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

 

[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'.

 

[Guest55]

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

eh ? not talkin to me like a pr*** are you ?

Everyone knows that stability has little to do with the size of the Ra value.

so tell me how you measure ground stability then ?
or do you just rely on earth rod resistance like everyone else ?

So is that a requirement or a suggestion?

well its an official suggestion , by the IET.

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'.

at no point in my posts did i state this.

 

[D Skelton]

eh ? not talkin to me like a pr*** are you ?

Not taking an obnoxious tone with me are you?

so tell me how you measure ground stability then ?
or do you just rely on earth rod resistance like everyone else ?

Personally, 99% of the time I rely on the Ra alone, hence why I would fit 30mA RCDs 99% of the time. Stability however, if needed can be measured over time taking into account different ground conditions, different seasons etc etc...

well its an official suggestion , by the IET.

Not a requirement then?

at no point in my posts did i state this.

I never said or implied that you did?

 

[Guest55]

yeah whatever lol.

 

[Engineer54]

Oh dear, some just never quite understand the workings of decent or crap TT systems, and the measures needed for it's installations protection!! Stability is ''ONLY'' achievable with depth (the deeper the better) when were talking about driven rods!! Anyone thinking of relying on a single up-front 100 or 300mA RCD on a TT system really does need to think again!!

 

[D Skelton]

Oh dear, some just never quite understand the workings of decent or crap TT systems, and the measures needed for it's installations protection!! Stability is ''ONLY'' achievable with depth (the deeper the better) when were talking about driven rods!! Anyone thinking of relying on a single up-front 100 or 300mA RCD on a TT system really does need to think again!!

Not quite sure who this is aimed at?

Stability is ''ONLY'' achievable with depth

There are a myriad of factors that come into play that will determine how stable a rod is. Yes, I agree, depth is one of the largest, but it's not the 'ONLY' factor. Whack a 3ft rod in the ground in the middle of the desert, it aint deep at all but it will be pretty darn stable! That's not to say it'd be any good mind.

 

[High Tower]

Gentlemen, please keep it civil.........thanks!

and whoever started E54 off on RA values and length of rods is getting a ban!!, you have all been warned do not mention TT systems and exporting PME's infront of him, you know what happens!! :lipsrsealed2:

 

[richy3333]

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!!

I have no idea why you have decided to have a 'pop' at me all of a sudden? The last time I looked this forum was still UK based (and not in a country with a dictatorship) and as such I am entitled to express my views and opinions? With that in mind I am perfectly entitled to make the comment that I did?

As the Regs are the minimum that we work to, I would once again ask you to highlight where in the Regs it stipulates that the use of a time delayed RCD is mandatory? Maybe you could do it without such vitriol?

BS7671 does not advocate an Ra of 200 ohms. It merely states that an Ra of 200 ohms COULD be unstable. I think anyone with experience of installing a TT system would know it is a totally undesirable figure to work towards and that it should be as low as possible. And I certainly would not disagree that BS7671 is lacking in its information about TT systems (something I think you have stated in other threads before).

It is admirable that you have such knowledge and passion for TT systems and that members of the forum can benefit from that, but being rude and obnoxious; using deliberate inflammatory language and attacking me is not warranted, or appreciated. If you feel so strongly about matters then why not use that knowledge and direct it towards those that write the Regs? Else if my previous post is, in your opinion so poor/dangerous/(insert your own expression of disdain) - then report it to the moderators.

 

[richy3333]

Slight error there mate

Not all circuits will need 30mA RCD protection.

Hi D Skelton.
In the context I made my post about the original question I believe the OP will need to use 30mA RCD's/RCBO's. I very much doubt he will get an Ra low enough to achieve disconnection times required. I agree that not all circuits in other types of TT system need 30mA RCD's/RCBO's.

On a general note (and not directed at you D Skelton):

Anyhow, as this thread has turned into the usual slanging match, I'm out.

 

[Engineer54]

I have no idea why you have decided to have a 'pop' at me all of a sudden? The last time I looked this forum was still UK based (and not in a country with a dictatorship) and as such I am entitled to express my views and opinions? With that in mind I am perfectly entitled to make the comment that I did?

As the Regs are the minimum that we work to, I would once again ask you to highlight where in the Regs it stipulates that the use of a time delayed RCD is mandatory? Maybe you could do it without such vitriol?

BS7671 does not advocate an Ra of 200 ohms. It merely states that an Ra of 200 ohms COULD be unstable. I think anyone with experience of installing a TT system would know it is a totally undesirable figure to work towards and that it should be as low as possible. And I certainly would not disagree that BS7671 is lacking in its information about TT systems (something I think you have stated in other threads before).

It is admirable that you have such knowledge and passion for TT systems and that members of the forum can benefit from that, but being rude and obnoxious; using deliberate inflammatory language and attacking me is not warranted, or appreciated. If you feel so strongly about matters then why not use that knowledge and direct it towards those that write the Regs? Else if my previous post is, in your opinion so poor/dangerous/(insert your own expression of disdain) - then report it to the moderators.

Exactly where is the rude, obnoxious, deliberate inflammatory language?? If i wanted to have a pop at you, you would know all about it!! I didn't, ...but it seems you have read differently!!

You can have your say, and state whatever opinion you want too, the same as i do. but why you would advocate not installing an S type RCD on a domestic TT system, is beyond me, and why i included my remarks!! ....It's that simple!!

 

[Engineer54]

Hi D Skelton.
In the context I made my post about the original question I believe the OP will need to use 30mA RCD's/RCBO's. I very much doubt he will get an Ra low enough to achieve disconnection times required. I agree that not all circuits in other types of TT system need 30mA RCD's/RCBO's.

On a general note (and not directed at you D Skelton):

Anyhow, as this thread has turned into the usual slanging match, I'm out.

Apart from mains and distribution sub-main cables etc, i can't think of a single example where a final circuit on any TT system would not require a 30mA RCD type protection. Nothing else is going to trip that circuit out in the disconnection times required on the occurrence of an earth fault...

 

[D Skelton]

Apart from mains and distribution sub-main cables etc, i can't think of a single example where a final circuit on any TT system would not require a 30mA RCD type protection. Nothing else is going to trip that circuit out in the disconnection times required on the occurrence of an earth fault...

How about a final circuit with a stable Zs below 200 ohms?

 

[Engineer54]

How about a final circuit with a stable Zs below 200 ohms?

Let me put it this way.... without a 30mA RCD/RCBO, Rather you than me!!!

 

[Deleted member 9648]

Apart from mains and distribution sub-main cables etc, i can't think of a single example where a final circuit on any TT system would not require a 30mA RCD type protection. Nothing else is going to trip that circuit out in the disconnection times required on the occurrence of an earth fault...

No reason why a final circuit not including socket outlets or buried cables could not be protected by a 100ma rcd rather than a 30ma rcd on any TT system.

 

[telectrix]

No reason why a final circuit not including socket outlets or buried cables could not be protected by a 100ma rcd rather than a 30ma rcd on any TT system.

no reason at all as long as disconnection times are achieved.

 

[Edd]

That was another question i ment to ask earlier as well but forgot. I have no idea why there is a time clock. Economy 7 does spring into mind but i can't see why they would have it at this property.

Anybody got any ideas??????

They dont have any off peak dedicated system like water heater or storage heaters but they most likely have a split bill and duel rate meter, so if they have a time clock on their water heater or washing machine they can benifit from cheap rate for the times the clock is set to.

 

[D Skelton]

Let me put it this way.... without a 30mA RCD/RCBO, Rather you than me!!!

I'm not questioning your personal preference, it is a wise choice, but it is still only a personal preference.