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]

But why not? the shed (well pretty big barn) would still have 300ma time delayed protection from the RCD upstream (at origin).

Difficult one I know

But from what you have said the incoming earth on the swa has been electrically isolated from the board .

 

[Rockingit]

The fact that elsewhere in the upstream installation you have a higher rated td RCD is irrelevant. That board needs 30mA RCD protection for final circuits. The td just gives separation of circuits for puposes of nuisance tripping.

I can't see any benefit in trying to sheath the tails once inside the box - they're inside an earthed mechanical protection unit so what's the point? (no different to a run of conduit). Ch 41 doesn't stipulate anything relevant here that I can immediately see.

 

[somersetsparks]

But from what you have said the incoming earth on the swa has been electrically isolated from the board .

It has, but an earth fault would still trip the 300ma

 

[somersetsparks]

The fact that elsewhere in the upstream installation you have a higher rated td RCD is irrelevant. That board needs 30mA RCD protection for final circuits. The td just gives separation of circuits for puposes of nuisance tripping.

I can't see any benefit in trying to sheath the tails once inside the box - they're inside an earthed mechanical protection unit so what's the point? (no different to a run of conduit). Ch 41 doesn't stipulate anything relevant here that I can immediately see.

I can't see why the board needs 30ma protection for all final ccts. It mainly feeds high level lighting and fixed equipment (e.g auger motor). These circuits are wired in SWA, and I don't think they require 30ma protection, I may be wrong. There is a masterseal socket which is on a 16A RCBO.
It is an old installation I am merely advising on since the conversion of the supply from TNS to PME and fixing any faults.

 

[sparks1973]

so the sub board being the one feeding ofice and workshop.....fineal circuits on that being given additional protection by 30mA RCD but the submain feeding this barn is on the 30mA RCD on this sub board?...if so then just move the submain feeding the barn board onto the an unprotected way in the office board, gland the submain feeding the barn at the upstream end (PME)....and then use 100 mA RCD at the barn end on rod....

 

[Deleted member 26818]

The 300mA RCD is fine.
You can't double insulate SWA.
It is only socket-outlet circuits not exceeding 32A that require 30mA RCD protection.
Circuits supplying socket-outlets that exceed 32A require 100mA RCD protection.
For all other circuits 300mA RCD protection is required.

 

[Richard Burns]

I would say that, in accordance with the regs all the circuits in the TT location are protected by an RCD (OK a long way away but still functional), if there are no circuits that require 30mA protection as additional protection then you should be OK as is. But you should fix the 100mA, either as an RCD to prevent nuisance tripping back at the origin or as a switch to save cost.

 

[somersetsparks]

so the sub board being the one feeding ofice and workshop.....fineal circuits on that being given additional protection by 30mA RCD but the submain feeding this barn is on the 30mA RCD on this sub board?...if so then just move the submain feeding the barn board onto the an unprotected way in the office board, gland the submain feeding the barn at the upstream end (PME)....and then use 100 mA RCD at the barn end on rod....

sorry for any confusion!

The layout is

ORIGIN
House DB1. SWF1 feeding workshop. DB2 (connected PME) SWF2 feeding Office DB3 (connected PME)

DB2 (PME) then feeds DB4 Cowshed (connected TT)

DB2 has an upfront 300ma S type RCD, mainly for fire protection (recommend for farms)
DB4 is fed from DB2 in 2c 16mm SWA and is connected TT due to the prescence of livestock. The earthing is isolated and the building staked adequately to earth (less then 2ohms).

My questions is, does DB4 require an upfront RCD as it is already protected by the 300ma in DB2.
or being a metalclad unit, double insulation of the SWA cores to the incomer (of DB4)

 

[somersetsparks]

I think you've got the jist of it Richard and spin,

I will probably just replace the 100ma then as has been said, it could reduce nuisance tripping of the 300ma which is a long way away and the building is usually locked.

 

[JD6400]

so the sub board being the one feeding office and workshop.....fineal circuits on that being given additional protection by 30mA RCD but the sub main feeding this barn is on the 30mA RCD on this sub board?...if so then just move the sub main feeding the barn board onto the an unprotected way in the office board, gland the sub main feeding the barn at the upstream end (PME)....and then use 100 mA RCD at the barn end on rod....

Hi Spark1973 , out of interest what is your take on going TT for a ag install , would it not include all sub mains as well that are going out of the offices , and to that point where would you draw the line at going TT , would not the offices need to go that way as well as they are on a property with livestock present ?
I would not personally do the offices , but as i said where do you draw the line ?

 

[Deleted member 26818]

How exactly do intend double insulating the SWA?
Are you going to remove the outer sheath, remove the armour, apply the double insulation, then replace the armour and the sheath?
The 300mA RCD in DB2 is providing protection for both the SWA distribution circuit and the final circuits fed from DB4.
You don't require the 100mA RCD main switch, but having one (or a 300mA) would help with discrimination.
As it stands, you would not know where the fault that caused the 300mA RCD at DB2, is on a circuit fed from DB2, or one fed from DB4.

 

[somersetsparks]

I would only say that TT'ing livestock buildings is necessary, I don't see any good reason for ditching a 0.08 Zs for a higher one were no risk to livestock etc. is present, at the end of the day the office is just the same as any office and the workshop as any industrial unit.
I suppose if the bulding was a combined workshop/office cowshed etc. Then I would TT the whole lot but as they are completely seperate buildings I see no need.

I seem to remember someone on here saying there is nothin preventing PME on farms as long as certain conditions are met.

 

[Deleted member 26818]

You would have to install a buried steel mesh, which would then have to be bonded to the PME earth.

 

[JD6400]

But what about the sub main feeding the cow shed , what if there was a fault down this ?
Remember those horses at the race course last year , i am talking about the TT regs and the sub not having a grid mesh over it , at the moment and not about it already having the 300ma upfront .
What are others thoughts on it ?

 

[sparks1973]

Hi Spark1973 , out of interest what is your take on going TT for a ag install , would it not include all sub mains as well that are going out of the offices , and to that point where would you draw the line at going TT , would not the offices need to go that way as well as they are on a property with livestock present ?
I would not personally do the offices , but as i said where do you draw the line ?

well i would suppose that theres nothing wrong with a stable TT system for offices is there....but it was just as its already on a PME connection my thinking was to keep it that way....but use the PME toto feed the barns...and TT it localy which would of course entail ensuring that the armourings downstream (barn end) could not be connected to the local earthing arrangements for said barn.....a bit like TTing a garage/outbuilding being fed from a C/U that has PME as its earthing arrangement....i would also consider yes...the presence of livestock and as such the layout of this farm/stud whatever would be important......i would go so far to say as long as the office area is well away from the presence of livestock.......now i may be talking cobblers here but your input would be appreciated....
With thanks.