Domestic DB

[Phaze-3]

Hi Guys,

Take a look at the picture below. (you may have to turn your head as i couldn't work out how to rotate it it).
As you can see, the DB has seen better days so i am going to replace it with a nice new one with plenty of ways.

I don't get to involved with domestic work as i really only work with industrial installations and bigger stuff. Few questions you guys hopefully will be able to help me with. I will just add that this is a TT System to this house.

1) How far does does the main intake and DB need to be away from the Gas Meter?(As you can see, it is pretty close)

2) I done a Ze on the existing board and was getting a silly reading! Do i need to be looking at installing a new earth rod? If so where is the best place to put it?

3) Choosing a new DB. I was thinking of getting the usual 2 x RCD split boards. Now do i need an RCD Main Switch to feed the Split board (if that makes sense). and if i do does that main switch ideally need to be 100ma?

or

4) can i put a normal DB in with a 100ma main switch and RCBO's to feed each circuit.

I've been a spark for 14years but only ever work on industrial installations and dont know that much about the domestic side of things these days. There are just a few little question im just not sure about when it comes to some of the regs that are involved in domestic installations.

Cheers Guys


Steve

 

[Phaze-3]

How close is the existing earth rod to the house? Under the eaves tends to result in dry ground and poor contact. I find a house for half an hour or so can help.

Not to sure where the existing one is. There is no evidence of where it is. The Earth just disappears into the ground.

 

[Octopus]

Not to sure where the existing one is. There is no evidence of where it is. The Earth just disappears into the ground.

And your view on Part P compliance with building regs is???????

 

[Engineer54]

First off install a new TT system, the existing one is a dead duck!! lol!! You need a minimum of 2 X 5/8'' extendable rods coupled together to give you the depth required to make the system a stable one, and with an added bonus of substantially lowering your Ra value. Locate the rod position at least 1m away (more is better) from any external house walls to miss original builders rubble etc. Oh, and use a flush to ground earth pit over the rod to protect the connection, and to facilitate future testing, as well as a visual means of identification of earthing location!!

Secondly, i wouldn't take any notice at all of richy3333 suggestion of NOT incorporating an S type 100 mA upfront RCD (which could be used in place of CU main isolator). This is a TT system, and totally relying on a single RCD for earth fault protection is not very sensible, The upfront S type RCD will be if you like, the Additional protection, that a normal RCD gives to a TN installation!! As for the type of CU you install that's up to you dual RCD, or the far better but costlier all RCBO...

 

[streetlighter]

Am i seeing it right in the photo, There appears to be 3x Wylex plug-in MCBs bodged in place of porcelain fuse carriers in that old MEM unit ? So the cover is left of all the time exposing Live parts ?

 

[richy3333]

First off install a new TT system, the existing one is a dead duck!! lol!! You need a minimum of 2 X 5/8'' extendable rods coupled together to give you the depth required to make the system a stable one, and with an added bonus of substantially lowering your Ra value. Locate the rod position at least 1m away (more is better) from any external house walls to miss original builders rubble etc. Oh, and use a flush to ground earth pit over the rod to protect the connection, and to facilitate future testing, as well as a visual means of identification of earthing location!!

Secondly, i wouldn't take any notice at all of richy3333 suggestion of NOT incorporating an S type 100 mA upfront RCD (which could be used in place of CU main isolator). This is a TT system, and totally relying on a single RCD for earth fault protection is not very sensible, The upfront S type RCD will be if you like, the Additional protection, that a normal RCD gives to a TN installation!! As for the type of CU you install that's up to you dual RCD, or the far better but costlier all RCBO...

I don't necessarily disagree about the 100mA RCD, but perhaps you could quote the REg that requires it? Just re-read my previous post I didn't say to NOT incorporate a time delayed 100mA RCD, but highlighted it is NOT a requirement of BS7671, unless of course you can advise differently?

 

[Engineer54]

I don't necessarily disagree about the 100mA RCD, but perhaps you could quote the REg that requires it? Just re-read my previous post I didn't say to NOT incorporate a time delayed 100mA RCD, but highlighted it is NOT a requirement of BS7671, unless of course you can advise differently?

