tt best method cheapest method?

[thekingiam]

i was just wondering where do people buy there s type td rcd's from? there seems alot of different prices on line from 30 pounds to 80 pounds.
i have split load cu with two 63 amp 30ma rcd's on a tt installation, original rcd is 80amp 30ma so trips out when a fault.
could i use 80 amp stype 100ma or 100amp s type 100ma? would a 63 amp also work?
do i need an upfront rcd at all as discrimination is already in cu via split load rcd?
​

 

[Deleted member 9648]

malcomsanford said,

TT is different because there is little chance you will get a low enough Zs to activate a normal protection device, so to get around this you would put you SWA on a 100Ma S type RCD and give you discrimination

your opinions?​

my ze readings was about 150 ohms new spike, 95 ohms combined with old one

Fairly typical Ra values......
One way or another an SWA will have to be RCD'd with those Ra values,what value of RCD would depend on the function of the SWA and how it would affect other circuits.

 

[spark 68]

malcomsanford said,

TT is different because there is little chance you will get a low enough Zs to activate a normal protection device, so to get around this you would put you SWA on a 100Ma S type RCD and give you discrimination

your opinions?​

Right leaving out for a moment about backup protection (which I partially agree with btw),

the general rule for TT systems where your Ze is too high to activate the OCPD, is 300mA for fire protection where it is required such as farms and the like,

100mA for distribution cables such a sub-mains and the like.

30mA for additional protection for final circuits.

This is greatly simplified btw.

 

[thekingiam]

here's a pic of incoming supply

 

[spark 68]

thekingiam,

As this is domestic, do you have a sub-main ?, or is it just normal tails at the CU ?

Could you provide a picture of the existing CU ?, and of the service head, and DNO fuse ?

Edit: that picture you have provided looks like a joint in the DNO's supply cable, and as such has nothing to do with you/us

 

[thekingiam]

Right leaving out for a moment about backup protection (which I partially agree with btw),

the general rule for TT systems where your Ze is too high to activate the OCPD, is 300mA for fire protection where it is required such as farms and the like,

100mA for distribution cables such a sub-mains and the like.

30mA for additional protection for final circuits.

This is greatly simplified btw.

so are you saying not a farm, not a sub main 30ma ok?

 

[spark 68]

so are you saying not a farm, not a sub main 30ma ok?

Yes, provided the CU is Plastic (class II).

Have a look at the picture Fig 2.1 (iii) in the OSG p.16, although this shows seperate RCBO's, a dual RCD board here is fine as well

 

[thekingiam]

 

[thekingiam]

happy days!

 

[spark 68]

Right mate,

your only concern is from the existing tails from the meter to the CU.

As your CU is plastic and contains two 30mA RCD's, there is no problem at all.

As I said, some sparks prefer an additional 100mA T/D upfront to provide some backup protection just incase one of the 30mA ones fail.
By the regs this is not a requirement (unless the CU was metal) in this situation, but in some circumstances I would agree with fitting it.
On such a small domestic install it could be argued that this would be overkill.

No, you would not fail an assessment for not providing it.

Just out of interest what is that additional box connected in series with the Tails ?, as the 3rd pic is too small to see.

If it is an RCD, is it just an upfront 30mA type ?, or just an isolator ?

 

[thekingiam]

upfront rcd 80 amp 30 ma, would trip all circuits as 2rcds in cu 30ma

 

[spark 68]

upfront rcd 80 amp 30 ma, would trip all circuits as 2rcds in cu 30ma

OK,
This has got to go!,

you have 4 choices here

1) You could replace the tails from the meter to go directly to the CU, but this would mean cutting the seals on the meter, not recommended.

2) just fit a 100A DP Isolator and enclosure, in place of it

3) fit a 100A (maybe 80A if you know what the DNO's fuse is) 100mA T/D RCD and enclosure

4) Henly block it to join the tails

 

[Taylortwocities]

Just a couple of points from the pictures - you'll need to do these before your ELECSA review:

• Those blank slots in the CU need blanking plates
• The loose cables need tidying up and given support
• The MET needs a "SAFETY EARTH, DO NOT DISCONNECT" label (Pinch one from a BS951 clamp)
• Can't see what is happening in the top of your CU bit it looks like the tails (maybe other cables) go into the top of the CU. Don't forget you must seal all holes in the top of the CU to meet the IP4x requirements

 

[Amp David]

I'd install proper earth pit around the top of the rod. You should also have do not disconnect on top of the rod to IMO

 

[Amp David]

And also Niglon do a well priced 100ma S type rcd

 

[thekingiam]

earth pit done,old pic.
ringing suplier to supply bigger fuse so install new tails to consumer unit dp isolator then,
consumer unit undergoing work hense no blanks labels yet.
presently old tails going into top of cu these needs replacing with new i could come from underneath and seal top with pvc and silicone? could i seal around tails from the top with silicone?