TT system and earthing

[geoffa]

Would appreciate some advice please.
I'm wiring a TT system and am un-sure of what earthing is required.
Reg 411.3 - 'Requiremnts for fault protection', says that protective equipotential bonding is required to extraneous-conductive-parts which i'd already done. Am i correct in saying that there is no requirement for cross bonding in the bathroom and kitchen now?
Also could someone clarify how Automatic dis-connection of supply is achieved?
My understanding of the reg's are that if the Zs value is too high, (which is most likely for all TT systems) to use tables 41.2 to 41.4 for overcurrent devices, then it's acceptable to rely soley on the operation of the RCD to trip within .2s at 2xI delta n or greater (Note 2 below Table 41.1)
Reg 411.5.2 says that either an RCD or overcurrent device may be used, pref' the RCD with overcurrent devices on circuits.
So as my Zs value is way too high to use overcurrent devices for ADS then its's fine to rely on the RCD for ADS within the required time....?
Thanks in advance

 

[Des 56]

could someone clarify how Automatic dis-connection of supply is achieved?
By the use of an Rcd.30m/amp with a max Ze of 1667 ohms for operation of that Rcd

The reading is however recomended to be under 200 ohms for purposes of stability

So as my Zs value is way too high to use overcurrent devices for ADS then its's fine to rely on the RCD for ADS within the required time....?
Yes


Am i correct in saying that there is no requirement for cross bonding in the bathroom and kitchen now?
Kitchens are not special locations and have no need of supplementary bonding.
Special locations such as a Bathroom still require supplementary bonding,but that can be omitted if the conditions for its ommision are met

 

[geoffa]

cheers,
thought i was correct just wanted to make sure as i've no experience with TT systems

 

[Engineer54]

Why you would not supplementary bond in a bathroom is beyond my understanding, ...again relying on a RCD for everything!!! Then best keep your fingers crossed, ...lol!!!!

 

[Chr!s]

Why you would not supplementary bond in a bathroom is beyond my understanding, ...again relying on a RCD for everything!!! Then best keep your fingers crossed

If you comply with the regs, in effect you are

 

[IQ Electrical]

If you comply with the regs, in effect you are

Yes, one of the 3 conditions that must be met is that 'all extraneous-conductive parts of the location must be effectively connected to the protective equipotential bonding according to Regulation 411.2.1.2

The application of 415.2.2 can be used to check the above:

R ≤ 50 V/Ia

Ia is the operating current of the protective device (I∆n for RCD's or the current causing operation in 5s for overcurrent devices).

I still, in all my years in this business, have not seen or read of 1 single incident attributed to the failure of an RCD/RCBO and believe me, I read electrical publications rather like a monk might study the bible!

 

[Engineer54]

I still, in all my years in this business, have not seen or read of 1 single incident attributed to the failure of an RCD/RCBO and believe me, I read electrical publications rather like a monk might study the bible![/SIZE]

Considering that RCD devices haven't been around in any numbers for the majority of your time in this business, it's not too surprising!!!
In fact, you could say the same about bathroom suplementry bonding, taken in the same context. In my humble opinion, it's better to have, than not have it....

 

[JD6400]

Yes, one of the 3 conditions that must be met is that 'all extraneous-conductive parts of the location must be effectively connected to the protective equipotential bonding according to Regulation 411.2.1.2

The application of 415.2.2 can be used to check the above:

R ≤ 50 V/Ia

Ia is the operating current of the protective device (I∆n for RCD's or the current causing operation in 5s for overcurrent devices).

I still, in all my years in this business, have not seen or read of 1 single incident attributed to the failure of an RCD/RCBO and believe me, I read electrical publications rather like a monk might study the bible!

Granted but i bet you have come across a knacked rcd ?
We come across at least one a year , even had brand spanky out the box ones not play ball !
Surely we are not alone with this ?
But in fairness we do a lot of agricultural !

 

[IQ Electrical]

Considering that RCD devices haven't been around in any numbers for the majority of your time in this business, it's not too surprising!!!
In fact, you could say the same about bathroom suplementry bonding, taken in the same context. In my humble opinion, it's better to have, than not have it....

The PowerBreaker RCD celebrated it's 35th birthday in 2010 and I'm 41 years old!

I'm still looking for evidence of 1 incident attributed to the failure of an RCD/RCBO....

 

[IQ Electrical]

Granted but i bet you have come across a knacked rcd ?
We come across at least one a year , even had brand spanky out the box ones not play ball !
Surely we are not alone with this ?
But in fairness we do a lot of agricultural !

Our main business is periodic inspections and we probably find 2-3 failed RCD/RCBO devices per week but it would be interesting to see failuire rates on faults of negligible impedance and even more interesting to see comparable failure rates for MCB's!

It's all about perspective.

 

[Engineer54]

The PowerBreaker RCD celebrated it's 35th birthday in 2010 and I'm 41 years old!

I'm still looking for evidence of 1 incident attributed to the failure of an RCD/RCBO....

I didn't say that RCDs haven't been around for any number of years, i said that they haven't been around in any ''numbers''. And that is very true, they have only become popular over fairly recent times. For the majority of those 35 years, the costs of such units were prohibitive and only considered on prestige jobs and projects....

 

[IQ Electrical]

I didn't say that RCDs haven't been around for any number of years, i said that they haven't been around in any ''numbers''. And that is very true, they have only become popular over fairly recent times. For the majority of those 35 years, the costs of such units were prohibitive and only considered on prestige jobs and projects....

Whatever views and opinions abound, I'll stick with the facts and the facts are that out of the many many thousands of RCD/RCBO's in use, I have never seen or read of 1 incident attributed to the failure of one of these devices.

 

[Des 56]

I dont think its a matter of any documented accounts of RCD failure resulting in an incident that should be the subject
It is the modern day reliance on Rcd back up protection that has now become primary in safety issues
The oft slandered fuse has one important advantage over an Rcd.given enough current flow it cant not do anything other than dis connect

Special locations now has possibility of omitting what was frontline protection by the inclusion of an Rcd
Is that a step forward or back

The inclusion of the Rcd for special locations would have been a step forward,but it has been almost universally and in correctly recognised as a means of not having supplementary bonding in a special location
because " its RCd d"instead of a straight additional requirement

 

[IQ Electrical]

But if the 3 conditions are satisfied, what do you achieve with the green/yellow everywhere?

All that regulation did was allow a competent person to decide if supplementary connections were necessary based on a sound formula.

The addition of rcd protection is then a step forward.

 

[Engineer54]

Whatever views and opinions abound, I'll stick with the facts and the facts are that out of the many many thousands of RCD/RCBO's in use, I have never seen or read of 1 incident attributed to the failure of one of these devices.

Unfortunately facts like figures can be made to say almost anything these days!!...lol!!! I wouldn't mind betting that there's a hell of a lot more properties without RCD protection in bathrooms but with supplementary bonding, Probably haven't seen any incidents concerning these either!!