Max Zs value for TT distribution circuit (NOT Final circuit!)

[zak325]

Apologies if this question sounds a bit basic but am a little confused at the mo and quite new to the game. Here goes -

I am trying to complete a NICEIC Highway Installation Certificate for a system where there is a 230 v TT supply coming into a roadside pillar into the main BS88 fused head. 6mm Meter tails then take this into a 30mA RCBO which protects the sole circuit to an accessory.

So on the certificate I have 2 circuits - 1. Supply (Distribution) and 2. Final circuit to accessory (Final)

I know there is some debate on using the 1667 ohm / 200 ohm as max Zs for the protected final circuit figure but what should I use for the max Zs of the actual supply / distribution circuit ?

If i was working on a TN system I could happily just look up the max Zs figures using Table 41.2 but am a bit stumped with this TT scenario.

Any help on this welcomed. Thanks

 

[i=p/u]

put in 1667

 

[zak325]

thanks i=p/u - do you think its okay to put this even though its not an rcd protected final cicuit?

 

[ashrow]

RCBO protects the circuit (or did i read wrong) ?

 

[i=p/u]

sorry reading this wrong, whatever fuse is protecting the dis circuit will have a value

 

[zak325]

A C6 BS61009-1 RCBO protects the final circuit so max Zs (from table 41.5) I can take as 1667 ohms.

but its the gG16 BS88 supply/distribution circuit that I am trying to find the max Zs value for.

 

[i=p/u]

Maximum Values of Loop Impedance (Zs) for comparison with test readings based on 80% of maximum values above (as per Appendix 14)
Rating of Protective Device
Protective device
(MCB/ Fuse)
Disconnection
time (s)
5A
6A
10
15A
16A
20A
25A
30A
32A
40A
45A
50A
60A
63A
80A
100A
BS EN 60898 Type B
0.4 & 5
7.36
6.14
3.68
2.46
2.30
1.84
1.47
1.28
1.15
0.92
0.82
0.74
0.58
0.46
0.37
BS EN 60898 Type C
0.4 & 5
3.06
1.84
1.15
0.92
0.74
0.58
0.46
0.37
0.29
0.23
0.18
BS EN 60898 Type D
0.4 & 5
1.54
0.92
0.58
0.46
0.37
0.29
0.23
0.18
0.14
0.11
0.09
0.4
8.36
2.62
1.36
0.92
0.48
BS 1361
Cartridge Fuse
5
13.12
4.00
2.24
0.77
1.47
0.56
0.77
0.56
0.40
0.29
0.4
6.82
4.09
2.16
1.42
1.15
0.83
0.66
0.48
BS 88
HBC fuse
5
10.8
5.94
3.34
2.33
1.84
1.47
1.08
0.83
0.66
0.46
0.34

 

[i=p/u]

3.34ohms is the 80% value .. sorry my copy and paste didnt come out right

 

[zak325]

should i be using simple ohms law then? so for example if it is a 16A fuse then 230v/16a = 14 ohms? my max Zs would be 14 ohms?

- - - Updated - - -

sorry i posted my last whilst you posted that last message

 

[i=p/u]

3 times 16 =48 , 230 /48 = 4.7 x 0.8 =3.8

but all type b breakers is 5 times, c = 10 times, d=20 times

- - - Updated - - -

you got the green book? big 1

 

[zak325]

sorry but why are you multiplying the fuse rating by 3?

I only have the big red book at the mo - green one on order

 

[i=p/u]

3 times the rated current of fuse it takes to blow instantly, then is why if your zs is over this it wont blow instantly in a fault...

 

[zak325]

thanks a lot for your help i=p/u. i will try and bounce a few thoughts off some of the guys from work on this one also.

 

[i=p/u]

it makes sense eventually... im only on year four but have done 2391 over ten weeks at a college which i think made it stick for me.... and never ending querstions..

 

[zak325]

i understand your formula but whereabouts can i find the "3 times rated current" in the regs do you know?

 

[Richard Burns]

You have a distribution circuit protected by a 16A BS88 fuse on a TT system so your disconnection time is one second.
The time current characteristic tables in APP3 of BGB (p300) show a table in the top right for disconnection currents for various times.
From the table a BS88-2 16A with 1s Disconnection time has a minimum current required to trip of 75A.
230V / 75A = 3.1 ohms
So max Zs of dist circuit is 3.1 ohms at 70ºC
(In BRB p248 and 72A)

 

[i=p/u]

he needs a book , i use me braIN LOL

 

[thekingiam]

another silly question of the day why 1 second disconection time? because street lighting?

 

[spark 68]

another silly question of the day why 1 second disconection time? because street lighting?

No, because it is a distribution circuit, 5 seconds on a TN system, 1 second on a TT system

 

[i=p/u]

I know all this stuff, but it aint easy remembering it all..... Someday aaa.

 

[zak325]

Thanks for your responses - the second part of Richard Burn's response has really hit the bullseye.
Thanks again for all the contributions!