16mm T&E used as supply to 2nd CU

[LeighSawczyn]

Customer has a 25mm sub main fed into DP isolator (as pictured) then 16mm T&E out of the top to feed 2nd consumer unit about 20m away. 2nd consumer unit has 8 circuits. (1 32A Ring, 2 socket radials, 1 lights, Shower (pump fed no heater), Immersion heater, 2 Ovens). Diversity & cable rating aside surely the supply to the 2nd CU should be 16mm CPC as per OSG 4.4 not 6mm?

• Is the 6mm earth conductor supply to the 2nd CU classed as "the" earthing conductor? If so surely OSG 4.4 applies (16/16)
• or is it considered as the main protective bonding conductor in which case 6mm is still inadequate for 16mm live conductor supply isn't it? (16/10)

If the earthing conductor is inadequate do I run a 10mm earth cable alongside the run? Or rip it all out and use 20m of "proper 16mm L,N,E tails"
Thanks in advance

 

[LeighSawczyn]

Oh it's TN-S not TT by the way

 

[telectrix]

get a pefc reading at the 2nd CU. then use the adiabatic to see if that 6mm is compliant.

 

[LeighSawczyn]

get a pefc reading at the 2nd CU. then use the adiabatic to see if that 6mm is compliant.

S= (SQRT(I2*T))/K I=1045.45 T=0.4 K=115
S=5.75mm
Presumably the disconnection time is 0.4s

Thanks telectrix. Just as well the Ze was 0.22 and not 0.2 then!

Does OSG 4.4(i) not apply here then?

 

[telectrix]

why use 0.4 secs for the value of t? bet that OCPD will dis. in 0.1 secs or less with a PEFC of 1kA.

 

[HappyHippyDad]

You say the 25mm is fed into the DP isolater (looks like a 100A main switch), but what is the breaker to the right of that, is it doing anything?

Cant quite work out if there is any OCPD apart from the main cut out for the 20m of 16mm?

 

[telectrix]

i would have thogh that that MCB would be 63A if protecting 16mm.

 

[HappyHippyDad]

And like you said Tel, I 'think' the disconnection time would be 0.1s at that Ze from Fig 3A4

Have to admit I always get a bit confused when I see that straight line on the disconnection time graphs! For the type B breakers (Fig 3A4) it goes straight at just over 10 seconds. For any PFC over that it states 'refer to manufacturers let through data'.

Also the table to the right of that graph shows the fault current required for disconnection time 0.1s-5s, thats quite a big spread. How do we narrow it down and get to this 0.1s figure?

 

[HappyHippyDad]

ps... Have scoured the forum a bit and anything after the straight line is 'instantaneous disconnection'. Sorry for hijacking the thread a bit Leigh although slightly on topic!

 

[telectrix]

i can't understand that table that the IET have put there. if a certain fault current causes disconnection within 0.1 secs, why state 0.1 - 5?

 

[HappyHippyDad]

Blimey Tel! If you don't know there's not a chance I will!

 

[D Skelton]

I'm observing the 'be nice' rule here!

 

[Guest77]

I'm observing the 'be nice' rule here!

Bah let it out man, speak your mind

 

[dingledong]

If you know the answers tell him
Don't keep it pent up just nicely make your point