please help with this question

[paulthemagpie]

hi guys,

i need some help with an assignment and id appreciate it so so much if anyone can help me.

A circut which is supplied from DB 3 is protected by a 16A type C circut breaker and has a measured earth fault loop impednace of 1.35 ohms. Please determine (with reasons were applicable)

a. if this value of earth fault loop impendance is acceptable
b. at least one method of achieving disconnection should the device provide unacceptable in (a.) above with out re-wiring the circut.
c. at least one method of confirming the integrity of the live conductors of that circut under short circut conditions.

thanks alot guys for any help

 

[plugsandsparks]

I think i would be tempted to try Ze + r1+r2 compensated just to see if i could get a better result as its so close.

 

[brman]

I think for C they are just looking for an insulation resistance test, this would confirm that the cables have suitable integrity, though why it mentions a short circuit I do not know.

that is what confused me.

 

[plugsandsparks]

that is what confused me.

dunno either, may be confirming conductor sizes ?

 

[brman]

The OP did state for b) without rewiring, so the only way you could conform with disconnection times in that case would be by RCD.

Agree with you about c) it is a strange question.

True, but to me "rewiring" means changing the cable. How about adding bonding to reduce R2?

 

[malcolmsanford]

That what threw me Ritch yes to confirm the cables integrity you carry out an IR .................but where does the short circuit come in, all I can think of was either

A) they are thinking about ADS to limit the time of the fault and therefore stress on the cable

B) Doing an IR test, after a fault occurred to check the cable

 

[brman]

dunno either, may be confirming conductor sizes ?

This is what gets me about the C&G. they complain about candidates not reading and understanding the question but half the time they don't actually word the question properly. Don't get me started.....

 

[ringer]

As plugandsparks says for the value in the regs books this is at 70oC and you need to correct as Malcolm has said in post 3 a for the temperature at which the EFLI has been measured.....

Richard, you risk here putting the OP on the wrong track. The 70oC is likely to be the maximum operating temperature of the cable, though the specific type of cable has not been specified. It might be 90oC cable for all we know! The point is that the note on Table 41.3 states the values given are for when the conductors are at their normal operating temperature which is very much unlikely to be 70oC. The reference to Appendix 14 then points the reader to the 0.8 'rule of thumb' correction factor. This is of course a compromise, because there are so many unknowns, the actual conductor temperature at the time of the test and the actual conductor temperature under normal conditions being just two of those unknowns. Hence the expression 'rule of thumb'.

 

[plugsandsparks]

Richard, you risk here putting the OP on the wrong track. The 70oC is likely to be the maximum operating temperature of the cable, though the specific type of cable has not been specified. It might be 90oC cable for all we know! The point is that the note on Table 41.3 states the values given are for when the conductors are at their normal operating temperature which is very much unlikely to be 70oC. The reference to Appendix 14 then points the reader to the 0.8 'rule of thumb' correction factor. This is of course a compromise, because there are so many unknowns, the actual conductor temperature at the time of the test and the actual conductor temperature under normal conditions being just two of those unknowns. Hence the expression 'rule of thumb'.

arrrr - the cat is out of the bag

 

[ringer]

...
c. at least one method of confirming the integrity of the live conductors of that circut under short circut conditions.
...

I can only assume that they mean confirm the continued integrity after short circuit conditions, which would be by doing an insulation resistance test.

 

[plugsandsparks]

I can only assume that they mean confirm the continued integrity after short circuit conditions, which would be by doing an insulation resistance test.

But at least one method, struggling to think of any more

 

[ringer]

... the 0.8 'rule of thumb' correction factor....

arrrr - the cat is out of the bag

Do you think using this expression might gain bonus marks in a 2391 exam then?

 

[plugsandsparks]

Do you think using this expression might gain bonus marks in a 2391 exam then?

all morning we had been trying to avoid giving the OP the answer, its such a simple rule but the regs behind it are perhaps too complex. Anyway its all about GN3 for mere mortals and thank god for that.

 

[Basil one eye]

A - meets maximum zs but exceeds rule of thumb, you probably should mention that
B - use a 16a btype breaker or 16a rcbo of via rcd protected way
c - very poor question, integrity of conductors ?? Test continuity and insulation resistance ? Calculate fault current and ensure your protective device will operate using regs ? (city & guilds question !!)

 

[JuniorSparky]

I'm doing the same course and on the drawing it's stated that DB 3 is security system. Isn't it considered low voltage system and if so we have to check table 41.6 for Zs. But then I don't know how to answer b) and c).

 

[sparkintraining]

Was this the 2365 exam?