Outside lights - Supply question

[timo]

Evening all.


Got a quote to do.
Guy wasnt some outside lights fitting around his garden borders. All going to be wired in SWA from a unit in a near by outbuilding.

At the mo the out building is fed from an old (passed it) SWA and isnt RCD protected on the main feed from the house. The CU in the outbuilding isnt RCD protected either. The SWA feed to the out building is to be replaced in the near future.
So what course would you take. Im thinking add a RCBO to the existing CU and recomend the other circuits be upgraded to RCBO's or new CU when the new SWA feed is done?

Also as this light circuit is coming from a CU not in the house and fed from the CU in the out building do i need to be checking/testing SWA feed to the out building. Also do i need to check/upgrade the Water/gas bonding to the house?

Just need to little clearing up as to other CU's fed not in the house etc eg dis boards supplied serperate from the house CU

thanks

 

[pushrod]

Definitely check main bonding. And check SWA to shed as by extending it you are effectively taking responsibility for the whole circuit -see below

SWA itself doesn't rcd protection but fairly certain the garden lights will ( high up permanent ones don't but think low ones do - would need to look it up ) Any sockets in the shed should definitely be rcd protected though

One thing to watch out for though is voltage drop. This is calculated from the original supply not the sub-main in the shed - it sounds like you could have a pretty long run.

The other thing to check is to make sure that the armouring of the SWA's is earthed at the supply ends even if it is not being used as the CPC

 

[johnboy6083]

Is'nt the ZS used to ensure that your circuit will comply with disconnection times and volt drop. so what im getting at is: say youve tested the ZS of the submain to the shed and its within 3/4 tabulated value, then as long as your ZS of any circuits from that are within 3/4 tab values from the MCB in the submain board,they will pass.

another question, when testing submain boards,

eg in plant rooms, or flats fed by riser mains,

where do you measure you ZE. I know it is the origin of the installation, but is that the installation of the flat, or plant room, or do you have to trace the mains back to the suppliers cut out?

thanks, john

 

[pushrod]

Is'nt the ZS used to ensure that your circuit will comply with disconnection times and volt drop. so what im getting at is: say youve tested the ZS of the submain to the shed and its within 3/4 tabulated value, then as long as your ZS of any circuits from that are within 3/4 tab values from the MCB in the submain board,they will pass.

another question, when testing submain boards,

eg in plant rooms, or flats fed by riser mains,

where do you measure you ZE. I know it is the origin of the installation, but is that the installation of the flat, or plant room, or do you have to trace the mains back to the suppliers cut out?
Measure the Ze at the DB beside the cut out so you are measuring the impedance external to the installation.

thanks, john

but you wouldn't install a circuit and then check the Zs to hope that it was ok and then have to change it if it wasn't - you would design it first to be sure that it was going to be correct.

 

[johnboy6083]

cheers pushrod. answered my question!

 

[pushrod]

No problem and btw just because a tested Zs is within the tabulated values it does not mean that volt drop is ok as well.
In future best to post separate questions in their own thread - keeps the mods happy

 

[timo]

So back to my orginal post guys

I need to carry out a ZE at the main cutout spliter block before the 60A Switch Fuse for suply to the dis board in the out building. and not a ZE from the out building?

If so how do i work out ZS. as R1+R2 is + ZE but i would have nothing for the SWA but if i did measure the SWA R1+R2 would i just add that to the others to find the total Zs ?

 

[pushrod]

check size of original swa, length of proposed runs,work out design current, and volt drop to see if the outside lighting circuit is a feasible project without replacing the original swa. Volt drops are added and only 3% is allowed for the lighting.