Metal Fuseboard on TT

[somersetsparks]

On one of my 'other jobs' which I mentioned the other day,
One building which is TT and fed by an SWA, although the earths are isolated and this building is protected by a rod, has single insulated cores feeding the incomer (100ma rcd).
However the whole installation is protected by a 300ma Time delay at source, so would the double insulation of the incoming cores still be necessary?

cheers

 

[JD6400]

well i think that was the 300mA feeding the lot....i would put a 100mA in at the barn end...for finals exeeding 32A of course....

But that goes back to it might as well be a 30ma as even if the 100 was s type i t may well still take the 300 out instead , depending on how low the Ze is on the both of them .

 

[JD6400]

I best leave this till tomorrow and when i finally get out of me pit and go get that regs book !
Thank Spark1973 , it is good to discuss !

 

[sparks1973]

reg 705.411.1 states that:
in circuits, whatever the type of earthing system, the following disconnection devices shall be provided:
(i) In final circuits supplying socket outlets with rated current not exceeding 32A, an RCD having the characteristics specified in regulation 415.1.1
(ii) In final circuits supplying socket outlets with a rated current more than 32A, an RCD with a rated residual operating current not exceeding 100mA
(iii) In all other circuits, RCDs with a rated residual operating current not exceeding 300mA
so how did they come to the conclusion that the 300mA RCD at origin would suffice?

 

[sparks1973]

this excerpt is taken out of the section (705) BGB that deals with agrecultural and hortecultural premises

 

[JD6400]

I think there was only one 16a socket in the cowshed which had a 30ma rcbo on it , It is the fact that the earth has been electrical isolated and a suggestion for just a main switch at the cow shed that i can not grasp at the moment , be it because of the alcohol or not ?

 

[sparks1973]

I think there was only one 16a socket in the cowshed which had a 30ma rcbo on it , It is the fact that the earth has been electrical isolated and a suggestion for just a main switch at the cow shed that i can not grasp at the moment , be it because of the alcohol or not ?

no it aint the booze DP...what about the install within the cowshed feeding this outlet?...its livestock....

 

[sparks1973]

the thing to take into account here is accumilated earth leakage.....but it would be doubtful if a fault furthest downstream would take out a 300mA TD RCD before being intercepted by a 30mA one....

 

[sparks1973]

But that goes back to it might as well be a 30ma as even if the 100 was s type i t may well still take the 300 out instead , depending on how low the Ze is on the both of them .

would depend on what Ra is...at cowshed...as opposed to Ze upstream....

 

[sparks1973]

but as theres parrallel paths available at the cowshed......

 

[spark 68]

Does this help SS ?

 

[sparks1973]

well...clearly circuits here within shed/s fed direct from 300 mA.....but the point i was making earlier is you have to survay the proposed install area first.....but a good diagram nontheless....see, thats what this forums really about.....not just answering piffling questions to barproppers that have robbed decent electricians out of work...

 

[sparks1973]

its also needed to know what the intended use of the install will be within the cowshed of course....this will define demand (Ib)..which will tell you how to procede.... In, Iz and fault protection.....

 

[spark 68]

I became more confused the more I read the preceding posts, so just decided to upload the page.

As SS said, it would be easier to TT the livestock shed and segregate the PME earth, as the supplementary bonding and grid buried in the floor (see fig 705 P220 BGB) is a fair amount of extra work for PME.

Oh! and any electrical accessories in the shed are to be IP44 minimum.

 

[sparks1973]

I became more confused the more I read the preceding posts, so just decided to upload the page.

As SS said, it would be easier to TT the livestock shed and segregate the PME earth, as the supplementary bonding and grid buried in the floor (see fig 705 P220 BGB) is a fair amount of extra work for PME.

Oh! and any electrical accessories in the shed are to be IP44 minimum.

sorry 68...i thought you refering to me for a mo (ss)...but its clear now..lol...

 

[sparks1973]

I became more confused the more I read the preceding posts, so just decided to upload the page.

As SS said, it would be easier to TT the livestock shed and segregate the PME earth, as the supplementary bonding and grid buried in the floor (see fig 705 P220 BGB) is a fair amount of extra work for PME.

Oh! and any electrical accessories in the shed are to be IP44 minimum.

yes..in hindesight it would be better to just TT the feed to the cowshed as this eliminates the issues described above...lol...