2391 question problem....help!!!!

[tommy.dale]

hey guys sorry to sound a numpty but i need help with a question in a last years past paper (june 2010) as i have my exam on thursday!

anyways any feedback would be grateful!

1)state 2 methods by which the external earth fault loop impedance (Ze) may be determined.

And

2) one method of determing Zs when direct measurement is not possible (Not Ze)


this is how its written i havent changed anything..

Cheers guys!!!!!

 

[truckster]

1) Measurement by loop tester or enquiry

2) Add Ze to R1+R2

 

[tommy.dale]

thanks mate, enquiry to who? is there a formula for the ze like there there is for zs or no??

 

[truckster]

Enquiry to the DNO who will probably tell you they don't know. There are stated maximum values for each type of supplier earthing
TNCS O.35
TNS 0.8

BTW For a ring cct Zs=(R1 +R2)/4 +Ze

 

[pushrod]

Hi truckster typed virtually an identical answer to the above then the phone rang before i hit the send button - came back, refreshed and there yours was - spooky magic lol

Tommy.Dale you should hit the "thanks" button for truckster for helping you out

 

[geofff]

Being as it's for the C&G exam you will lose marks for incorrect terminology, the markers seem to have got their knickers in a twist over it.
The correct test instrument to measure Ze is an "earth fault loop impedance tester"
and the calculated Zs for a ring is (R1+R2)+Ze

The (r1+r2)/4 is to get a target value to be measured at each socket outlet, just beware of using upper case R which has a different meaning to lower case r.