and seen most of the interesting places in HK, Changsha, Zhuhai and surrounding areas, and more than a few in areas that i haven't been living/working in, like Beijing and Shanghai etc.

When this project is over, that's it for me, and will be spending at least 6 months touring the rest of China to areas we haven't seen. It's a bloody big country with more history than many other countries put together.

If you notice, the times i come on here, is normally either early morning or late evening (in China which is 8 hours ahead of you) and normally the Forum is running in the background, while i'm working at home or doing other things on the Laptop/PC.

 

[jamie-spark]

I have it in the van. I dont have a scanner, but I do have a hi res camera so will try that and upload it later. I also looked at PE online, could not find the article though.