single isolation point

[johnad]

hi everyone i would appreciate your thoughts
i have a TT system (no main earth) and a old voltage trip controlling the cu
i cant deicide whether to use two 4 way units both with rcd's as the incomer or a split load board (main switch and rcd)
"would a 100ma rcd stop nuisance tripping" ?
so if i use two units would i still need the main trip to give me a single point of isolation, ?
if i use the split load option i would be in darkness in the event of a fault
i have had some tripping due to a garden pond and i wish to avoid putting the whole house out if this happends,
on this system is there a regulation to say the rcd's must be 30ma (life savers), ?
sorry if you have been over this before in a previous post but i am now ready to do the job and still cant decide whats best
many thanks

 

[breezer]

i have had some tripping due to a garden pond

I would get your pond sorted

 

[Lectrician]

As good as RCD's are, I would trust one as a 'life saver'

A split load board with a time delayed 100mA main sw with 30mA split would be the best option.

If you had two seperate boards, one 30mA and one 100mA (no need for time delay), then you would need to put these two boards into a henley block to the main tails. You should also place a second INSULATED main switch before the tails split, as there should always be one point of isolation for a property. (although this is very often overlooked, and nearly always overlooked with off-peak econ7)

 

[andy]

or have a larger CU with an isolator feeding a 100mA RCD for lights etc and a 30mA RCD for sockets etc

 

[johnad]

so with both options i can then out the exsisting main trip ?
i was thinking of getting a MK unit pre assembled from screwfix
with all breakers (around £80)
then all i need to do is swap the main switch for a 100ma rcd,
or if i use a big unit with a main switch and two rcd's how do i connect them
what size tails ???
thanks again for the info

 

[andy]

use 16mm or 25mm for the main tails. providing your CU has an RCD protecting everything then you can take away the old ELCB and replace it with an isolator, but you might not be able to do this w/o pulling the service fuse

 

[Big_Spark]

use 16mm or 25mm for the main tails. providing your CU has an RCD protecting everything then you can take away the old ELCB and replace it with an isolator, but you might not be able to do this w/o pulling the service fuse

#Andrew, what he has is not an ELCB, this is an Earth Leakage Circuit Breaker, an early form of RCD. What he has is an archaic Fault Voltage Operated Device (FVOD). These were banned from installation after 1 January 1985 although if they had already been installed they could be left in place.

The confusion is not your fault, it is the bloody manufacturers, the BSi and the Industry as a whole who all called the original RCD's different things depending on their role in the installation and their current rating, there was also some confusion caused by the early ELCB's sensing DC currents on the Earth, like DC sensing RCD's do today, and this l;ed to many thinking they were the same as the FVOD's of the '70's, early '80's, and this caused a lot of confusion. I remember the sparks at work going mad over the confusion as they would often get sent the wrong thing by the wholesalers due to manufacturer nomenclature inaccuracies (not a lot changes!)

 

[andy]

use 16mm or 25mm for the main tails. providing your CU has an RCD protecting everything then you can take away the old ELCB and replace it with an isolator, but you might not be able to do this w/o pulling the service fuse

#Andrew, what he has is not an ELCB, this is an Earth Leakage Circuit Breaker, an early form of RCD. What he has is an archaic Fault Voltage Operated Device (FVOD). These were banned from installation after 1 January 1985 although if they had already been installed they could be left in place.

learn something new every day....

 

[johnad]

still confused

A split load board with a time delayed 100mA main sw with 30mA split would be the best option.

If you had two seperate boards, one 30mA and one 100mA (no need for time delay),

why no need for time delay on two separate boards, ?
and if nuisance tripping can occur with fridges etc would it be better to put the kitchen sockets on the 100ma rcd, ?

or have a larger CU with an isolator feeding a 100mA RCD for lights etc and a 30mA RCD for sockets etc

i am thinking this is a good option but 100ma time delay type or not ?
and if i leave the old trip in place will the rcds time delay or not, trip first.
many thanks

 

[andy]

you use the 100mA time delay IF it is then wired to the 30mA. this is so the 30mA can trip before the 100mA trips.

if you do as i said, and have an isolator feeding a 30mA and 100mA RCD then it does not need to be time delay, since the 30mA will NOT trip the 100mA

 

[ban-all-sheds]

and if nuisance tripping can occur with fridges etc would it be better to put the kitchen sockets on the 100ma rcd?

No - it would be better to have the F/F on a non-RCD circuit.

 

[johnad]

No - it would be better to have the F/F on a non-RCD circuit.

but you cant have a non rcd circuit on a tt system. ???
if i use the time delay option and a fault occurs on the time delay rcd then im still without power to everything,
so if i get myself a split load board and swap the main incomer for a 100ma rcd (so 100ma and 30ma) can i not feed these independently so no need for time delay, and if so how, ?
and would i still need a single point of isolation (ie, my exsisting trip)
or would this trip 1st in the event of a fault...
or could i use the main switch from the unit to feed both (how)???
sorry to be a bore with this but your help is appreciated.

 

[ban-all-sheds]

No - it would be better to have the F/F on a non-RCD circuit.

but you cant have a non rcd circuit on a tt system. ???

Sorry - you're absolutely right - too early on a Monday morning, and too few cups of tea by then...

if i use the time delay option and a fault occurs on the time delay rcd then im still without power to everything,

Yup

so if i get myself a split load board and swap the main incomer for a 100ma rcd (so 100ma and 30ma) can i not feed these independently so no need for time delay, and if so how, ?

No - the main incomer is the 100mA RCD, that feds the 30mA one for the sockets, hence why you use a time delayed one - then a fault on a socket circuit does not take out all your lights etc...

You can get boards with an incomer and then two independent RCDs - Hager for example do one, but there will surely be others.

You still use a 100mA device for the non-socket circuits, but it no longer needs to be a time-delayed one - the delay is to ensure discrimination, and since the 30mA one is not downstream of the 100mA one, a fault on a socket circuit could not trip the 100mA one.

and would i still need a single point of isolation (ie, my exsisting trip)
or would this trip 1st in the event of a fault...
or could i use the main switch from the unit to feed both (how)???
sorry to be a bore with this but your help is appreciated.

You could not feed a second board from the incomer of the first - if you had 2 boards you'd have to split the tails, so you would install an isolation switch:

[code:1]
[METER]=====[SWITCH]=====[HENLEY BLOCK]===[CU1]
||
||
||========[CU2]
[/code:1]

Neither CU1 nor CU2 would be a split load, and neither would need an incomer as well as an RCD, and the 100mA one would not need to be time delayed.

As ever, and this applies to split-load or separate CUs, the best way to avoid nuisance trips, and give the most fine-grained control, is to use RCBOs for each circuit. But that is not a cheap solution...

 

[johnad]

now it sounds like i would be better off using two units both with rcds
so would it be possible to leave my exsisting trip in place as a main isolation point or would this trip before the rcds, ?
and must all the sockets go on the 30ma or can i split them, ?

 

[ban-all-sheds]

now it sounds like i would be better off using two units both with rcds

Depends on space required, numbers of circuits, relative prices..

so would it be possible to leave my exsisting trip in place as a main isolation point or would this trip before the rcds, ?

Replace it with a simple isolation switch.

and must all the sockets go on the 30ma or can i split them, ?

They've all got to be on a 30mA, but you can use RCBOs for better discrimination. In fact, if you take an aggressive view of the Wiring Regulations, RCBOs are compulsory..