Then why would you highlight something like that, that should be a part of every TT system. So what if there is no regulation in your bible, the same bible advocates a numbty 200Ohm Ra for a TT system!!
Can you not think for yourself, does everything have to have a reg attached for you??

An understanding of a TT system coupled with some commonsense, should tell you an S type upfront RCD is a prudent additional protection to any (dual 30mA RCD protected or otherwise) TT system. As i stated before, your comment on the no requirement of a time delayed RCD should not be taken any notice of at all!!

 

[FAA Greenie]

The cut out looks like it has been installed as a TNC system. A quick phone call and you could find yourself not needing a TT system.

 

[Heisenberg]

The cut out looks like it has been installed as a TNC system. A quick phone call and you could find yourself not needing a TT system.

Don't think so, no sign of any connection to the supply cable...



EDIT: Actually I presumed you meant TNS not TNC, so I apologise if you actually did mean TNC :lipsrsealed2:

 

[D Skelton]

circuits need to be protected by 30mA RCD/RCBO.

Slight error there mate

Not all circuits will need 30mA RCD protection.

 

[Guest55]

Slight error there mate

Not all circuits will need 30mA RCD protection.

but circuits without 30ma protection will still require an upfront 100ma rcd in TT installs.
;-)

 

[Octopus]

but circuits without 30ma protection will still require an upfront 100ma rcd in TT installs.
;-)

Biff, have you got your sensible head on. lol

 

[Guest55]

Biff, have you got your sensible head on. lol

it wont last

 

[D Skelton]

but circuits without 30ma protection will still require an upfront 100ma rcd in TT installs.
;-)

No they won't. All you need to ensure is that the maximum permitted Zs for each circuit isn't exceeded. If for example you had a stable Ra value of 150 ohms you would be fine with 300mA RCD protection for all circuits other than those that require additional protection by 30mA RCD.

Hell, if you could guarantee a low enough and stable enough Ra value you wouldn't need RCD protection at all!

I'm just trying to get people to think a bit more about things, I'm not neccessarily advocating ommision of 30mA RCD protection

 

[Guest55]

No they won't. All you need to ensure is that the maximum permitted Zs for each circuit isn't exceeded.

and what are the chances of getting a Ze/ Ra below 0.5 ohms from a rod to give you a fighting chance of meeting the max Zs of 1.16 for a 32A type b ? none i'd say.


If for example you had a stable Ra value of 150 ohms you would be fine with 300mA RCD protection for all circuits other than those that require additional protection by 30mA RCD.

150 ohms isnt that stable at all , most highways authorities accept no more than 20 , and if that 150 creeps up then that 300ma wont be doing its job , hence the 100ma suggested in the OSG ;-)

Hell, if you could guarantee a low enough and stable enough Ra value you wouldn't need RCD protection at all!
ok.

I'm just trying to get people to think a bit more about things, I'm not neccessarily advocating ommision of 30mA RCD protection

...............

 

[D Skelton]

and what are the chances of getting a Ze/ Ra below 0.5 ohms from a rod to give you a fighting chance of meeting the max Zs of 1.16 for a 32A type b ? none i'd say.

In your average domestic setting the money spent on achieving such a low result would far outweigh the benefits of such a result however in larger installations I'd say it would be quite easy to achieve such a low Ra. Oh, and the max Zs is 1.44. Yes I'm a pedant, what of it?! lol

150 ohms isnt that stable at all , most highways authorities accept no more than 20 , and if that 150 creeps up then that 300ma wont be doing its job

C'mon Biff, you're smarter than that.

Everyone knows that stability has little to do with the size of the Ra value. There are so called 'sparks' up and down the country pi$$ing on rods to achieve low Ra values but does that value do anything for it's stability? No.

but circuits without 30ma protection will still require an upfront 100ma rcd in TT installs

hence the 100ma suggested in the OSG

So is that a requirement or a suggestion?


Anyway, you're missing my point entirely. I'm not advising on a solution, I'm not sitting here saying that I would go for a low Ra value just so I can get away with a higher rated RCD?!, I'm not even saying what I would or wouldn't do myself... I'm merely pointing out the common misconception that TT systems need 30mA RCD protection and exposing the misguided link in peoples minds between the phrases 'TT' and '30mA'.