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thekingiam

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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?
 
K

Knobhead

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  • #2
If the CU is non metallic and close to the meter I don’t think you require an up front S type RCD. You’ve got the protection you need in the board already.
 
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Deleted member 9648

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  • #3
As long as 30ma protection is in place where required you dont need another RCD assuming the CU is insulated etc.....the advantage of having a 100ma S type as a main switch is it provides back up earth fault protection if the other RCD fails,as the EFLI is likely to be too high to operate an OCPD on a usual UK TT.....if it's a main switch I'd go for a 100a.....but if you decide to go lower you need to check it wont be overloaded
 
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thekingiam

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  • #4
i posted this elsewhere as well it was mentioned if a outside supply rcd might be needed any comments?
 

spark 68

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Arms
Esteemed
Hi mate,

The 63A rating on the RCD is just the max current capacity it can safely carry, it does not relate to tripping, as RCD only trip on imbalance (earth leakage faults etc.) and do not provide any over current protection.

All final circuits require 30mA protection, so you cannot just replace a 30mA device with an RCD @100mA T/D or not.

As to whether you need 100mA up front, this depends on whether the CU is metal and is used on a TT system where the tails require additional protection or not.

Some of the older Sparks of say they prefer 100mA T/D up front on TT systems, but see above, this is not neccessarily so.

One last point, if your 30mA devices are tripping regularly, then you either have a fault, or too many High (normal) leakage Items on one RCD, either way further investigation is required.
 
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thekingiam

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  • #6
i am using this install to join elecsa would could inspector fail me for not providing one?
 
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Deleted member 9648

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  • #8
Hi mate,

The 63A rating on the RCD is just the max current capacity it can safely carry, it does not relate to tripping, as RCD only trip on imbalance (earth leakage faults etc.) and do not provide any over current protection.

All final circuits require 30mA protection, so you cannot just replace a 30mA device with an RCD @100mA T/D or not.

As to whether you need 100mA up front, this depends on whether the CU is metal and is used on a TT system where the tails require additional protection or not.

Some of the older Sparks of say they prefer 100mA T/D up front on TT systems, but see above, this is not neccessarily so.

One last point, if your 30mA devices are tripping regularly, then you either have a fault, or too many High (normal) leakage Items on one RCD, either way further investigation is required.
He's not replacing 30ma protection....he's using a twin RCD CU.....the question concerns an additional 100 ma S type main switch unless I've misunderstood.
 
T

thekingiam

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  • #9
cu pvc domestic just put earth rod in as cable undersized, supply via concrete floor outside into a black bicc box connector thingy then into wall into pantry.

- - - Updated - - -

He's not replacing 30ma protection....he's using a twin RCD CU.....the question concerns an additional 100 ma S type main switch unless I've misunderstood.
correct.
 

spark 68

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Arms
Esteemed
Sorry,

He seemed to imply replacing the 63A 30mA ones with a 100mA type, and then went on to ask about upfront protection ?

Maybe I misunderstood ?
 
T

thekingiam

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  • #11
sorry, i was asking what type i could use upfront but then asked if i needed one at all as split load pvc cu 2 x 63 amp 30ma.
 

spark 68

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Arms
Esteemed
Right back on track, lol

Not if your CU is Class II (plastic), then no you won't fail for not providing an additional up front T/D 100mA RCD, as this is generally not required for domestic if you are providing a split load twin RCD board containing the two 30mA RCD's, as this will take care of your requirements.
 
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Deleted member 9648

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  • #13
I'm all for an additional 100ma S type main switch purely as back-up as stated....although some may not wear the additional cost. RCD's do fail,and on a normal TT you are totally reliant on RCD's for earth fault protection,IMO the chances of two failing is miniscule. Of course if you are one of those who insist on TN Ra values this isnt a concern to you.
 
T

thekingiam

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  • #14
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?
 
T

thekingiam

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  • #15
my ze readings was about 150 ohms new spike, 95 ohms combined with old one
 
D

Deleted member 9648

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  • #16
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

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Arms
Esteemed
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.
 
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T

thekingiam

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  • #18
here's a pic of incoming supplyIMG_20120625_105544.jpg
 

spark 68

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Arms
Esteemed
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
 
T

thekingiam

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  • #20
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

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Arms
Esteemed
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
 

spark 68

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Arms
Esteemed
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 ?
 
T

thekingiam

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  • #25
upfront rcd 80 amp 30 ma, would trip all circuits as 2rcds in cu 30ma
 

spark 68

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Arms
Esteemed
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

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Arms
Esteemed
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

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Mentor
Arms
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
 
T

thekingiam

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  • #30
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?
 

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