Sub Mains

stretchyboy · May 13, 2009

The requirement for protecting cables with an RCD when they are less than 50mm from the surface is for walls only though?
I know that I would personally favour an SWA anyday on sub-mains and have a dislike of the use of T&E for this purpose. Also the electricity boards will not accept SWA as a sub-main prior to metering if that is relevant here. But strictly speaking the RCD protection is only necessary for sub-mains within walls. The use of an RCD on the sub-main will also contravene 314:1 All circuits of an installation should not be connected to a single upstream RCD in order to avoid unwanted tripping, avoid hazards and minimize inconvenience in the event of a fault (reg 314.1)

I hope this isn't a spanner in the works ;-)

Plus, even if there was a time-delayed 30mA RCD it wouldn't meet the requirements anyway as 30mA RCD's are for fault protection and to limit the voltage/current flowing into someone during a fault or direct contact, but the time delay would not allow this to happen would it?

MTI · May 13, 2009

hi all, just been reading this thread and im not trying to hijack but i have found this really usefull! cheers! i have to do a sub main tomorrow and was wondering the same. i intended on using swa but my questions are..
1)will i need an isolator at the mains cut out possition? my sub is to another room but only 5m in length.obviously length is not important/part of discussion
2) what size swa? i am led to believe 7671 states 25mm tails but will i need 25mm? can i use 16mm as it will more than suffice considering single phase supply only provides 80A?
3) what size isolator at mains to cover appropriate swa size?
4) if swa is 3 core, i take it no seperate earth is needed as mentioned above? plus, surely a main earth as part of a sub would need mechanical protection?

sorry to but in and hope i dont cause us to go off topic! thanks, martin

oh, and another..if cu is plastic, should i just earth sheath at cut out end and tape neatly at cu or should i put clamp on for the sake of it?

electro · May 13, 2009

stretchyboy said:
The requirement for protecting cables with an RCD when they are less than 50mm from the surface is for walls only though?
I know that I would personally favour an SWA anyday on sub-mains and have a dislike of the use of T&E for this purpose. Also the electricity boards will not accept SWA as a sub-main prior to metering if that is relevant here. But strictly speaking the RCD protection is only necessary for sub-mains within walls. The use of an RCD on the sub-main will also contravene 314:1 All circuits of an installation should not be connected to a single upstream RCD in order to avoid unwanted tripping, avoid hazards and minimize inconvenience in the event of a fault (reg 314.1)

I hope this isn't a spanner in the works ;-)

Plus, even if there was a time-delayed 30mA RCD it wouldn't meet the requirements anyway as 30mA RCD's are for fault protection and to limit the voltage/current flowing into someone during a fault or direct contact, but the time delay would not allow this to happen would it?

what a good post, i have read the thread from start to finish, here is your answer in a nut shell.
quality

regards electro

MTI · May 13, 2009

electro said:
what a good post, i have read the thread from start to finish, here is your answer in a nut shell.
quality

regards electro

any ideas on my situation please electro? im running sub from original metre and cut out to allow cu in another/next room along so my swa is not a sub for elec board to use as my tails into their metre if you get my drift.

cheers, martin

electro · May 13, 2009

MTI said:
hi all, just been reading this thread and im not trying to hijack but i have found this really usefull! cheers! i have to do a sub main tomorrow and was wondering the same. i intended on using swa but my questions are..
1)will i need an isolator at the mains cut out possition? my sub is to another room but only 5m in length.obviously length is not important/part of discussion
2) what size swa? i am led to believe 7671 states 25mm tails but will i need 25mm? can i use 16mm as it will more than suffice considering single phase supply only provides 80A?
3) what size isolator at mains to cover appropriate swa size?
4) if swa is 3 core, i take it no seperate earth is needed as mentioned above? plus, surely a main earth as part of a sub would need mechanical protection?

sorry to but in and hope i dont cause us to go off topic! thanks, martin

oh, and another..if cu is plastic, should i just earth sheath at cut out end and tape neatly at cu or should i put clamp on for the sake of it?

Hi MTI

1. you will need a fused switched isolator as over 2M - 3M rule dependant on DNO
2. i would use 25mm or you will need to do calcs for 16mm, (and single phase can be 100amp)
3. it needs to be metal or use an earth tag to earth the outer armour. 80amp overcurrent if this ties in with your MD.
4. not sure of question? more info please (you do not need a seperate earth, mark cable both ends)

regards

MTI · May 13, 2009

cheers electro for the info so far. ignore the seperate earth question-was just checking. as swa is 3 core and not relying on sheath as main earth then all is well.

single phase 100A? correct me if wrong but although some propertys have 80 or 100 amp slow blow cut out, you can only physically draw 80A from any single phase supply? an engineer told me this and i just took his word for it.

think ill go with 25mm for what its worth and no questions asked. price is good so not bothered really and will avoid calcs as you mentioned

if i have seperate earth conductor combined within swa (3 core) and its earthed via nut, banjo and gland on isolator, surely its not mandatory to bond at plastic cu? may do it anyway but any advice most appreciated mate.
all companys i have worked for, have fitted 2 core subs and seperate earth to mutli story appartments etc but not bonded swa sheath at final cu.same goes with 3 core subs too. suppose its good practice but not as necessary with 2core and seperate earth

also, wont swa of say 5m take 100A? correct me if wrong? still going with 25mm but just out of interest

sorry, meant 16mm swa taking 100A+ ?

electro · May 13, 2009

MTI said:
cheers electro for the info so far. ignore the seperate earth question-was just checking. as swa is 3 core and not relying on sheath as main earth then all is well.

single phase 100A? correct me if wrong but although some propertys have 80 or 100 amp slow blow cut out, you can only physically draw 80A from any single phase supply? an engineer told me this and i just took his word for it.

think ill go with 25mm for what its worth and no questions asked. price is good so not bothered really and will avoid calcs as you mentioned

if i have seperate earth conductor combined within swa (3 core) and its earthed via nut, banjo and gland on isolator, surely its not mandatory to bond at plastic cu? may do it anyway but any advice most appreciated mate.
all companys i have worked for, have fitted 2 core subs and seperate earth to mutli story appartments etc but not bonded swa sheath at final cu.same goes with 3 core subs too. suppose its good practice but not as necessary with 2core and seperate earth

also, wont swa of say 5m take 100A? correct me if wrong? still going with 25mm but just out of interest

sorry, meant 16mm swa taking 100A+ ?

Hi MTI

A 100amp fuse will take 100 amps +
bonding a Cu if its plastic No,you will need to terminate the earth into the MET
25mm swa 145amps
16mm saw 110amps
these are rough guides no CF taken into account, as i dont know your exact run.

regards

MTI · May 13, 2009

hi electro, sorry if we got our lines crossed-bonding plastic cu? all i meant was protecting/earthing the sheath of swa at both ends. obviously with plastic cu and swa, there is nothing to terminate swa into. nobody i know would put a gland on swa at cu just to finish the job off nicely. i dont mean glanding it to plastic cu obvoiusly but just 'leaving loose' within a plastic 50x50 trunking or something as it comes out of floor as an example.
right, so metal clad isolator at mains cut out,
swa glanded into isolator,
25mm d. insulated tails from metal clad isolator into metre,
banjo on gland too with flying lead onto met in isolator box?
earthing conductor within swa onto met in isolator box or straight to met at supply and one back to metal box?
other thing is, i have a tns with clamp on incoming feed to house-no pme-bummer!
plus to make it worse, as normal in my area, next doors house is daisy chained into my job! would you link both clamps? i think supply company say no.

i ended up with this on a job last week and had to use a spike as no incoming earth