TT distribution circuit

[soulman]

Hi, a bit of advice required. I need to install another d.b to a garage, the tails are to be split. The earthing system is TT, the cable is armoured, clipped to the side of the building & over to the garage on a small cantenary wire. As it is TT would the distribution cable need to have rcd protection? I have been told it doesn't, but the regs stipulate 1 second disconnection time.

Cheers

 

[guest119]

What is your plan for over current protection?

Boydy

 

[soulman]

Hi Boydy,

There will be a switched fuse at the origin.

 

[telectrix]

bit you'll need the RCD for fault protection to achieve your disconnection time.

 

[sjm]

I would put a 100ma type s (time delayed) then your switched fuse to protect your distribution circuit. Then 30ma RCDs to cover circuits from 2nd DB.

 

[sjm]

bit you'll need the RCD for fault protection to achieve your disconnection time.

Not if the earth rod was installed by eng54.

 

[GLENNSPARK]

I would put a 100ma type s (time delayed) then your switched fuse to protect your distribution circuit. Then 30ma RCDs to cover circuits from 2nd DB.

yep...was going to say 100mA s type RCD top end for the feed...

30mA RCD/s for the finals..

 

[soulman]

Thanks for advice, much appreciated,

I was going to use a 100ma as the cable was clipped, i would have used 30ma if cable was buried.

 

[telectrix]

Thanks for advice, much appreciated,

I was going to use a 100ma as the cable was clipped, i would have used 30ma if cable was buried.

why use 30mA for buried cable - mole protection?

 

[GLENNSPARK]

why use 30mA for buried cable - mole protection?

well...you dont know whoos ferreting and rummaging about down there nowadays...

 

[GLENNSPARK]

Thanks for advice, much appreciated,

I was going to use a 100ma as the cable was clipped, i would have used 30ma if cable was buried.

you are using an SWA...so this cable type enjoyes an earthed metallic covering yes...

so no need for the 30mA for it...and dont forget as well if you gave the SWA 30mA top end for fault protection...then how are you going to split the finals and acheave discrimination & diversity?..

got to think these things through mate ....

 

[soulman]

I always thought 30ma had to be used when buried on TT, incase someone cut through the cable. Thanks for that appreciated.

 

[sparks1234]

Not if the earth rod was installed by eng54.

Don't forget your 0.1 ohm reading on the earth rods….LOL

 

[telectrix]

I always thought 30ma had to be used when buried on TT, incase someone cut through the cable. Thanks for that appreciated.

no. the 30mA RCD is for all final circuits, not distribution circuits. it's the fault protection that protects SWA form damage. in this case an RCD is used on TT systems as the EFLI is too high to achieve disconnection times, generally 5 secs. for a dist. cct.

 

[GLENNSPARK]

no. the 30mA RCD is for all final circuits, not distribution circuits. it's the fault protection that protects SWA form damage. in this case an RCD is used on TT systems as the EFLI is too high to achieve disconnection times, generally 5 secs. for a dist. cct.

and finals over 32A...which begs me to ask after looking at a recent condition report carried out by a so called `competant person`...he had put 63A OPDs down as .4
as well as a multitude of other made up crap..

well i guess you sees stuff every day....

 

[telectrix]

deleted.

 

[soulman]

no. the 30mA RCD is for all final circuits, not distribution circuits. it's the fault protection that protects SWA form damage. in this case an RCD is used on TT systems as the EFLI is too high to achieve disconnection times, generally 5 secs. for a dist. cct.

That's really helpful.

Cheers

 

[driverman]

Hi, a bit of advice required. I need to install another d.b to a garage, the tails are to be split. The earthing system is TT, the cable is armoured, clipped to the side of the building & over to the garage on a small cantenary wire. As it is TT would the distribution cable need to have rcd protection? I have been told it doesn't, but the regs stipulate 1 second disconnection time.

Cheers

depending on over current protection i think you find it 0.2 sec

 

[telectrix]

distribution circuit, 1 sec. (see 411.3.2.4.) it's 0.2 secs. for finals.