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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?
 
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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.
 
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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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D

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.
 
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thekingiam

  • Thread Starter Thread Starter
  • #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?
 
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thekingiam

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