trouble with 2394/2395 revision queston

stevo · Jul 4, 2014

hey guys....

ive downloaded some revision questions for my 2394/95 course i have booked for august and ive come across a question im a little unsure about......even when i have used the regs and GN3 to help. a little help would be greatly appreciated as there is no answer sheet for the questions. Heres the question:

consider the following circuit: A TNS system in a domestic property has a earth fault loop impedance measured at the origin of 0.46ohm. A 4mm2 twin and earth cable with a 1.5mm2 cpc feeds a cooker which is operated by a control unit which has an integral socket outlet. the cooker cable is 35mtrs long and is fed through a 30amp BS3036 fuse in a consumer unit at the origin.

1) state the max disconnection time for the circuit? ( i put 0.4 seconds)
2)state the maximum permissible earth loop impedance if the rule of thumb method is used?

( i used Zs < 0.8 x U/Ia , which gave 230/210 x 0.8 = 0.876ohm)

3)Using the more accurate method state the value of actual earth loop impedance?

(i Used 1/F(Zs +Ze(F-1)) , which gave me 1/1.2x(2.64+0.46(1.2-1) = 2.407ohm)

4) State the approximate disconnection time of this circuit should a fault of negligible impedance occur between phase and earth at the extremity of the circuit when the conductors are at their operating temperature?

(i put "wont trip")

5) state if the disconnection time is acceptable?

(i put "no")

6)what simple change could be made if the circuit does not comply?

(i put "decrease the size of the fuse")

If some is kind enough and has the time, could they check through this for me and if ive gone wrong let me know where.

thank you in advance

Richard Burns · Jul 4, 2014

stevo said:
hey guys....

ive downloaded some revision questions for my 2394/95 course i have booked for august and ive come across a question im a little unsure about......even when i have used the regs and GN3 to help. a little help would be greatly appreciated as there is no answer sheet for the questions. Heres the question:

consider the following circuit: A TNS system in a domestic property has a earth fault loop impedance measured at the origin of 0.46ohm. A 4mm2 twin and earth cable with a 1.5mm2 cpc feeds a cooker which is operated by a control unit which has an integral socket outlet. the cooker cable is 35mtrs long and is fed through a 30amp BS3036 fuse in a consumer unit at the origin.

1) state the max disconnection time for the circuit? ( i put 0.4 seconds)
2)state the maximum permissible earth loop impedance if the rule of thumb method is used?

( i used Zs < 0.8 x U/Ia , which gave 230/210 x 0.8 = 0.876ohm)

3)Using the more accurate method state the value of actual earth loop impedance?

(i Used 1/F(Zs +Ze(F-1)) , which gave me 1/1.2x(2.64+0.46(1.2-1) = 2.407ohm)

4) State the approximate disconnection time of this circuit should a fault of negligible impedance occur between phase and earth at the extremity of the circuit when the conductors are at their operating temperature?

(i put "wont trip")

5) state if the disconnection time is acceptable?

(i put "no")

6)what simple change could be made if the circuit does not comply?

(i put "decrease the size of the fuse")

If some is kind enough and has the time, could they check through this for me and if ive gone wrong let me know where.

thank you in advance

1) state the max disconnection time for the circuit? ( i put 0.4 seconds)
A final circuit less than 32A at a Uo of 230V on a TNS is indeed 0.4s
2)state the maximum permissible earth loop impedance if the rule of thumb method is used?

( i used Zs < 0.8 x U/Ia , which gave 230/210 x 0.8 = 0.876ohm)
Correct
3)Using the more accurate method state the value of actual earth loop impedance?

(i Used 1/F(Zs +Ze(F-1)) , which gave me 1/1.2x(2.64+0.46(1.2-1) = 2.407ohm)
You have used the Zs for the 5 s disconnection time for a BS3036 fuse instead of the Zs for the 0.4 s disconnection time.
Your answer should be fairly close to the value in BS7671 of 1.09 Ω.
I think the question may have been asking for the actual value of the earth loop impedance i.e. what you would expect to measure.
based on Zs = Ze + (R1+R2) however the use of the phrase "using the more accurate method" may mean I am wrong.
4) State the approximate disconnection time of this circuit should a fault of negligible impedance occur between phase and earth at the extremity of the circuit when the conductors are at their operating temperature?

(i put "wont trip")
If you calculate the expected Zs value from the (R1+R2) resistance of the cable at 16.71 mΩ/m and the given Ze.
and then calculate the expected fault current at that point using ohms law I = Uo / Zs and read the disconnection time off the BS3036 graph for that current this should give you the approximate disconnection time.
(the answer It will not trip would be incorrect)

5) state if the disconnection time is acceptable?

(i put "no")
check the answer you got above and see if the time is less than 0.4s.

6)what simple change could be made if the circuit does not comply?

(i put "decrease the size of the fuse")
If you decrease the size of the fuse then you may be subject to unwanted tripping of the circuit as the circuit should have been designed using this fuse as the minimum rating that is greater than the design current (after diversity).
Changing the length of the circuit or the size of the conductors should help but would not be a simple change, but replacing the fuse with a BSEN60898 type B circuit breaker would give you more leeway.