2391 exam question

[ta-electrical]

Hi, got a question from the 2391 exam that i need answers for, any help would be appreciated.

You'll need the following information to help answer the question;
- It's a TT system
- Electrode resistance (Ze) = 29 ohms
- BSEN 60898 maximum Zs = 8.0 ohms
- Length = 60m to furthest point of circuit
___________________________
Conductor CSA | mΩ/m at 20°C |
__1.0mm ____|___ 18.1______ |
__1.5mm ____|___ 12.1______ |


a) Calculate R1 + R2 for circuit if CPC is 1mm2 copper conductor (show all calculations)

b) Determine the value of Zs and say why it meets BS7671 requirements for the BSEN 60898 device (show all calculations)

c) State with reason what action would be taken if circuit fails to meet requirements

Thanks in advance.

 

[IQ Electrical]

Assuming 1.5mm line, 1.0mm cpc:

R1=60X12.1/1000=0.73 Ohms

R2=60X18.1/1000=1.09 Ohms

R1+R2=0.73 Ohms + 1.09 Ohms = 1.82 Ohms

Zs=Ze+(R1+R2) 29+1.82 = 30.82 Ohms

Circuit exceeds the max Zs for the OPD however, the addition of RCD protection will ensure compliance.

 

[martysparky]

am I correct thinking without looking etc, the cpc is 0.65 greater resistance than line.

 

[telectrix]

yes its more but 0.36

 

[IQ Electrical]

No, in that scenario, the cpc resistance (R2) should be 1.5 X line resistance (R1).

 

[telectrix]

true. 0.73 x 1.5 =1.09 , 18:12, 3:2 , 1.5

 

[martysparky]

so, if your doing an end to end test on a ring what would you expect to find?

the cpc = 1.5 x line??

 

[IQ Electrical]

No, on a ring you'd have 2.5/1.5 so r2 would be 1.67 X r1

 

[martysparky]

nice one is that in the gn3?

 

[IQ Electrical]

Yes, it does mention the ratio of 2.5/1.5 in step 1 of the 3 steps of the ring continuity test.

 

[martysparky]

Just had a look, on the ball m8.