TT installation help

[Welshsparky]

Hi all

Job on a farm, which is TT, The supply conductors go into a 100 amp 4 pole RCD Mainswitch ( In a plastic Enclosure) Then off to an MCCb Board which then feeds Several sub mains around the farm that all Have 30 Milliamp RCD Mainswitch

Problem the customer has is when their is a fault at one of the sub mains IE - House or office, This trips the Main RCD for the installation before tripping the 30 Ma's Rcd at the submains, Would installing a a 100ma time S type solve this problem? If not Can anybody advise me what would?

Many Thanks

 

[sjm]

That should do it I would have thought, surprised it's not type S already.

 

[ackbarthestar]

I assume that there is a BS 4293 30mA RCCB in the main board?
If so there is not much chance of earth fault discrimination between two RCDs in series.

IMO, I would try either a 100mA or 300mA 'S' at the main board and test downstream at each of the other 30mA protected boards....

 

[Welshsparky]

The Supply RCCB is 100 Ma , but trips most times before the 30 Ma 's at the sub mains

 

[IQ Electrical]

The Supply RCCB is 100 Ma , but trips most times before the 30 Ma 's at the sub mains

The only way to achieve discrimination with RCDs in series is by using a time delay upstream.

 

[alarm man]

The Supply RCCB is 100 Ma , but trips most times before the 30 Ma 's at the sub mains

does it trip before the others or is it the only rcd that trips..

 

[i=p/u]

Ra...............................

 

[Welshsparky]

The Rcd were the fault occurs will trip, but the Rcd at the supply will always trip aswell. resulting in having to reset the rcd at the sub mains, Then having to go out to the barn to reset the one at the supply also

 

[i=p/u]

time delaqyed and put in the max I delta n you can go with

 

[alarm man]

The Rcd were the fault occurs will trip, but the Rcd at the supply will always trip aswell. resulting in having to reset the rcd at the sub mains, Then having to go out to the barn to reset the one at the supply also

may be worth testing the rcds,is there a reason for the tripping any signs of rodent damage in the outbuildings,had the same with rat chewed cables....
ps wear gloves when looking

 

[sedgy34]

yeh try the time delay option should be the last to trip

 

[Guitarist]

Out of interest, how is the earthing system arranged?

 

[ackbarthestar]

Had this problem with a garden install. 100mA RCD protecting the main installation and a 30mA covering the garden shed.

Tested out Ok at 1/2 and 1 times IAN but tripped both out on 5 times IAN.

IMO to reduce nonsense tripping then replace the main RCD with eaither 100mA or 300mA 'S'

 

[Guitarist]

Had this problem with a garden install. 100mA RCD protecting the main installation and a 30mA covering the garden shed.

Tested out Ok at 1/2 and 1 times IAN but tripped both out on 5 times IAN.

IMO to reduce nonsense tripping then replace the main RCD with eaither 100mA or 300mA 'S'

I hope not all of the house installation was relying on 100mA

 

[ackbarthestar]

I hope not all of the house installation was relying on 100mA

So do I !
I separated them out with separate feeds....