Installing On A Tt System

Hi,

Could someone please confirm that I am thinking along the right lines with this. (Ive never worked on a TT system)

Just as an example........Say I install a circuit on a TT system.

The Ze reading is 16 ohms 

The R1 + R2 reading is 0.25 ohms.

So if i calculated the Zs, it would be 16.25 ohms.

Obviously this is going to be alot higher than the max Zs values in the BGB,

But because every TT circuit should be RCD protected, my disconnection time would be less than 0.2 seconds.

So.....would this be the way to conform to the current regs ?

Cheers

Just before the discussion starts about your reads,

are your RCD devices double or single pole?

basically yes. although in reality, your Zs will usually be lower due to parallel paths but not enough that you dont need the RCD

Technician said:

Just before the discussion starts about your reads,

are your RCD devices double or single pole?

Click to expand...

This is just in theory,not an actual job.

But I would use double pole.

your max permitted Zs on a TT system is dependant upon your RCD size, and not your OCPD size, 

its a different table in the big green comic,

HTH

steptoe said:

your max permitted Zs on a TT system is dependant upon your RCD size, and not your OCPD size, 

its a different table in the big green comic,

HTH

Click to expand...

Thanks for that,

I`ll have another look in the BGB.

steptoe said:

your max permitted Zs on a TT system is dependant upon your RCD size, and not your OCPD size, 

its a different table in the big green comic,

HTH

Click to expand...

It is,,,, but you have to be aware of the L-N loop otherwise a MCB might not disconnect in the event of a short circuit!

I notice you didn't scoob me there Canoey

NozSpark said:

It is,,,, but you have to be aware of the L-N loop otherwise a MCB might not disconnect in the event of a short circuit!

Click to expand...

well added,

NozSpark said:

I notice you didn't scoob me there Canoey

Click to expand...

it wasnt that good, 

Ok heres one for you. 

An eicr (a few actually)  gets a Unsatisfactory because the RCBO's used on all socket circuits are above the 80% & knocking on for being on the 100% value for Zs for the over current / mcb portion of the device (they would they are type C), yet the same company that did some of this work (fitting RCBO's) 2 years ago filled out the eic as 1667 for Zs  :C

M107, was it TT installation or TN.

Tn

its NOT 1667 anyway, no matter how you measure it, so they were negligent in their first attempt anyways.......

just...... being correct

The maximum permissible on a TT would be 1667 for 30mA, Type C would be ok depending on L-N loop.

However it's TN.

NozSpark said:

It is,,,, but you have to be aware of the L-N loop otherwise a MCB might not disconnect in the event of a short circuit!

Click to expand...

+1