# Is this Question Right ?

Discuss Is this Question Right ? in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

A

#### acestu

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Hi guys could you help me please i have been given this question but i don't think it is right:

An A2 radial circuit is wired in 4mm 2 thermoplastic twin and earth cable. It is 23 meteres long. The circuit has on it 4 twin 13 amp sockets, Protection is by a 30 amp BS 3036 semi enclosed fuse, Ze for the instalation is 0.6 ohms.
socket 1 is 12 metres from the consumer unit, socket 2 is 6 metres from socket 1 and socket3 is is 2.5 metres from socket 2

Calculate the r1 & r2 value at each socket outlet
Will the protective device be suitable

could you let me know what you think the answer is so that i can compare mine

acestu

C

#### cctbreaker

It doesn't say what size the cpc is? Do we assume it's also 4mm?

#### fengine

##### -
Given Ze= 0.6 ohms
A few assumptions here
No rating factors
4mm fte with 2.5cpc
20 c temp
R1+R2 table 9A OSG=12.02mohm/m
Thermoplastic max temp70 c table 52.1
Table41.2 max EFL zs 1.09 for BS 3036 semi enclosed
R1+R2
Sct 1 12m x12.02/1000=0.144ohm
Sct2 18m x12.02/1000=0.216ohm
Sct3 20.5m x12.02/1000=0.246ohm
Sct4 23m x12.02/1000=o.276ohm
Max R1+R2=0.276x1.20(correction factor OSG table 9c)=.3312
Zs=0.6+.3312=0.9312
Calculated Zs =0.9312
Max Zs allowable =1.09

#### ian.settle1

##### -
Mentor
Arms
It doesn't say what size the cpc is? Do we assume it's also 4mm?
It does tell you!

He is using twin and earth

C

#### cctbreaker

Yeah, but given it sounds like an exam question I guess you could in theory have 1.5, 2.5 or 4 for the cpc;-)

A

#### acestu

Ok fellas this is what the answer says:

From table 9a in the on-site guide r1 & r2 value for the 4mm 2 / 1.5mm 2 conductors is 16.71 m ohm /M. The r1 & r2 value at the sockets is

socket 1 = 12 x 16.71 divided by 1000 = 0.2 ohms

socket 2 = 18 x 16.71 divided by 1000 = 0.3 ohms

socket 3 = 20.5 x 16.75 divided by 1000 = 0.34 ohms

Zs will be 0.34 + 0.6 = 0.94 ohms. The maximum zs for 30 amp BS 3036 fuse from table 41B1 in BS7671 (0.4s) is given as 1.14 ohms.

Using the rule of thumb for temperature correction the maximum permitted value is 1.14 x 0.75 = 0.86 ohms. This value is acceptable.

Ok so what does everybody think of th?

Thanks
Acestu

C

#### cctbreaker

Well it's using 16th edition regs for a start.

A

#### acestu

I am only a newbie student but my thoughts first of all were the same as Mr cctbreaker ie what is the csa of the protective conductor, and then when i looked at the answer i thought what has happened to the 4th 13 amp socket surely i use the resistance reading at the end of the radial circuit in order to determine Zs, I thought i was going mad, it seems that this publication is full of mistakes because the first couple of questions seem to have errors

Acestu 